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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/37350-8.txt b/37350-8.txt new file mode 100644 index 0000000..6aa037d --- /dev/null +++ b/37350-8.txt @@ -0,0 +1,1476 @@ +The Project Gutenberg EBook of The Ancestry of Modern Amphibia: A Review +of the Evidence, by Theodore H. Eaton + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: The Ancestry of Modern Amphibia: A Review of the Evidence + +Author: Theodore H. Eaton + +Release Date: September 8, 2011 [EBook #37350] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK THE ANCESTRY OF MODERN *** + + + + +Produced by Chris Curnow, Charlene Taylor, Joseph Cooper +and the Online Distributed Proofreading Team at +http://www.pgdp.net + + + + + + + + + + University of Kansas Publications + Museum of Natural History + + + Volume 12, No. 2, pp. 155-180, 10 figs. + -----------July 10, 1959--------------- + + + The Ancestry of Modern Amphibia: + A Review of the Evidence + + BY + + THEODORE H. EATON, JR. + + + University of Kansas + Lawrence + 1959 + +University of Kansas Publications, Museum of Natural History + +Editors: E. Raymond Hall, Chairman, Henry S. Fitch, Robert W. Wilson + + + Volume 12, No. 2, pp. 155-180 + Published July 10, 1959 + + + University of Kansas + Lawrence, Kansas + + + PRINTED IN + THE STATE PRINTING PLANT + TOPEKA, KANSAS + 1959 + + [Illustration] + + 27-8362 + + + + +[Transcriber's Notes: Several typos have been regulated. + +One typo of "ancester" for "ancestor" was corrected. + +One instance of "salamanderlike" corrected to "salamaner-like".] + + + + +The Ancestry of Modern Amphibia: A Review of the Evidence + +BY +THEODORE H. EATON, JR. + + + + +INTRODUCTION + + +In trying to determine the ancestral relationships of modern orders of +Amphibia it is not possible to select satisfactory structural ancestors +among a wealth of fossils, since very few of the known fossils could +even be considered possible, and scarcely any are satisfactory, for such +a selection. The nearest approach thus far to a solution of the problem +in this manner has been made with reference to the Anura. Watson's paper +(1940), with certain modifications made necessary by Gregory (1950), +provides the paleontological evidence so far available on the origin of +frogs. It shows that several features of the skeleton of frogs, such as +the enlargement of the interpterygoid spaces and orbits, reduction of +the more posterior dermal bones of the skull, and downward spread of the +neural arches lateral to the notochord, were already apparent in the +Pennsylvanian _Amphibamus_ (Fig. 1), with which Gregory synonymized +_Miobatrachus_ and _Mazonerpeton_. But between the Pennsylvanian and the +Triassic (the age of the earliest known frog, _Protobatrachus_) there +was a great lapse of time, and that which passed between any conceivable +Paleozoic ancestor of Urodela and the earliest satisfactory +representative of this order (in the Cretaceous) was much longer still. +The Apoda, so far as known, have no fossil record. + +Nevertheless it should be possible, first, to survey those characters of +modern Amphibia that might afford some comparison with the early +fossils, and second, to discover among the known Paleozoic kinds those +which are sufficiently unspecialized to permit derivation of the modern +patterns. Further circumstantial evidence may be obtained by examining +some features of Recent Amphibia which could not readily be compared +with anything in the fossils; such are the embryonic development of the +soft structures, including cartilaginous stages of the skeleton, the +development and various specializations of the ear mechanism, adaptive +characters associated with aquatic and terrestrial life, and so on. + + + + +COMPARISON OF MODERN ORDERS WITH THE +LABYRINTHODONTS AND LEPOSPONDYLS + + +[Illustration: Fig. 1. _Saurerpeton_ (× 1/2, after Romer, 1930, fig. 6); +_Amphibamus_, the palatal view × 2-1/4, from Watson, 1940, fig. 4 (as +_Miobatrachus_), the dorsal view × 2-1/2, from Gregory's revised figure +of _Amphibamus_ (1950, Fig. 1); _Protobatrachus_, × 1, from Watson, +1940, fig. 18, 19.] + +In both Anura and Urodela the skull is short, broad, relatively flat, +with reduced pterygoids that diverge laterally from the parasphenoids +leaving large interpterygoid vacuities, and with large orbits. (These +statements do not apply to certain larval or perennibranchiate forms.) +The skull in both orders has lost a number of primitive dermal bones in +the posterior part; these are: basioccipital, supraoccipital, +postparietal, intertemporal, supratemporal, and tabular. The +exoccipitals form the two condyles but there are no foramina for the +11th and 12th nerves, since these are not separate in modern Amphibia. +The opisthotic is missing in all except Proteidae (but see discussion +of the ear). Although the skull is normally autostylic, a movable +basipterygoid articulation is present among Hynobiid salamanders and in +at least the metamorphic stages of primitive frogs, and therefore should +be expected in their ancestors. The vertebrae are, of course, complete; +see discussion in later section. The quadratojugal, lost in salamanders, +is retained in frogs, and conversely the lacrimal, absent in frogs, +occurs in a few primitive salamanders. The situation in Apoda is +different, but postfrontal and jugal should be noted as bones retained +in this order while lost in the others. + +Thus, in spite of minor differences, the above list shows that there are +numerous and detailed similarities between Anura and Urodela with +respect to the features in which they differ from the Paleozoic orders. +Pusey (1943) listed 26 characters which _Ascaphus_ shares with +salamanders but not with more advanced frogs; a few of these might be +coincidental, but most of them are of some complexity and must be taken +to indicate relationship. The main adaptive specializations of Anura, +however, including loss of the adult tail, extreme reduction in number +of vertebrae, formation of urostyle, elongation of the ilium and +lengthening of the hind legs, must have appeared at a later time than +the separation of that order from any possible common stem with Urodela, +although they are only partially developed in the Triassic +_Protobatrachus_. + +Turning to the Paleozoic Amphibia, there are two groups in which some +likelihood of a relationship with modern order exists. In the +Pennsylvanian Trimerorhachoidea (Labyrinthodontia, order Temnospondyli) +some members, such as _Eugyrinus_, _Saurerpeton_, and notably +_Amphibamus_ (Fig. 1) had short, broad heads, an expansion of palatal +and orbital openings, posterior widening of the parasphenoid associated +with divergence of the pterygoids, a movable basipterygoid articulation, +and reduction in size (but not loss) of the more posterior dermal bones +of the skull. In recognition of Watson's (1940) evidence that these +animals make quite suitable structural ancestors of frogs, Romer (1945) +placed _Amphibamus_ in an order, Eoanura, but Gregory (1950) indicated +that it might better be left with the temnospondyls. Association of the +urodele stem with this group does not seem to have been proposed +hitherto. + +The other group of Paleozoic Amphibia that has been considered probably +ancestral to any modern type is the subclass Lepospondyli, containing +three orders, Aistopoda, Nectridia and Microsauria. In these the +vertebrae are complete (holospondylous), the centra presumably formed by +cylindrical ossification around the notochord, and there is no evidence +as to the contributions from embryonic cartilage units. It is important +to note at this point that precisely the same statement can be made +regarding the vertebrae of _adults_ of all three Recent orders, yet for +all of them, as shown in a later section, we have ample evidence of the +part played by cartilage elements in vertebral development. Therefore +(a) we cannot say that there were no such elements in embryonic stages +of lepospondyls, and (b) it would take more than the evidence from adult +vertebrae to relate a particular modern order (for example, Urodela) to +the Lepospondyli. Vague similarities to Urodela have been noted by many +authors in the Nectridia, Aistopoda and Microsauria, but these are not +detailed and refer mainly to the vertebrae. The skulls do not show, +either dorsally or in the palate, any striking resemblance to those of +generalized salamanders, and certainly most known lepospondyls are too +specialized to serve as the source of Urodela. It is true that the +elongate bodies, small limbs, and apparent aquatic habitus of some +lepospondyls accord well with our usual picture of a salamander, but +such a form and way of life have appeared in many early Amphibia, +including the labyrinthodonts. The family Lysorophidae (Fig. 2), usually +placed among microsaurs, is sufficiently close in skull structure to the +Apoda to be a possible ancestor of these, but it probably has nothing to +do with Urodela, by reason of the numerous morphological specializations +that were associated with its snakelike habitus. + +[Illustration: Fig. 2. _Lysorophus tricarinatus_, lateral and posterior +views × 2-1/2, modified after Sollas, 1920, Figs. 8 and 12, +respectively; palatal view after Broom, 1918, × 1-1/2. For explanation +of abbreviations see Fig. 3.] + +McDowell's (1958) suggestion that it would be profitable to look among +the Seymouriamorpha for the ancestors of frogs seems to be based upon a +few details of apparent resemblance rather than a comprehensive view of +the major characters of the animals. In most points which he mentions +(limb girdles, form of ear, pterygoid articulation) the present writer +does not see a closer similarity of frogs to Seymouriamorpha than to +Temnospondyli. + +Still other opinions have been expressed. Herre (1935), for instance, +concludes "on anatomical, biological and paleontological grounds" that +the orders of Urodela, Anura, Apoda and Stegocephali were all +independently evolved from fish, but beyond citing the opinions of a +number of other authors he does not present tangible evidence for this +extreme polyphyletic interpretation. + +More notable are the views of several Scandinavian workers +(Säve-Söderbergh, 1934; Jarvik, 1942; Holmgren, 1933, 1939, 1949a, b), +of whom Jarvik, in a thorough analysis of the ethmoid region, would +derive the Urodela from Porolepid Crossopterygii, and all other +tetrapods from the Rhipidistia; Säve-Söderbergh and Holmgren, the latter +using the structure of carpus and tarsus, see a relationship of Urodela +to Dipnoi, but accept the derivation of labyrinthodonts and other +tetrapods from Rhipidistia. All of this work is most detailed and +laborious, and has produced a great quantity of data useful to +morphologists, but the diphyletic theory is not widely adopted; the +evidence adduced for it seems to consist largely of minutiae which, +taken by themselves, are inconclusive, or lend themselves to other +interpretation. For instance Holmgren's numerous figures of embryonic +limbs of salamanders show patterns of cartilage elements that he would +trace to the Dipnoan type of fin, yet it is difficult to see that the +weight of evidence requires this, when the pattern does not differ in +any fundamental manner from those seen in other embryonic tetrapods, and +the differences that do appear may well be taken to have ontogenetic +rather than phylogenetic meaning. Further, the Dipnoan specialization of +dental plates and autostylic jaw suspension, already accomplished early +in the Devonian, would seem to exclude Dipnoi from possible ancestry of +the Urodela, an order unknown prior to the Mesozoic, in which the teeth +are essentially similar to those of late Paleozoic Amphibia, and the jaw +suspension is not yet in all members autostylic. + + + + +THE EAR + + +[Illustration: Fig. 3. Occipital region of skulls of _Megalocephalus +brevicornis_ (× 3/10, after Watson, 1926, as _Orthosaurus_), +_Dvinosaurus_ (× 1/4, modified after Bystrow, 1938; the lower figure +after Sushkin, 1936), and _Necturus maculosus_ (× 3, original, from K. +U., No. 3471). + +Abbreviations Used in Figures + + b'd.c.--basidorsal cartilage (neural arch) + b'oc.--basioccipital + ce._{1-4}--centrale_{1-4} + ch.--ceratohyal + clav.--clavicle + clei.--cleithrum + cor.--coracoid + d.c._{1-4}--distal carpal_{1-4} + diap.--diapophysis + exoc.--exoccipital + ep.--episternum + hyost.--hyostapes + i.--intermedium + Mk.--Meckel's cartilage + n.--notochord + om.--omosternum + op.--operculum + opis.--opisthotic + par.--parietal + par. proc.--paroccipital process + peri. cent.--perichordal centrum + p'p.--postparietal + prep.--prepollex + pro.--prootic + p'sp.--parasphenoid + pt.--pterygoid + p.t.f.--post-temporal fossa + postzyg.--postzygapophysis + qj.--quadratojugal + qu.--quadrate + ra.--radiale + r.hy.--hyomandibular ramus of VII + rib-b.--rib-bearer + r.md.--mandibular ramus of VII + sc.--scapula + sc'cor.--scapulocoracoid + s'd.--supradorsal cartilage + s'd.(postzyg.)--supradorsal (postzygapophysis) + soc.--supraoccipital + sp.c.--spinal cord + sq.--squamosal + s'sc.--suprascapula + s't.--supratemporal + sta.--stapes + ster.--sternum + tab.--tabular + uln.--ulnare + v.a.--vertebral artery + xiph.--xiphisternum + I,IV--digits I and IV + V, VII, X, XII--foramina for cranial nerves of these numbers (in + Fig. 4, VII is the facial nerve) +] + +In temnospondylous Amphibia the tympanum generally occupied an otic +notch, at a high level on the skull, bordered dorsomedially by the +tabular and ventrolaterally by the squamosal. In this position the +tympanum could receive airborne sounds whether the animal were entirely +on land or lying nearly submerged with only the upper part of its head +exposed. Among those Anura in which the ear is not reduced the same is +true, except that the tabular is lost. In Temnospondyli (Fig. 3) the +posterior wall of the otic capsule was usually formed by the opisthotic, +which extended up and outward as a buttress from the exoccipital to the +tabular, and sometimes showed a paroccipital process for the insertion, +presumably, of a slip or tendon of the anterior axial musculature. The +stapes, in addition to its foot in the fenestra ovalis and its tympanic +or extrastapedial process to the tympanum, bore a dorsal process (or +ligament) to the tabular, an "internal" process (or ligament) to the +quadrate or an adjacent part of the squamosal, and a ligament to the +ceratohyal. Some of these attachments might be reduced or absent in +special cases, but they seem to have been the ones originally present +both phylogenetically and embryonically in Amphibia. + +Among typical frogs (Fig. 4) the base, or otostapes, is present and +bony, the extrastapedial process (extracolumella, or hyostapes) is +usually cartilaginous, the dorsal process (processus paroticus) is of +cartilage or ligament, but the other two attachments are absent in the +adult. The exoccipital extends laterally, occupying the posterior face +of the otic capsule. Between it and the otostapes is a small disc, +usually ossified, the operculum, which normally fits loosely in a +portion of the fenestral membrane, and is developed from the otic +capsule. The opercularis muscle extends from this disc to the +suprascapula, in many but by no means all families of Anura. + +[Illustration: Fig. 4. Diagram of middle ear structures in _Rana_ (upper +figure, after Stadtmüller, 1936, and lower left after DeBeer, 1937), and +_Ambystoma_ (lower right, after DeBeer, 1937); all × 4. For explanation +of abbreviations see Fig. 3.] + +Among Urodela (Fig. 4) the middle ear cavity and tympanum are lacking, +and the stapes (columella) consists of no more than its footplate and +the stylus, which is attached to the border of the squamosal, thus +corresponding to the "internal" process. In families in which +individuals metamorphose and become terrestrial (Hynobiidae, +Ambystomidae, Salamandridae, Plethodontidae), an operculum and +opercularis muscle appear in the adult, just as in frogs, except that in +Plethodontidae, the most progressive family, the operculum fuses with +the footplate of the stapes. Among neotenous or perennibranchiate +urodeles there is no separate operculum or opercularis. The evidence +given by Reed (1915) for fusion of the operculum with the columella in +_Necturus_ appears inconclusive, in spite of the great care with which +his observations were made. On the other hand, _Necturus_ and _Proteus_ +alone among living salamanders have a distinct opisthotic on the +posterior wall of the otic capsule (Fig. 3), as do the Cretaceous +_Hylaeobatrachus_ and the Eocene _Palaeoproteus_. Probably these +Proteidae should be regarded as primitive in this respect, although many +other features may be attributed to neoteny. + +There is a contrast between Anura and most Urodela in the relative +positions of the stapes and facial nerve, as shown in DeBeer's (1937) +diagrams. In the latter (_Ambystoma_) the nerve is beneath, and in the +former (_Rana_) above, the stapes. Judging by figures of _Neoceratodus_, +_Hypogeophis_, and several types of reptiles and mammals, the Urodela +are exceptional. _Necturus_, however, has the nerve passing above its +stapes, and this may be primitive in the same sense as the persistent +opisthotic. There can be, of course, no question of the nerve having +worked its way through or over the obstructing stapes in order to come +below it in salamanders; rather, the peripheral growth of neuron fibers +in the embryo must simply pursue a slightly different course among the +partially differentiated mesenchyme in the two contrasting patterns. + +Although DeBeer (1937) shows in his figure of _Hypogeophis_ (one of the +Apoda) an operculum, this is apparently a mistake. The stapes has a +large footplate, and its stylus articulates with the quadrate, but no +true operculum or opercularis has been described in the Apoda. The +facial nerve passes above the stapes. It does not seem necessary to +regard the conditions in this order as related directly to those of +either salamanders or frogs, but a reduction of the stapes comparable to +that in salamanders has occurred. + +The presence in both frogs and terrestrial salamanders of a special +mechanism involving the opercularis muscle and an operculum cut out in +identical fashion from the wall of the otic capsule behind the stapes +seems to require some other explanation than that of a chance +convergence or parallelism. Although the stapes and otic region are +readily visible in a number of labyrinthodonts and lepospondyls, no +indication of an operculum seems to be reported among them. But in the +Triassic _Protobatrachus_ (Fig. 1), which is unmistakably a frog in its +skull, pelvis and some other features, Piveteau (1937) has shown, +immediately behind the foot of the stapes, a small bony tubercle, which +he and Watson (1940) designated opisthotic. Very clearly it served for +insertion of a muscle, and it is equally clear that the bone is a +reduced opisthotic, carrying the paroccipital process already mentioned +as characteristic of it in some temnospondyls. Since the remainder of +the posterior wall of the otic capsule consists of cartilage, meeting +the exoccipital, it may be that the opisthotic becomes the operculum in +frogs. _Protobatrachus_ was too far specialized in the Anuran direction, +although it still had a tail, and the forelegs and hind legs were nearly +the same size, to be considered a possible ancestor of the Urodeles. But +at one stage in the general reduction of the skull in the ancestry of +both groups, a condition similar to that in _Protobatrachus_ may have +characterized the otic region, long before the Triassic. + +In the argument thus far we have considered terrestrial, adult +amphibians, since it is only in these that either the normal middle ear +and tympanum, or the opercular apparatus, is present. But among the +urodeles several neotenic types occur (this term applies also to the +perennibranchs). For most of these there is nothing about the otic +region that would be inconsistent with derivation, by neoteny, from +known families in which adults are terrestrial; for example, +_Cryptobranchus_ could have had a Hynobiid-like ancestor. But this, as +mentioned above, does not hold for the Proteidae, which possess an +opisthotic of relatively large size, distinctly separate from the +exoccipital and prootic. Either this bone is a neomorph, which seems +improbable, or there has not been in the ancestry of this particular +family an episode of reduction comparable to that seen in the +terrestrial families, where there is an operculum instead of a normal +opisthotic. Therefore the Proteidae probably are not derived from the +general stem of other salamanders, but diverged sufficiently long ago +that the bones of the otic region were reduced on a different pattern. +They need not be removed from the order, but, in this respect, +recognized as more primitive than any other existing Urodela or Anura. A +recent paper by Hecht (1957) discusses many features of _Necturus_ and +_Proteus_, and shows that they are remote from each other; his evidence +does not seem to prove, however, that they were of independent origin or +that they need be placed in separate families. + + + + +VERTEBRAE AND RIBS + + +Development of the vertebrae and ribs of Recent Amphibia has been +studied by Gamble (1922), Naef (1929), Mookerjee (1930 a, b), Gray +(1930) and Emelianov (1936), among others. MacBride (1932) and Remane +(1938) provide good summaries. In this section reference will be made to +the embryonic vertebral cartilages by the names used for them in these +studies, although the concept of "arcualia" is currently considered of +little value in comparative anatomy. + +[Illustration: Fig. 5. Development of Anuran vertebrae. Upper left, late +tadpole of _Xenopus laevis_; lower left, same just after metamorphosis; +upper right, diagram of general components of primitive Anuran vertebra. +(After MacBride, 1932, Figs. 35, 38, 47D, respectively.) Lower right, +section through anterior portion of urostyle, immediately posterior to +sacral vertebra, in transforming _Ascaphus truei_ (original, from +specimen collected on Olympic Peninsula, Washington). All × 20 approx. +For explanation of abbreviations see Fig. 3.] + +The centrum in Anura (Fig. 5) is formed in the perichordal sheath +(_Rana_, _Bufo_) or only in the dorsal portion thereof (_Bombinator_, +_Xenopus_). The neural arch develops from the basidorsal cartilages that +rest upon, and at first are entirely distinct from, the perichordal +sheath. Ribs, present as separate cartilages associated with the 2nd, +3rd and 4th vertebrae in the larvae of _Xenopus_ and _Bombinator_, fuse +with lateral processes (diapophyses) of the neural arches at +metamorphosis, but in _Leiopelma_ and _Ascaphus_ the ribs remain freely +articulated in the adult. Basiventral arcualia have been supposed to be +represented by the hypochord, a median rod of cartilage beneath the +shrinking notochord in the postsacral region, which at metamorphosis +ossifies to produce the bulk of the urostyle. Fig. 5, lower right, a +transverse section taken immediately posterior to the sacral ribs in a +transforming specimen of _Ascaphus_, shows that the "hypochord" is a +mass of cartilage formed in the perichordal sheath itself, and very +obviously is derived from the ventral part of postsacral perichordal +centra; there are, then, no basiventral arcualia, and the discrete +hypochord shown in MacBride's diagram (Fig. 5, upper right) of a frog +vertebra does not actually occur below the centrum, but only below the +notochord in the postsacral region. + +[Illustration: Fig. 6. Development of Urodele vertebrae. Upper figures, +_Triton_: at left, larva at 20 mm., at right, diagram of components of +vertebra (from MacBride, 1932, figs. 17, 47C). Middle figures, _Molge +vulgaris_ larva: left, at 18 mm.; middle, at 20-22 mm.; right, at 25 mm. +(from Emelianov, 1936, figs. 33, 36, 38 respectively). Lower figures, +_Necturus maculosus_ larva: left, at 21 mm.; right, at 20 mm. (from +MacBride, 1932, figs. 41.5, 41.3 respectively, after Gamble, 1922). All +× 20 approx. For explanation of abbreviations see Fig. 3.] + +In Urodela (Fig. 6) the pattern of vertebral and rib development is more +complex, and there has been much controversy over its interpretation. +Neural arches and perichordal centra form in the same manner as in +frogs, but with the addition in certain cases (_Triton_) of a median +supradorsal cartilage, which gives rise to the zygapophyses of each +neural arch. Difficulty comes, however, in understanding the +relationship of the ribs to the vertebrae. Each rib, usually +two-headed, articulates with a "transverse process" that in its early +development seems to be separate from both the vertebra and the rib, and +is therefore known, noncommittally, as "rib-bearer." This lies laterally +from the centrum, neural arch, and vertebral artery; upon fusing with +the vertebra it therefore encloses the artery in a foramen separate from +the one between the capitulum and tuberculum of the rib (the usual +location of the vertebral artery). At least four different +interpretations of these structures have been suggested: + +(1) Naef (1929) considered the rib-bearer a derivative of the +basiventral, which, by spreading laterally and dorsally to meet the +neural arch, enclosed the vertebral artery. He then supposed that by +reduction of the rib-bearer in other tetrapods (frogs and amniotes) the +vertebrarterial foramen and costal foramen were brought together in a +single foramen transversarium. The implication is that the Urodele +condition is primitive, but it cannot now be supposed that Urodela are +ancestral to any other group, and the rib-bearer is most probably a +specialization limited to salamanders. This does not, of course, +invalidate the first part of his interpretation. + +(2) Remane (1938), noting that rib insertions of early Amphibia are +essentially as in Amniota, argued that the rib-bearer is not from the +basiventral but is a neomorph which originates directly from the neural +arch and grows ventrally. This he inferred mainly from Gamble's (1922) +observation on _Necturus_, but his assumption that _Necturus_ is more +primitive than other salamanders (such as the Salamandridae), where the +pattern differs from this, is not necessarily correct. Rather, the +perennibranchs are distinguished mainly by their neotenous features, and +their development is likely to show simplifications which are not +necessarily primitive. The suggestion of a "neomorph" ought not to be +made except as a last resort, for it is simply an acknowledgment that +the author does not recognize homology with any structure already known; +sometimes further information will make such recognition possible. + +(3) Gray (1930), using _Molge taeniatus_, concluded that the normal +capitulum of the rib was lost, but that the tuberculum bifurcated to +make the two heads seen in Urodela, thus accounting for the failure of +the costal foramen to coincide with that of the vertebral artery. This +answer, too, seems to entail an unprovable assumption which should not +be made without explicit evidence. + +(4) Finally, Emelianov (1936) regarded the rib-bearer as a rudimentary +_ventral_ rib, on account of its relationship to the vertebral artery, +and considered the actual rib to be a neomorph in the _dorsal_ position +characteristic of tetrapod ribs in general. This argument would fit the +ontogenetic picture satisfactorily, provided that (_a_) there were some +evidence of ventral, rather than dorsal, ribs in early Amphibia, and +(_b_) we accept the invention of another neomorph in modern Amphibia as +an unavoidable necessity. Emelianov's conclusion (p. 258) should be +quoted here (translation): "The ribs of Urodela are shown to be upper +ribs, yet we find besides these in Urodela rudimentary lower ribs fused +with the vertebral column. The ribs of Apoda are lower ribs. In Anura +ribs fail to develop fully, but as rare exceptions rudiments of upper +ribs appear." + +Of these various interpretations, that of Naef seems to involve the +minimum of novelty, namely, that the rib-bearer is the basiventral, +expanded and external to the vertebral artery. It is not necessary to +take this modification as the ancestral condition in tetrapods, of +course. The basiventral (=intercentrum) would merely have expanded +sufficiently to provide a diapophysis for the tuberculum as well as the +(primitive) facet for the capitulum. No neomorph appears under this +hypothesis, which has the distinct advantage of simplicity. + +Figures of early stages in vertebral development by the authors +mentioned show that the basidorsals chondrify first, as neural arches, +while a separate mass of mesenchyme lies externally and ventrally from +these. This mesenchyme may chondrify either in one piece (on each side) +or in two; in _Molge_ the part adjacent to the centrum is ossified in +the 20-mm. larva, and subsequently unites with the more dorsal and +lateral cartilaginous part, while the rib, appearing farther out, grows +inward to meet this composite "rib-bearer." In _Necturus_ the mesenchyme +below the neural arch differentiates into a cartilage below the +vertebral artery (position proper to a basiventral), a bridge between +this and the neural arch, and a rib, the latter two chondrifying later +than the "basiventral" proper. In the "axolotl" (presumably _Ambystoma +tigrinum_) the rib-bearer grows downward from its first center of +chondrification at the side of the neural arch (Emelianov, 1936). + +Thus it appears that the simplest hypothesis to account for the +rib-bearer is that (_a_) it is the basiventral, (_b_) it is recognizable +just before chondrification as a mass of mesenchyme in contact with both +the notochordal sheath and the basidorsal cartilage, (_c_) it may +chondrify or ossify first in its ventral portion or in its dorsal +portion, the two then joining before it fuses with the rest of the +vertebra, (_d_) the enclosure of the vertebral artery is a consequence +of the extension of the basiventral beyond the position occupied by it +in primitive Amphibia, and (_e_) there is no indication that this took +place in other orders than the Urodela. + +It seems that the vertebrae in Urodela have at least the following +components: perichordal centra, separate basidorsal cartilages, and +basiventrals, which are somewhat specialized in their manner of +development. The vertebrae of Anura develop in the fashion just +described except that basiventrals are lacking. It would seem no more +difficult to accept the derivation of salamander vertebrae from the +temnospondylous type than it is in the case of frogs, if other evidence +points to such an ancestry. + +[Illustration: Fig. 7. Vertebrae of _Eusthenopteron_ (×1) and +_Ichthyostega_ (×2/3, after Jarvik, 1952), _Trimerorhachis_ (×1-1/2, +after Case), and _Amphibamus_ (×10, after Watson, 1940) in lateral and +end views; the two lower right-hand figures are from Watson (1940, as +_Miobatrachus_); the lower left is from a cast of the "_Miobatrachus_" +specimen in Chicago Natural History Museum, No. 2000, in the presacral +region (original, ×10).] + +Fig. 7, lower right, is Watson's (1940) illustration of the anterior +trunk vertebrae of _Amphibamus_ (_Miobatrachus_), in which the +intercentrum is shown as a single median piece. Fig. 7, lower left, +shows two of the more posterior trunk vertebrae seen as impressions in a +cast of the type of "_Miobatrachus romeri_;" evidently the inter-centra +were paired at about the level of the 16th vertebra, and relatively +large. Gregory's (1950) figure of the type specimen of "_Mazonerpeton_" +(also equivalent to _Amphibamus_) shows the anterior trunk vertebrae in +relation to the ribs essentially as they appear to me in the cast of +_Miobatrachus_, and rather differently from Watson's figure of the +latter. Gregory is probably right in considering the specimens to +represent various degrees of immaturity. So far as present information +goes, then, the vertebrae of salamanders and frogs show no _clear_ +evidence of derivation from those of any particular group among the +early Amphibia, but their features are not inconsistent with a +simplification of the pattern of Temnospondyli. + +[Illustration: Fig. 8. Pectoral girdles of _Protobatrachus_ (after +Piveteau, 1937), _Notobatrachus_ (after Stipanicic and Reig, 1956), +Ascaphus (after Ritland, 1955 a) and _Rana_ (original); all ×2. For +explanation of abbreviations see Fig. 3.] + + + + +PECTORAL GIRDLE + + +Hecht and Ruibal (Copeia, 1928:242) make a strong point of the nature of +the pectoral girdle in _Notobatrachus_, as described recently by +Stipanicic and Reig (1955, 1956) from the Jurassic of Patagonia, and +quite rightly recommend that the significance of the arciferal and +firmisternal types of girdle be restudied. That of _Notobatrachus_ is +said to be firmisternal; in view of the arciferal condition in the +supposedly primitive _Leiopelma_, _Ascaphus_, _Bombinator_, etc., this +comes as a surprise. Is the firmisternal girdle, as seen in _Rana_, +_Bufo_, and others, actually the ancestral type, and has the arciferal +been derived from something like this? + +In the figures given by Stipanicic and Reig the ossified parts of the +girdle are figured in detail (Fig. 8) and Reig's discussion of it is +thorough. The decision to call it firmisternal was taken with some +hesitancy, for no median elements are indicated, and the position and +shape of those seen is closely similar to the ossified parts in +_Ascaphus_ and _Leiopelma_; there is no bony sternum or omosternum. It +is safe to suppose that some cartilage lay in the midline between the +clavicles and coracoids, but there is no evidence as to its extent, +rigidity, or degree of overlapping if any. Apparently, then, there is +not sufficient reason to infer that this Jurassic frog had a pectoral +girdle comparable with the modern firmisternal type. + +Piveteau (1955:261) remarks that the only living Anuran that can be +compared usefully with _Protobatrachus_ (Triassic) with regard to its +pectoral girdle is _Ascaphus_. Again, the extent of cartilage in +_Protobatrachus_ (Fig. 8) can only be inferred, and there are no median +elements. The agreement with _Ascaphus_ includes the presence, in both, +of a separate coracoid ossification situated posterior to the ossified +"scapulocoracoid" (actually scapula). This ossification is evidently +that shown in _Notobatrachus_ as "coracoid." Direct comparison of the +three genera with one another suggests that if we use the term arciferal +for any, we should use it for all. + +In the remote predecessor of Anura, _Amphibamus_ of the Pennsylvanian, +the pectoral girdle was less substantial than in many of its +contemporaries, but it contained the primitive median interclavicle in +addition to the clavicle, cleithrum, and scapulocoracoid. (The figure of +Watson, 1940, and that by Gregory, 1950, are of individuals of different +ages, the latter being older.) It is clear that the paired elements of +such a girdle were held rigid by their attachment to the interclavicle, +_via_ the clavicles. Subsequent elimination of the interclavicle in the +Anuran line of descent, and decrease of ossification, left a girdle like +that of _Protobatrachus_, _Notobatrachus_, _Ascaphus_ and _Leiopelma_. +But in several advanced families a more rigid median "sternum," of one +or two bony pieces plus cartilage, is developed secondarily, possibly +(as Cope, 1889: 247, suggested) in correlation with axillary amplexus. + +Among Urodela no dermal bones occur in the pectoral girdle. There is +usually a scapulocoracoid ossified as a single piece, from which a thin +cartilaginous suprascapula extends dorsally and a broad cartilaginous +coracoid plate extends medially, overlapping the one from the opposite +side; a precoracoid lobe of this reaches forward on either side, and a +median, posterior "sternum" of cartilage may make contact with the +edges of the two coracoids. In _Siren_ and _Amphiuma_ two centers of +ossification are found for each scapulocoracoid, and in _Triton_ and +_Salamandra_ three. Probably the more dorsal and lateral of these +represents the primitive scapula and the other one (or two) the +primitive coracoid. + +Comparing the girdle of a salamander with that of a frog, the closest +similarity can be seen between _Ascaphus_ and a salamander in which the +scapula and coracoid ossify separately. Both have the median "sternum" +in contact with the coracoid plates. The major difference, of course, is +the lack of clavicle and cleithrum in the salamander. + + + + +CARPUS AND TARSUS + + +In _Ascaphus_ (Ritland, 1955a; cleared and stained specimens of nearly +grown males) distal carpals 1, 2, 3 and 4 are present and separate, +increasing in size in the order given (Fig. 9). A prepollex rests +against centrale 1; centralia 2 and 3 are fused; the radiale fuses with +centrale 4, and the intermedium fuses with the ulnare; radius and ulna +are fused with each other as in other frogs. The digits (and +metacarpals) are considered by Ritland to be 1-4, in addition to the +prepollex, rather than 2-5. + +[Illustration: Fig. 9. Skeleton of fore foot of _Notobatrachus_ (after +Stipanicic and Reig, 1956, terminology revised) and _Ascaphus_ (after +Ritland, 1955 a); all ×5. For explanation of abbreviations see Fig. 3.] + +In the Jurassic _Notobatrachus_ Stipanicic and Reig (1956) have shown +the carpus with surprising clarity (Fig. 9). If their nomenclature of +the parts be revised, we obtain a fairly close resemblance to +_Ascaphus_, except that centralia 2 and 3 are not fused, distal carpals +1 and 2 do not show (which would easily be understood if they were of +the size of those in _Ascaphus_, or not ossified), and the intermedium +remains separate from the ulnare. + +In _Salamandra_ (Francis, 1934; Nauck, 1938) distal carpals 1 and 2 are +fused in both larva and adult, and 3 and 4 are separate; the radiale, +intermedium and ulnare are separate in the larva but the latter two fuse +in the adult; centrale 1 (labelled prepollical cartilage by Francis) and +centrale 2 are separate. Francis considers the digits (and metacarpals) +to be 1-4. Apparently the arrangement here indicated for the larva is +characteristic of other larval salamanders, except where further +reduced, and reduction below the number given for the adult is common in +other terrestrial forms. The radius and ulna are, of course, separate. + +The ossification of carpals is more likely to be complete in adult frogs +than in salamanders, but some ossification of all parts named is found +in several of the latter. A common ancestor of frogs and salamanders +could be expected to have the following elements present and ossified in +the adult: distal carpals 1-4 separate; 3 centralia; radiale, +intermedium and ulnare separate. Comparison with fossils older than +_Notobatrachus_ is fruitless on these points, unless we go back to forms +too distant to have any special value, such as _Eryops_. This is because +of inadequate preservation and because the elements are not fully +ossified in many immature specimens. + +For the purpose of this review there is no special value in a comparison +of the tarsi of frogs and salamanders, since the leaping adaptation of +the former leaves very little common pattern between them. Even in +_Protobatrachus_, where the legs were not yet conspicuously lengthened, +the tibiale and fibulare ("astragalus" and "calcaneum" respectively) +were already considerably elongated. The carpus and tarsus of +_Amphibamus_ are as yet undecipherable. + + + + +THE LARVA + + +Considering the postembryonic developmental stages of modern Amphibia, +there can be no doubt that a gill-bearing, four-legged larva of a +salamander, in which lateral line pores and a gular fold are present, +represents much more closely the type of larva found in labyrinthodonts +than does the limbless, plant-nibbling tadpole of the Anura. +Salamander-like larvae of labyrinthodonts are well known, especially +those formerly supposed to comprise the order Branchiosauria. Many, +perhaps the majority of, labyrinthodonts show some features associated +with aquatic life even when full-grown, as do the lepospondyls. These +features may include impressions of sensory canals on the dermal bones +of the skull, persistence of visceral arches, reduction in size of +appendages, and failure of tarsal and carpal elements to ossify. In +fact, it appears that very few of the Paleozoic Amphibia were successful +in establishing themselves as terrestrial animals even as adults. + +Nevertheless, in the ancestry of Anura, and that of at least the +Hynobiid, Ambystomid, Salamandrid and Plethodontid salamanders, there +must certainly have been a terrestrial adult, transforming from an +aquatic larva. The leaping mechanism of Anura, shown in so many features +of their anatomy, is perhaps to be explained as a device for sudden +escape from land into the water, but it was not yet perfected in the +Triassic _Protobatrachus_ or the Jurassic _Notobatrachus_. + +The middle ear, its sound-transmitting mechanism, and the tympanum, well +developed in most Anura, are readily derived from those of early +labyrinthodonts, and are presumably effective for hearing airborne +sounds whether on land or while floating in the water. Reduction of +these organs in Urodela may be correlated with their customary +restriction to subsurface habitats and inability to maintain a floating +position while in water. + +Some light may be shed on the significance of the tadpole of Anura by +considering the early stages of the ribbed frogs, Liopelmidae. +_Leiopelma_ and _Ascaphus_ are so closely similar in the adult that +there is no doubt that they belong in one family, primitive in some +respects (including articulated ribs; pyriformis and +caudalipuboischiotibialis muscles) but not in others (absence of +tympanum and middle ear). In both genera the eggs are large, 5 mm. in +_Leiopelma_, 4.5 mm. in _Ascaphus_, and unpigmented; but at this point +the resemblance ends. + +[Illustration: Fig. 10. _Leiopelma hochstetteri_ larva, lateral and +ventral (after Stephenson, 1955), ×4.] + +Stephenson (1955) showed that embryos of _L. hochstetteri_ develop +equally well on land (in damp places) or in the water, and that embryos +prematurely released from egg capsules develop successfully in the +water. The larvae possess both pairs of legs (Fig. 10) and a broad gular +fold similar to that of larval salamanders. In _L. hochstetteri_ the +fold grows back over the forelegs temporarily, but remains free from the +body and presently the legs reappear, whereas in _L. archeyi_ the +forelegs are not covered at any time. No branchial chamber or spiracle +is formed. Of course direct development, without a tadpole, occurs in +several other groups of Anura, but in each case terrestrial adaptations +are obvious. This is not true of _Leiopelma_, which Stephenson regards +as more nearly comparable with Urodela in its development than with +other Anura, and he sees in it a "primary and amphibious" mode instead +of a terrestrial specialization. + +The _Ascaphus_ tadpole bears no outward resemblance to the larva of +_Leiopelma_, but is a normal tadpole in form, although sluggish in +activity. Its greatly expanded labial folds bear numerous rows of horny +epidermal "teeth," which, with the lips, serve to anchor the tadpole to +stones in the swift water of mountain brooks. Noble (1927) noticed that +particles of food were taken in through the external nares, and that a +current of water passed through these openings and out by way of the +median spiracle. It appears that any action by the teeth and jaws in +scraping algae from the rocks (which were bare in the stream where I +have collected _Ascaphus_) would be quite incidental, and that the lips +and teeth must be primarily a clinging mechanism. Certain other mountain +brook tadpoles (for example, _Borborocoetes_) show similar devices, but +these are developed independently, as specializations from the usual +sort of tadpole. + +May it not be that closure of the gill-chamber by the opercular (=gular) +fold, retardation of limb development, expansion of the lips, growth of +parallel rows of horny teeth, and other correlated features that make a +tadpole, were brought about as an adaptation of the primitive Anuran +larva to a swift-stream habitat, and that this "basic patent" then later +served to admit the tadpoles of descendant types to an alga-scraping +habit in quiet water as well? The tadpole, as a unique larval type among +vertebrates, bears the hallmarks of an abrupt adaptive shift, such as +might have occurred within the limits of a single family, and it seems +difficult to imagine the enclosed branchial chamber, the tooth-rows, and +lips of a familiar tadpole as having evolved without some kind of +suctorial function along the way. + + + + +SUMMARY + + +The Anura probably originated among temnospondylous labyrinthodonts, +through a line represented approximately by _Eugyrinus_, _Amphibamus_, +and the Triassic frog _Protobatrachus_, as shown by Watson, Piveteau and +others. The known Paleozoic lepospondyls do not show clear indications +of a relationship with Urodela, but _Lysorophus_ may well be on the +ancestral stem of the Apoda. + +Between Urodela and Anura there are numerous resemblances which seem to +indicate direct relationship through a common stock: (1) a similar +reduction of dermal bones of the skull and expansion of palatal +vacuities; (2) movable basipterygoid articulation in primitive members +of both orders; (3) an operculum formed in the otic capsule, with +opercularis muscle; (4) many details of cranial development, cranial +muscles, and thigh muscles, especially between _Ascaphus_ and the +Urodela, as shown by Pusey and Noble; (5) essentially similar manner of +vertebral development, quite consistent with derivation of both orders +from Temnospondyli; (6) presence in the larva of _Leiopelma_ of a +salamander-like gular fold, four limbs, and no suggestion of +modification from a tadpole (Stephenson). + + + + +LITERATURE CITED + + +Broom, R. + + 1918. Observations on the genus _Lysorophus_ Cope. Ann. Mag. Nat. + Hist., (9)2:232-239. + +Bystrow, A. P. + + 1938. Dvinosaurus als neotenische Form der Stegocephalen. Acta + Zool., 19:209-295. + +Case, E. C. + + 1935. Description of a collection of associated skeletons of + _Trimerorhachis_. Contrib. Mus. Pal. Univ. Michigan, 4:227-274. + +Cope, E. D. + + 1889. The Batrachia of North America. Bull. U. S. Nat. Mus., + 34:1-525. + +de Beer, G. R. + + 1937. The development of the vertebrate skull. Pp. xxiii + 552. + Oxford, Clarendon Press. + +de Villiers, C. G. S. + + 1934. Studies of the cranial anatomy of _Ascaphus truei_ + Stejneger, the American "Liopelmid." Bull. Mus. Comp. Zool., + 77:1-38. + +Emelianov, S. W. + + 1936. Die Morphologie der Tetrapodenrippen. Zool. Jahrb. (Anat.), + 62:173-274. + +Francis, E. T. B. + + 1934. The anatomy of the salamander. Pp. xxxi + 381. Oxford, + Clarendon Press. + +Gamble, D. L. + + 1922. The morphology of the ribs and the transverse processes of + _Necturus maculatus_. Jour. Morph., 36:537-566. + +Gray, P. + + 1930. On the attachments of the Urodele rib to the vertebra and + their homologies with the capitulum and tuberculum of the Amniote + rib. Proc. Zool. Soc. London, 1930(1931):907-911. + +Gregory, J. T. + + 1950. Tetrapods from the Pennsylvanian nodules from Mazon Creek, + Illinois. Am. Jour. Sci., 248:833-873. + +Hecht, M. E. + + 1957. A case of parallel evolution in salamanders. Proc. Zool. + Soc. Calcutta, Mookerjee Mem.:283-292. + +Holmgren, N. + + 1933. On the origin of the tetrapod limb. Acta Zool., 14:185-295. + + 1939. Contributions to the question of the origin of the tetrapod + limb. Acta Zool., 20:89-124. + + 1949a. Contributions to the question of the origin of tetrapods. + Acta Zool., 30:459-484. + + 1949b. On the tetrapod limb problem again. Acta Zool., 30:485-508. + +Herre, W. + + 1935. Die Schwanzlurche der mitteleocänen (oberlutetischen) + Braunkohle des Geiseltales und die Phylogenie der Urodelen unter + Einschluss der fossilen Formen. Zoologica, 33:87, 1-5. + +Jarvik, E. + + 1942. On the structure of the snout of Crossopterygians and lower + gnathostomes in general. Zool. Bidrag fran Upsala, 21:235-675. + 1952. On the fish-like tail in the Ichthyostegid Stegocephalians. + Meddelelser on Grønland, 114(12):1-90. + +Mookerjee, H.K. + + 1930a. On the development of the vertebral column of the Urodela. + Phil. Trans. Roy. Soc. London, B 218:415-446. + + 1930b. On the development of the vertebral column of the Anura. + Philos. Trans. Royal Soc. London, B 219:165-196. + +MacBride, E.W. + + 1932. Recent work on the development of the vertebral column. + Cambridge, Biol. Rev., 7:108-148. + +McDowell, S.B. + + 1958. Are the frogs specialized seymouriamorphs? (Abstract) Anat. + Rec., 132(3):472. + +Naef, A. + + 1929. 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The sound-transmitting apparatus in Necturus. Anat. Rec., + 9:581-590. + +Remane, A. + + 1936. Wirbelsäule und ihre Abkömmlinge. In: Handbuch der + vergleichenden Anatomie der Wirbeltiere, L. Bolk _et al._, + 4:1-206. Urban and Schwarzenberg, Berlin, Vienna. + +Ritland, R. M. + + 1955a. Studies on the post-cranial morphology of Ascaphus truei. + I. Skeleton and spinal nerves. Jour. Morph., 97:119-174. + + 1955b. Studies on the post-cranial morphology of Ascaphus truei. + II. Myology. Jour. Morph., 97:215-282. + +Romer, A. S. + + 1945. Vertebrate paleontology. 2nd edition. Pp. viii + 687. Univ. + Chicago Press. + + 1947. Review of the Labyrinthodontia. Bull. Mus. Comp. Zool., + 99:3-368. + +Säve-Söderbergh, G. + + 1934. Some points of view concerning the evolution of the + vertebrates and the classification of this group. Arkiv för + Zoologi, 26A:1-20. + +Stadtmüller, F. + + 1936. Kranium und Visceralskelett der Stegocephalen und Amphibien. + In: Handbuch der vergleichenden Anatomie der Wirbeltiere, by L. + Bolk _et al._, 4:501-698. + +Stephenson, N. G. + + 1955. On the development of the frog, _Leiopelma hochstetteri_ + Fitzinger. Proc. Zool. Soc. London, 124(4):785-795. + +Stipanicic, P. N. and Reig, O. A. + + 1955. Breve noticia sobre el hallazgo de anuros en el denominado + "Complejo Porfirico de la Patagonia Extraandina," con + consideraciones acerca de la composicion geologica del mismo. + Revista de la Asoc. Geol. Argentina, 10(4):215-233. + + 1956. El "complejo porfirico de la Patagonia extraandina" y su + fauna de Anuros. Acta Geol. Lilloana (Univ. Nac. del Tucuman), + 1:185-297. + +Sushkin, P. P. + + 1936. Notes on the pro-Jurassic Tetrapoda from U. S. S. R. III. + Dvinosaurus Amalitzky, a perennibranchiate stegocephalian from the + Upper Permian of North Dvina. Trav. Inst. Pal. Acad. Sci. URSS, + 5:43-91. + +Watson, D. M. S. + + 1940. The origin of frogs. Trans. Roy. Soc. Edinburgh, + 40(7):195-231. + + + + +_Transmitted April 7, 1959._ + + + + + +End of the Project Gutenberg EBook of The Ancestry of Modern Amphibia: A +Review of the Evidence, by Theodore H. Eaton + +*** END OF THIS PROJECT GUTENBERG EBOOK THE ANCESTRY OF MODERN *** + +***** This file should be named 37350-8.txt or 37350-8.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/3/7/3/5/37350/ + +Produced by Chris Curnow, Charlene Taylor, Joseph Cooper +and the Online Distributed Proofreading Team at +http://www.pgdp.net + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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Eaton, Jr. + </title> + <style type="text/css"> + + p { margin-top: .75em; + text-align: justify; + margin-bottom: .75em; + } + + h1,h2,h3,h4,h5,h6 { + text-align: center; /* all headings centered */ + clear: both; + } + + hr { width: 33%; + margin-top: 2em; + margin-bottom: 2em; + margin-left: auto; + margin-right: auto; + clear: both; + } + + body { margin-left: 10%; + margin-right: 10%; + } + + ul {list-style-type: none; + padding-left: 2em; + } + + .pagenum { /* uncomment the next line for invisible page numbers */ + /* visibility: hidden; */ + position: absolute; + left: 92%; + font-size: smaller; + text-align: right; + } /* page numbers */ + + .blockquot{margin-left: 5%; margin-right: 10%;} + + .center {text-align: center;} + + .smcap {font-variant: small-caps;} + + .caption {font-weight: bold;} + + .figcenter {margin: auto; text-align: center;} + + .figleft {float: left; clear: left; margin-left: 0; margin-bottom: 1em; margin-top: + 1em; margin-right: 1em; padding: 0; text-align: center;} + + .figright {float: right; clear: right; margin-left: 1em; margin-bottom: 1em; + margin-top: 1em; margin-right: 0; padding: 0; text-align: center;} + + </style> + </head> +<body> + + +<pre> + +The Project Gutenberg EBook of The Ancestry of Modern Amphibia: A Review +of the Evidence, by Theodore H. Eaton + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: The Ancestry of Modern Amphibia: A Review of the Evidence + +Author: Theodore H. Eaton + +Release Date: September 8, 2011 [EBook #37350] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK THE ANCESTRY OF MODERN *** + + + + +Produced by Chris Curnow, Charlene Taylor, Joseph Cooper +and the Online Distributed Proofreading Team at +http://www.pgdp.net + + + + + + +</pre> + + + + + +<h2><span class="smcap">University of Kansas Publications<br /> +Museum of Natural History</span></h2> + + +<p class="center">Volume 12, No. 2, pp. 155-180, 10 figs.</p> + +<p class="center">-----------July 10, 1959-----------</p> + + +<h1>The Ancestry of Modern Amphibia:<br /> +A Review of the Evidence</h1> + +<p><b>BY</b></p> + +<h2>THEODORE H. EATON, JR.</h2> + + +<p class="center"><span class="smcap">University of Kansas</span><br /> +<span class="smcap">Lawrence</span><br /> +1959</p> + +<p class="center"><span class="smcap">University of Kansas Publications, Museum of Natural History</span></p> + +<p class="center">Editors: E. Raymond Hall, Chairman, Henry S. Fitch, +Robert W. Wilson</p> + + +<p class="center">Volume 12, No. 2, pp. 155-180<br /> +Published July 10, 1959</p> + + +<p class="center"><span class="smcap">University of Kansas</span><br /> +Lawrence, Kansas</p> + + +<p class="center">PRINTED IN THE STATE PRINTING PLANT<br /> +TOPEKA, KANSAS<br /> +1959</p> + +<div class="figcenter" style="width: 160px;"> +<img src="images/title.png" width="160" height="45" alt="Publisher's Decorative" id="coverpage" /> +</div> + +<p class="center">27-8362</p> + +<p> +[Transcriber's Notes: Several typos have been regulated.<br /> +One typo of "ancester" for "ancestor" was corrected.<br /> +One instance of "salamanderlike" corrected to "salamander-like".<br /> +The captions on some images give relative size. Due to differences in monitor size/resolution, do not +consider the images to be scalable.] +</p> + +<hr style="width: 65%;" /> +<p><span class='pagenum'><a name="Page_157" id="Page_157">[Pg 157]</a></span></p> + +<h2>The Ancestry of Modern Amphibia:</h2> + +<h2>A Review of the Evidence</h2> + +<p class="center">BY</p> + +<p class="center">THEODORE H. EATON, JR.</p> + + + +<hr style="width: 65%;" /> + +<h3>INTRODUCTION</h3> + + +<p>In trying to determine the ancestral relationships of modern orders of +Amphibia it is not possible to select satisfactory structural ancestors +among a wealth of fossils, since very few of the known fossils could +even be considered possible, and scarcely any are satisfactory, for such +a selection. The nearest approach thus far to a solution of the problem +in this manner has been made with reference to the Anura. Watson's paper +(1940), with certain modifications made necessary by Gregory (1950), +provides the paleontological evidence so far available on the origin of +frogs. It shows that several features of the skeleton of frogs, such as +the enlargement of the interpterygoid spaces and orbits, reduction of +the more posterior dermal bones of the skull, and downward spread of the +neural arches lateral to the notochord, were already apparent in the +Pennsylvanian <i>Amphibamus</i> (Fig. 1), with which Gregory synonymized +<i>Miobatrachus</i> and <i>Mazonerpeton</i>. But between the Pennsylvanian and the +Triassic (the age of the earliest known frog, <i>Protobatrachus</i>) there +was a great lapse of time, and that which passed between any conceivable +Paleozoic ancestor of Urodela and the earliest satisfactory +representative of this order (in the Cretaceous) was much longer still. +The Apoda, so far as known, have no fossil record.</p> + +<p>Nevertheless it should be possible, first, to survey those characters of +modern Amphibia that might afford some comparison with the early +fossils, and second, to discover among the known Paleozoic kinds those +which are sufficiently unspecialized to permit derivation of the modern +patterns. Further circumstantial evidence may be obtained by examining +some features of Recent Amphibia which could not readily be compared +with anything in the fossils; such are the embryonic development of the +soft structures, including cartilaginous stages of the skeleton, the +development and various specializations of the ear mechanism, adaptive +characters associated with aquatic and terrestrial life, and so on.<span class='pagenum'><a name="Page_158" id="Page_158">[Pg 158]</a></span></p> + + + + +<hr style="width: 65%;" /> +<h3>COMPARISON OF MODERN ORDERS WITH THE +LABYRINTHODONTS AND LEPOSPONDYLS</h3> + + +<div class="figcenter" style="width: 800px;"> +<img src="images/fig1.png" width="800" height="923" alt="Fig. 1." title="Fig. 1" /> +<span class="caption">Fig. 1. Saurerpeton (× 1/2, after Romer, 1930, fig. 6); +Amphibamus, the palatal view × 2-1/4, from Watson, 1940, fig. 4 (as +Miobatrachus), the dorsal view × 2-1/2, from Gregory's revised figure +of Amphibamus (1950, Fig. 1); Protobatrachus, × 1, from Watson, +1940, fig. 18, 19.</span> +</div> + +<p>In both Anura and Urodela the skull is short, broad, relatively flat, +with reduced pterygoids that diverge laterally from the parasphenoids +leaving large interpterygoid vacuities, and with large orbits. (These +statements do not apply to certain larval or perennibranchiate forms.) +The skull in both orders has lost a number of primitive dermal bones in +the posterior part; these are: basioccipital, supraoccipital, +postparietal, intertemporal, supratemporal, and tabular. The +exoccipitals form the two condyles but there are no foramina for the +11th and 12th nerves, since these are not separate in modern Amphibia. +The opisthotic is missing in all<span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span> except Proteidae (but see discussion +of the ear). Although the skull is normally autostylic, a movable +basipterygoid articulation is present among Hynobiid salamanders and in +at least the metamorphic stages of primitive frogs, and therefore should +be expected in their ancestors. The vertebrae are, of course, complete; +see discussion in later section. The quadratojugal, lost in salamanders, +is retained in frogs, and conversely the lacrimal, absent in frogs, +occurs in a few primitive salamanders. The situation in Apoda is +different, but postfrontal and jugal should be noted as bones retained +in this order while lost in the others.</p> + +<p>Thus, in spite of minor differences, the above list shows that there are +numerous and detailed similarities between Anura and Urodela with +respect to the features in which they differ from the Paleozoic orders. +Pusey (1943) listed 26 characters which <i>Ascaphus</i> shares with +salamanders but not with more advanced frogs; a few of these might be +coincidental, but most of them are of some complexity and must be taken +to indicate relationship. The main adaptive specializations of Anura, +however, including loss of the adult tail, extreme reduction in number +of vertebrae, formation of urostyle, elongation of the ilium and +lengthening of the hind legs, must have appeared at a later time than +the separation of that order from any possible common stem with Urodela, +although they are only partially developed in the Triassic +<i>Protobatrachus</i>.</p> + +<p>Turning to the Paleozoic Amphibia, there are two groups in which some +likelihood of a relationship with modern order exists. In the +Pennsylvanian Trimerorhachoidea (Labyrinthodontia, order Temnospondyli) +some members, such as <i>Eugyrinus</i>, <i>Saurerpeton</i>, and notably +<i>Amphibamus</i> (Fig. 1) had short, broad heads, an expansion of palatal +and orbital openings, posterior widening of the parasphenoid associated +with divergence of the pterygoids, a movable basipterygoid articulation, +and reduction in size (but not loss) of the more posterior dermal bones +of the skull. In recognition of Watson's (1940) evidence that these +animals make quite suitable structural ancestors of frogs, Romer (1945) +placed <i>Amphibamus</i> in an order, Eoanura, but Gregory (1950) indicated +that it might better be left with the temnospondyls. Association of the +urodele stem with this group does not seem to have been proposed +hitherto.</p> + +<p>The other group of Paleozoic Amphibia that has been considered probably +ancestral to any modern type is the subclass Lepospondyli, containing +three orders, Aistopoda, Nectridia and Microsauria. In<span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span> these the +vertebrae are complete (holospondylous), the centra presumably formed by +cylindrical ossification around the notochord, and there is no evidence +as to the contributions from embryonic cartilage units. It is important +to note at this point that precisely the same statement can be made +regarding the vertebrae of <i>adults</i> of all three Recent orders, yet for +all of them, as shown in a later section, we have ample evidence of the +part played by cartilage elements in vertebral development. Therefore +(a) we cannot say that there were no such elements in embryonic stages +of lepospondyls, and (b) it would take more than the evidence from adult +vertebrae to relate a particular modern order (for example, Urodela) to +the Lepospondyli. Vague similarities to Urodela have been noted by many +authors in the Nectridia, Aistopoda and Microsauria, but these are not +detailed and refer mainly to the vertebrae. The skulls do not show, +either dorsally or in the palate, any striking resemblance to those of +generalized salamanders, and certainly most known lepospondyls are too +specialized to serve as the source of Urodela. It is true that the +elongate bodies, small limbs, and apparent aquatic habitus of some +lepospondyls accord well with our usual picture of a salamander, but +such a form and way of life have appeared in many early Amphibia, +including the labyrinthodonts. The family Lysorophidae (Fig. 2), usually +placed among microsaurs, is sufficiently close in skull structure to the +Apoda to be a possible ancestor of these, but it probably has nothing to +do with Urodela, by reason of the numerous morphological specializations +that were associated with its snakelike habitus.</p> + +<div class="figleft" style="width: 400px;"> +<img src="images/fig2.png" width="400" height="428" alt="Fig. 2." title="Fig. 2." /> +<span class="caption">Fig. 2. Lysorophus tricarinatus, lateral and posterior +views × 2-1/2, modified after Sollas, 1920, Figs. 8 and 12, +respectively; palatal view after Broom, 1918, × 1-1/2. For explanation +of abbreviations see Fig. 3.</span> +</div> + +<p>McDowell's (1958) suggestion that it would be profitable to look among +the Seymouriamorpha for the ancestors of frogs seems to be based upon a +few details of apparent resemblance rather than a comprehensive view of +the major characters of the animals. In most points which he mentions +(limb girdles, form of ear, pterygoid<span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span> articulation) the present writer +does not see a closer similarity of frogs to Seymouriamorpha than to +Temnospondyli.</p> + +<p>Still other opinions have been expressed. Herre (1935), for instance, +concludes "on anatomical, biological and paleontological grounds" that +the orders of Urodela, Anura, Apoda and Stegocephali were all +independently evolved from fish, but beyond citing the opinions of a +number of other authors he does not present tangible evidence for this +extreme polyphyletic interpretation.</p> + +<p>More notable are the views of several Scandinavian workers +(Säve-Söderbergh, 1934; Jarvik, 1942; Holmgren, 1933, 1939, 1949a, b), +of whom Jarvik, in a thorough analysis of the ethmoid region, would +derive the Urodela from Porolepid Crossopterygii, and all other +tetrapods from the Rhipidistia; Säve-Söderbergh and Holmgren, the latter +using the structure of carpus and tarsus, see a relationship of Urodela +to Dipnoi, but accept the derivation of labyrinthodonts and other +tetrapods from Rhipidistia. All of this work is most detailed and +laborious, and has produced a great quantity of data useful to +morphologists, but the diphyletic theory is not widely adopted; the +evidence adduced for it seems to consist largely of minutiae which, +taken by themselves, are inconclusive, or lend themselves to other +interpretation. For instance Holmgren's numerous figures of embryonic +limbs of salamanders show patterns of cartilage elements that he would +trace to the Dipnoan type of fin, yet it is difficult to see that the +weight of evidence requires this, when the pattern does not differ in +any fundamental manner from those seen in other embryonic tetrapods, and +the differences that do appear may well be taken to have ontogenetic +rather than phylogenetic meaning. Further, the Dipnoan specialization of +dental plates and autostylic jaw suspension, already accomplished early +in the Devonian, would seem to exclude Dipnoi from possible ancestry of +the Urodela, an order unknown prior to the Mesozoic, in which the teeth +are essentially similar to those of late Paleozoic Amphibia, and the jaw +suspension is not yet in all members autostylic.</p> + + + +<hr style="width: 65%;" /> +<h3>THE EAR</h3> + + +<div class="figcenter" style="width: 800px;"> +<img src="images/fig3.png" width="800" height="483" alt="Fig. 3." title="Fig. 3." /> +<span class="caption">Fig. 3. Occipital region of skulls of Megalocephalus +brevicornis (× 3/10, after Watson, 1926, as Orthosaurus), +Dvinosaurus (× 1/4, modified after Bystrow, 1938; the lower figure +after Sushkin, 1936), and Necturus maculosus (× 3, original, from K. +U., No. 3471).</span> +</div> + +<p><b>Abbreviations Used in Figures</b></p> + +<ul> +<li>b'd.c.—basidorsal cartilage (neural</li> +<li>b'oc.—basioccipital</li> +<li>ce.<sub>1-4</sub>—centrale<sub>1-4</sub></li> +<li>ch.—ceratohyal</li> +<li>clav.—clavicle</li> +<li>clei.—cleithrum</li> +<li>cor.—coracoid</li> +<li>d.c.<sub>1-4</sub>—distal carpal<sub>1-4</sub></li> +<li>diap.—diapophysis</li> +<li>exoc.—exoccipital</li> +<li>ep.—episternum</li> +<li>hyost.—hyostapes</li> +<li>i.—intermedium</li> +<li>Mk.—Meckel's cartilage</li> +<li>n.—notochord</li> +<li>om.—omosternum</li> +<li>op.—operculum</li> +<li>opis.—opisthotic</li> +<li>par.—parietal</li> +<li>par. proc.—paroccipital process</li> +<li>peri. cent.—perichordal centrum</li> +<li>p'p.—postparietal</li> +<li>prep.—prepollex</li> +<li>pro.—prootic</li> +<li>p'sp.—parasphenoid</li> +<li>pt.—pterygoid</li> +<li>p.t.f.—post-temporal fossa</li> +<li>postzyg.—postzygapophysis</li> +<li>qj.—quadratojugal</li> +<li>qu.—quadrate</li> +<li>ra.—radiale</li> +<li>r.hy.—hyomandibular ramus of VII</li> +<li>rib-b.—rib-bearer</li> +<li>r.md.—mandibular ramus of VII</li> +<li>sc.—scapula</li> +<li>sc'cor.—scapulocoracoid</li> +<li>s'd.—supradorsal cartilage</li> +<li>s'd.(postzyg.)—supradorsal (postzygapophysis)</li> +<li>soc.—supraoccipital</li> +<li>sp.c.—spinal cord</li> +<li>sq.—squamosal</li> +<li>s'sc.—suprascapula</li> +<li>s't.—supratemporal</li> +<li>sta.—stapes</li> +<li>ster.—sternum</li> +<li>tab.—tabular</li> +<li>uln.—ulnare</li> +<li>v.a.—vertebral artery</li> +<li>xiph.—xiphisternum</li> +<li>I, IV—digits I and IV</li> +<li>V, VII, X, XII—foramina for cranial nerves of these numbers (in Fig. 4, VII is the facial nerve)</li> +</ul> + +<p>In temnospondylous Amphibia the tympanum generally occupied an otic +notch, at a high level on the skull, bordered dorsomedially by the +tabular and ventrolaterally by the squamosal. In this position the +tympanum could receive airborne sounds whether the animal were entirely +on land or lying nearly submerged with only the upper part of its head +exposed. Among those Anura in which the ear is not reduced the same is +true, except that the tabular is<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span> lost. In Temnospondyli (Fig. 3) the +posterior wall of the otic capsule was usually formed by the opisthotic, +which extended up and outward as a buttress from the exoccipital to the +tabular, and sometimes showed a paroccipital process for the insertion, +presumably, of a slip or tendon of the anterior axial musculature. The<span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span> +stapes, in addition to its foot in the fenestra ovalis and its tympanic +or extrastapedial process to the tympanum, bore a dorsal process (or +ligament) to the tabular, an "internal" process (or ligament) to the +quadrate or an adjacent part of the squamosal, and a ligament to the +ceratohyal. Some of these attachments might be reduced or absent in +special cases, but they seem to have been the ones originally present +both phylogenetically and embryonically in Amphibia.</p> + +<p>Among typical frogs (Fig. 4) the base, or otostapes, is present and +bony, the extrastapedial process (extracolumella, or hyostapes) is +usually cartilaginous, the dorsal process (processus paroticus) is of +cartilage or ligament, but the other two attachments are absent in the +adult. The exoccipital extends laterally, occupying the posterior face +of the otic capsule. Between it and the otostapes is a small disc, +usually ossified, the operculum, which normally fits loosely in a +portion of the fenestral membrane, and is developed from the otic +capsule. The opercularis muscle extends from this disc to the +suprascapula, in many but by no means all families of Anura.</p> + +<div class="figright" style="width: 400px;"> +<img src="images/fig4.png" width="400" height="412" alt="Fig. 4." title="Fig. 4." /> +<span class="caption">Fig. 4. Diagram of middle ear structures in Rana (upper +figure, after Stadtmüller, 1936, and lower left after DeBeer, 1937), and +Ambystoma (lower right, after DeBeer, 1937); all × 4. For explanation +of abbreviations see Fig. 3.</span> +</div> + +<p>Among Urodela (Fig. 4) the middle ear cavity and tympanum are lacking, +and the stapes (columella) consists of no more than its footplate and +the stylus, which is attached to the border of the squamosal, thus +corresponding to the "internal" process. In families in which +individuals metamorphose and become terrestrial (Hynobiidae, +Ambystomidae, Salamandridae, Plethodontidae), an operculum and +opercularis muscle appear in the adult, just as in frogs, except that in +Plethodontidae, the most progressive family, the operculum fuses with +the footplate of the stapes. Among neotenous or perennibranchiate +urodeles there is no separate operculum or opercularis. The evidence +given by Reed (1915) for fusion of the operculum with the columella in +<i>Necturus</i> appears inconclusive, in spite of the great care with which +his observations were<span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span> made. On the other hand, <i>Necturus</i> and <i>Proteus</i> +alone among living salamanders have a distinct opisthotic on the +posterior wall of the otic capsule (Fig. 3), as do the Cretaceous +<i>Hylaeobatrachus</i> and the Eocene <i>Palaeoproteus</i>. Probably these +Proteidae should be regarded as primitive in this respect, although many +other features may be attributed to neoteny.</p> + +<p>There is a contrast between Anura and most Urodela in the relative +positions of the stapes and facial nerve, as shown in DeBeer's (1937) +diagrams. In the latter (<i>Ambystoma</i>) the nerve is beneath, and in the +former (<i>Rana</i>) above, the stapes. Judging by figures of <i>Neoceratodus</i>, +<i>Hypogeophis</i>, and several types of reptiles and mammals, the Urodela +are exceptional. <i>Necturus</i>, however, has the nerve passing above its +stapes, and this may be primitive in the same sense as the persistent +opisthotic. There can be, of course, no question of the nerve having +worked its way through or over the obstructing stapes in order to come +below it in salamanders; rather, the peripheral growth of neuron fibers +in the embryo must simply pursue a slightly different course among the +partially differentiated mesenchyme in the two contrasting patterns.</p> + +<p>Although DeBeer (1937) shows in his figure of <i>Hypogeophis</i> (one of the +Apoda) an operculum, this is apparently a mistake. The stapes has a +large footplate, and its stylus articulates with the quadrate, but no +true operculum or opercularis has been described in the Apoda. The +facial nerve passes above the stapes. It does not seem necessary to +regard the conditions in this order as related directly to those of +either salamanders or frogs, but a reduction of the stapes comparable to +that in salamanders has occurred.</p> + +<p>The presence in both frogs and terrestrial salamanders of a special +mechanism involving the opercularis muscle and an operculum cut out in +identical fashion from the wall of the otic capsule behind the stapes +seems to require some other explanation than that of a chance +convergence or parallelism. Although the stapes and otic region are +readily visible in a number of labyrinthodonts and lepospondyls, no +indication of an operculum seems to be reported among them. But in the +Triassic <i>Protobatrachus</i> (Fig. 1), which is unmistakably a frog in its +skull, pelvis and some other features, Piveteau (1937) has shown, +immediately behind the foot of the stapes, a small bony tubercle, which +he and Watson (1940) designated opisthotic. Very clearly it served for +insertion of a muscle, and it is equally clear that the bone is a +reduced opisthotic, carrying the paroccipital process already mentioned +as characteristic of it in<span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span> some temnospondyls. Since the remainder of +the posterior wall of the otic capsule consists of cartilage, meeting +the exoccipital, it may be that the opisthotic becomes the operculum in +frogs. <i>Protobatrachus</i> was too far specialized in the Anuran direction, +although it still had a tail, and the forelegs and hind legs were nearly +the same size, to be considered a possible ancestor of the Urodeles. But +at one stage in the general reduction of the skull in the ancestry of +both groups, a condition similar to that in <i>Protobatrachus</i> may have +characterized the otic region, long before the Triassic.</p> + +<p>In the argument thus far we have considered terrestrial, adult +amphibians, since it is only in these that either the normal middle ear +and tympanum, or the opercular apparatus, is present. But among the +urodeles several neotenic types occur (this term applies also to the +perennibranchs). For most of these there is nothing about the otic +region that would be inconsistent with derivation, by neoteny, from +known families in which adults are terrestrial; for example, +<i>Cryptobranchus</i> could have had a Hynobiid-like ancestor. But this, as +mentioned above, does not hold for the Proteidae, which possess an +opisthotic of relatively large size, distinctly separate from the +exoccipital and prootic. Either this bone is a neomorph, which seems +improbable, or there has not been in the ancestry of this particular +family an episode of reduction comparable to that seen in the +terrestrial families, where there is an operculum instead of a normal +opisthotic. Therefore the Proteidae probably are not derived from the +general stem of other salamanders, but diverged sufficiently long ago +that the bones of the otic region were reduced on a different pattern. +They need not be removed from the order, but, in this respect, +recognized as more primitive than any other existing Urodela or Anura. A +recent paper by Hecht (1957) discusses many features of <i>Necturus</i> and +<i>Proteus</i>, and shows that they are remote from each other; his evidence +does not seem to prove, however, that they were of independent origin or +that they need be placed in separate families.</p> + + + +<hr style="width: 65%;" /> +<h3>VERTEBRAE AND RIBS</h3> + + +<p>Development of the vertebrae and ribs of Recent Amphibia has been +studied by Gamble (1922), Naef (1929), Mookerjee (1930 a, b), Gray +(1930) and Emelianov (1936), among others. MacBride (1932) and Remane +(1938) provide good summaries. In this section reference will be made to +the embryonic vertebral cartilages by the names used for them in these +studies, although the concept<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span> of "arcualia" is currently considered of +little value in comparative anatomy.</p> + +<div class="figcenter" style="width: 800px;"> +<img src="images/fig5.png" width="800" height="632" alt="Fig. 5." title="Fig. 5." /> +<span class="caption">Fig. 5. Development of Anuran vertebrae. Upper left, late +tadpole of Xenopus laevis; lower left, same just after metamorphosis; +upper right, diagram of general components of primitive Anuran vertebra. +(After MacBride, 1932, Figs. 35, 38, 47D, respectively.) Lower right, +section through anterior portion of urostyle, immediately posterior to +sacral vertebra, in transforming Ascaphus truei (original, from +specimen collected on Olympic Peninsula, Washington). All × 20 approx. +For explanation of abbreviations see Fig. 3.</span> +</div> + +<p>The centrum in Anura (Fig. 5) is formed in the perichordal sheath +(<i>Rana</i>, <i>Bufo</i>) or only in the dorsal portion thereof (<i>Bombinator</i>, +<i>Xenopus</i>). The neural arch develops from the basidorsal cartilages that +rest upon, and at first are entirely distinct from, the perichordal +sheath. Ribs, present as separate cartilages associated with the 2nd, +3rd and 4th vertebrae in the larvae of <i>Xenopus</i> and <i>Bombinator</i>, fuse +with lateral processes (diapophyses) of the neural arches at +metamorphosis, but in <i>Leiopelma</i> and <i>Ascaphus</i> the ribs remain freely +articulated in the adult. Basiventral arcualia have been supposed to be +represented by the hypochord, a median rod of cartilage beneath the +shrinking notochord in the postsacral region, which at metamorphosis +ossifies to produce the bulk of the urostyle. Fig. 5, lower right, a +transverse section taken immediately posterior to the sacral ribs in a +transforming specimen of <i>Ascaphus</i>,<span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span> shows that the "hypochord" is a +mass of cartilage formed in the perichordal sheath itself, and very +obviously is derived from the ventral part of postsacral perichordal +centra; there are, then, no basiventral arcualia, and the discrete +hypochord shown in MacBride's diagram (Fig. 5, upper right) of a frog +vertebra does not actually occur below the centrum, but only below the +notochord in the postsacral region.</p> + +<div class="figcenter" style="width: 800px;"> +<img src="images/fig6.png" width="800" height="693" alt="Fig. 6." title="Fig. 6" /> +<span class="caption">Fig. 6. Development of Urodele vertebrae. Upper figures, +Triton: at left, larva at 20 mm., at right, diagram of components of +vertebra (from MacBride, 1932, figs. 17, 47C). Middle figures, Molge +vulgaris larva: left, at 18 mm.; middle, at 20-22 mm.; right, at 25 mm. +(from Emelianov, 1936, figs. 33, 36, 38 respectively). Lower figures, +Necturus maculosus larva: left, at 21 mm.; right, at 20 mm. (from +MacBride, 1932, figs. 41.5, 41.3 respectively, after Gamble, 1922). All +× 20 approx. For explanation of abbreviations see Fig. 3.</span> +</div> + +<p>In Urodela (Fig. 6) the pattern of vertebral and rib development is more +complex, and there has been much controversy over its interpretation. +Neural arches and perichordal centra form in the same manner as in +frogs, but with the addition in certain cases (<i>Triton</i>) of a median +supradorsal cartilage, which gives rise to the zygapophyses of each +neural arch. Difficulty comes, however, in understanding the +relationship of the ribs to the vertebrae. Each<span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span> rib, usually +two-headed, articulates with a "transverse process" that in its early +development seems to be separate from both the vertebra and the rib, and +is therefore known, noncommittally, as "rib-bearer." This lies laterally +from the centrum, neural arch, and vertebral artery; upon fusing with +the vertebra it therefore encloses the artery in a foramen separate from +the one between the capitulum and tuberculum of the rib (the usual +location of the vertebral artery). At least four different +interpretations of these structures have been suggested:</p> + +<p>(1) Naef (1929) considered the rib-bearer a derivative of the +basiventral, which, by spreading laterally and dorsally to meet the +neural arch, enclosed the vertebral artery. He then supposed that by +reduction of the rib-bearer in other tetrapods (frogs and amniotes) the +vertebrarterial foramen and costal foramen were brought together in a +single foramen transversarium. The implication is that the Urodele +condition is primitive, but it cannot now be supposed that Urodela are +ancestral to any other group, and the rib-bearer is most probably a +specialization limited to salamanders. This does not, of course, +invalidate the first part of his interpretation.</p> + +<p>(2) Remane (1938), noting that rib insertions of early Amphibia are +essentially as in Amniota, argued that the rib-bearer is not from the +basiventral but is a neomorph which originates directly from the neural +arch and grows ventrally. This he inferred mainly from Gamble's (1922) +observation on <i>Necturus</i>, but his assumption that <i>Necturus</i> is more +primitive than other salamanders (such as the Salamandridae), where the +pattern differs from this, is not necessarily correct. Rather, the +perennibranchs are distinguished mainly by their neotenous features, and +their development is likely to show simplifications which are not +necessarily primitive. The suggestion of a "neomorph" ought not to be +made except as a last resort, for it is simply an acknowledgment that +the author does not recognize homology with any structure already known; +sometimes further information will make such recognition possible.</p> + +<p>(3) Gray (1930), using <i>Molge taeniatus</i>, concluded that the normal +capitulum of the rib was lost, but that the tuberculum bifurcated to +make the two heads seen in Urodela, thus accounting for the failure of +the costal foramen to coincide with that of the vertebral artery. This +answer, too, seems to entail an unprovable assumption which should not +be made without explicit evidence.</p> + +<p>(4) Finally, Emelianov (1936) regarded the rib-bearer as a rudimentary +<i>ventral</i> rib, on account of its relationship to the vertebral artery, +and considered the actual rib to be a neomorph in the <i>dorsal</i><span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span> position +characteristic of tetrapod ribs in general. This argument would fit the +ontogenetic picture satisfactorily, provided that (<i>a</i>) there were some +evidence of ventral, rather than dorsal, ribs in early Amphibia, and +(<i>b</i>) we accept the invention of another neomorph in modern Amphibia as +an unavoidable necessity. Emelianov's conclusion (p. 258) should be +quoted here (translation): "The ribs of Urodela are shown to be upper +ribs, yet we find besides these in Urodela rudimentary lower ribs fused +with the vertebral column. The ribs of Apoda are lower ribs. In Anura +ribs fail to develop fully, but as rare exceptions rudiments of upper +ribs appear."</p> + +<p>Of these various interpretations, that of Naef seems to involve the +minimum of novelty, namely, that the rib-bearer is the basiventral, +expanded and external to the vertebral artery. It is not necessary to +take this modification as the ancestral condition in tetrapods, of +course. The basiventral (=intercentrum) would merely have expanded +sufficiently to provide a diapophysis for the tuberculum as well as the +(primitive) facet for the capitulum. No neomorph appears under this +hypothesis, which has the distinct advantage of simplicity.</p> + +<p>Figures of early stages in vertebral development by the authors +mentioned show that the basidorsals chondrify first, as neural arches, +while a separate mass of mesenchyme lies externally and ventrally from +these. This mesenchyme may chondrify either in one piece (on each side) +or in two; in <i>Molge</i> the part adjacent to the centrum is ossified in +the 20-mm. larva, and subsequently unites with the more dorsal and +lateral cartilaginous part, while the rib, appearing farther out, grows +inward to meet this composite "rib-bearer." In <i>Necturus</i> the mesenchyme +below the neural arch differentiates into a cartilage below the +vertebral artery (position proper to a basiventral), a bridge between +this and the neural arch, and a rib, the latter two chondrifying later +than the "basiventral" proper. In the "axolotl" (presumably <i>Ambystoma +tigrinum</i>) the rib-bearer grows downward from its first center of +chondrification at the side of the neural arch (Emelianov, 1936).</p> + +<p>Thus it appears that the simplest hypothesis to account for the +rib-bearer is that (<i>a</i>) it is the basiventral, (<i>b</i>) it is recognizable +just before chondrification as a mass of mesenchyme in contact with both +the notochordal sheath and the basidorsal cartilage, (<i>c</i>) it may +chondrify or ossify first in its ventral portion or in its dorsal +portion, the two then joining before it fuses with the rest of the +vertebra, (<i>d</i>) the enclosure of the vertebral artery is a con<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span>sequence +of the extension of the basiventral beyond the position occupied by it +in primitive Amphibia, and (<i>e</i>) there is no indication that this took +place in other orders than the Urodela.</p> + +<p>It seems that the vertebrae in Urodela have at least the following +components: perichordal centra, separate basidorsal cartilages, and +basiventrals, which are somewhat specialized in their manner of +development. The vertebrae of Anura develop in the fashion just +described except that basiventrals are lacking. It would seem no more +difficult to accept the derivation of salamander vertebrae from the +temnospondylous type than it is in the case of frogs, if other evidence +points to such an ancestry.</p> + +<div class="figcenter" style="width: 800px;"> +<img src="images/fig7.png" width="800" height="659" alt="Fig. 7." title="Fig. 7" /> +<span class="caption">Fig. 7. Vertebrae of Eusthenopteron (×1) and +Ichthyostega (×2/3, after Jarvik, 1952), Trimerorhachis (×1-1/2, +after Case), and Amphibamus (×10, after Watson, 1940) in lateral and +end views; the two lower right-hand figures are from Watson (1940, as +Miobatrachus); the lower left is from a cast of the "Miobatrachus" +specimen in Chicago Natural History Museum, No. 2000, in the presacral +region (original, ×10).</span> +</div> + +<p>Fig. 7, lower right, is Watson's (1940) illustration of the anterior +trunk vertebrae of <i>Amphibamus</i> (<i>Miobatrachus</i>), in which the +intercentrum is shown as a single median piece. Fig. 7, lower left, +shows two of the more posterior trunk vertebrae seen as impressions in a +cast of the type of "<i>Miobatrachus romeri</i>;" evidently the inter-centra +were paired at about the level of the 16th vertebra, and<span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span> relatively +large. Gregory's (1950) figure of the type specimen of "<i>Mazonerpeton</i>" +(also equivalent to <i>Amphibamus</i>) shows the anterior trunk vertebrae in +relation to the ribs essentially as they appear to me in the cast of +<i>Miobatrachus</i>, and rather differently from Watson's figure of the +latter. Gregory is probably right in considering the specimens to +represent various degrees of immaturity. So far as present information +goes, then, the vertebrae of salamanders and frogs show no <i>clear</i> +evidence of derivation from those of any particular group among the +early Amphibia, but their features are not inconsistent with a +simplification of the pattern of Temnospondyli.</p> + +<div class="figcenter" style="width: 800px;"> +<img src="images/fig8.png" width="800" height="580" alt="Fig. 8." title="Fig. 8." /> +<span class="caption">Fig. 8. Pectoral girdles of Protobatrachus (after +Piveteau, 1937), Notobatrachus (after Stipanicic and Reig, 1956), +Ascaphus (after Ritland, 1955 a) and Rana (original); all ×2. For +explanation of abbreviations see Fig. 3.</span> +</div> + + + +<hr style="width: 65%;" /> +<h3>PECTORAL GIRDLE</h3> + + +<p>Hecht and Ruibal (Copeia, 1928:242) make a strong point of the nature of +the pectoral girdle in <i>Notobatrachus</i>, as described recently by +Stipanicic and Reig (1955, 1956) from the Jurassic of Patagonia, and +quite rightly recommend that the significance of the arciferal and +firmisternal types of girdle be restudied. That of <i>Notobatrachus</i> is +said to be firmisternal; in view of the arciferal condition in the +supposedly primitive <i>Leiopelma</i>, <i>Ascaphus</i>, <i>Bombinator</i>, etc., this +comes as a surprise. Is the firmisternal girdle, as seen in <i>Rana</i>, +<i>Bufo</i>, and others, actually the ancestral type, and has the arciferal +been derived from something like this?<span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span></p> + +<p>In the figures given by Stipanicic and Reig the ossified parts of the +girdle are figured in detail (Fig. 8) and Reig's discussion of it is +thorough. The decision to call it firmisternal was taken with some +hesitancy, for no median elements are indicated, and the position and +shape of those seen is closely similar to the ossified parts in +<i>Ascaphus</i> and <i>Leiopelma</i>; there is no bony sternum or omosternum. It +is safe to suppose that some cartilage lay in the midline between the +clavicles and coracoids, but there is no evidence as to its extent, +rigidity, or degree of overlapping if any. Apparently, then, there is +not sufficient reason to infer that this Jurassic frog had a pectoral +girdle comparable with the modern firmisternal type.</p> + +<p>Piveteau (1955:261) remarks that the only living Anuran that can be +compared usefully with <i>Protobatrachus</i> (Triassic) with regard to its +pectoral girdle is <i>Ascaphus</i>. Again, the extent of cartilage in +<i>Protobatrachus</i> (Fig. 8) can only be inferred, and there are no median +elements. The agreement with <i>Ascaphus</i> includes the presence, in both, +of a separate coracoid ossification situated posterior to the ossified +"scapulocoracoid" (actually scapula). This ossification is evidently +that shown in <i>Notobatrachus</i> as "coracoid." Direct comparison of the +three genera with one another suggests that if we use the term arciferal +for any, we should use it for all.</p> + +<p>In the remote predecessor of Anura, <i>Amphibamus</i> of the Pennsylvanian, +the pectoral girdle was less substantial than in many of its +contemporaries, but it contained the primitive median interclavicle in +addition to the clavicle, cleithrum, and scapulocoracoid. (The figure of +Watson, 1940, and that by Gregory, 1950, are of individuals of different +ages, the latter being older.) It is clear that the paired elements of +such a girdle were held rigid by their attachment to the interclavicle, +<i>via</i> the clavicles. Subsequent elimination of the interclavicle in the +Anuran line of descent, and decrease of ossification, left a girdle like +that of <i>Protobatrachus</i>, <i>Notobatrachus</i>, <i>Ascaphus</i> and <i>Leiopelma</i>. +But in several advanced families a more rigid median "sternum," of one +or two bony pieces plus cartilage, is developed secondarily, possibly +(as Cope, 1889: 247, suggested) in correlation with axillary amplexus.</p> + +<p>Among Urodela no dermal bones occur in the pectoral girdle. There is +usually a scapulocoracoid ossified as a single piece, from which a thin +cartilaginous suprascapula extends dorsally and a broad cartilaginous +coracoid plate extends medially, overlapping the one from the opposite +side; a precoracoid lobe of this reaches forward on either side, and a +median, posterior "sternum" of carti<span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span>lage may make contact with the +edges of the two coracoids. In <i>Siren</i> and <i>Amphiuma</i> two centers of +ossification are found for each scapulocoracoid, and in <i>Triton</i> and +<i>Salamandra</i> three. Probably the more dorsal and lateral of these +represents the primitive scapula and the other one (or two) the +primitive coracoid.</p> + +<p>Comparing the girdle of a salamander with that of a frog, the closest +similarity can be seen between <i>Ascaphus</i> and a salamander in which the +scapula and coracoid ossify separately. Both have the median "sternum" +in contact with the coracoid plates. The major difference, of course, is +the lack of clavicle and cleithrum in the salamander.</p> + + + +<hr style="width: 65%;" /> +<h3>CARPUS AND TARSUS</h3> + + +<p>In <i>Ascaphus</i> (Ritland, 1955a; cleared and stained specimens of nearly +grown males) distal carpals 1, 2, 3 and 4 are present and separate, +increasing in size in the order given (Fig. 9). A prepollex rests +against centrale 1; centralia 2 and 3 are fused; the radiale fuses with +centrale 4, and the intermedium fuses with the ulnare; radius and ulna +are fused with each other as in other frogs. The digits (and +metacarpals) are considered by Ritland to be 1-4, in addition to the +prepollex, rather than 2-5.</p> + +<div class="figcenter" style="width: 799px;"> +<img src="images/fig9.png" width="799" height="548" alt="Fig. 9." title="Fig. 9" /> +<span class="caption">Fig. 9. Skeleton of fore foot of Notobatrachus (after +Stipanicic and Reig, 1956, terminology revised) and Ascaphus (after +Ritland, 1955 a); all ×5. For explanation of abbreviations see Fig. 3.</span> +</div> + +<p>In the Jurassic <i>Notobatrachus</i> Stipanicic and Reig (1956) have shown +the carpus with surprising clarity (Fig. 9). If their nomenclature of +the parts be revised, we obtain a fairly close resemblance to +<i>Ascaphus</i>, except that centralia 2 and 3 are not fused, distal<span class='pagenum'><a name="Page_174" id="Page_174">[Pg 174]</a></span> carpals +1 and 2 do not show (which would easily be understood if they were of +the size of those in <i>Ascaphus</i>, or not ossified), and the intermedium +remains separate from the ulnare.</p> + +<p>In <i>Salamandra</i> (Francis, 1934; Nauck, 1938) distal carpals 1 and 2 are +fused in both larva and adult, and 3 and 4 are separate; the radiale, +intermedium and ulnare are separate in the larva but the latter two fuse +in the adult; centrale 1 (labelled prepollical cartilage by Francis) and +centrale 2 are separate. Francis considers the digits (and metacarpals) +to be 1-4. Apparently the arrangement here indicated for the larva is +characteristic of other larval salamanders, except where further +reduced, and reduction below the number given for the adult is common in +other terrestrial forms. The radius and ulna are, of course, separate.</p> + +<p>The ossification of carpals is more likely to be complete in adult frogs +than in salamanders, but some ossification of all parts named is found +in several of the latter. A common ancestor of frogs and salamanders +could be expected to have the following elements present and ossified in +the adult: distal carpals 1-4 separate; 3 centralia; radiale, +intermedium and ulnare separate. Comparison with fossils older than +<i>Notobatrachus</i> is fruitless on these points, unless we go back to forms +too distant to have any special value, such as <i>Eryops</i>. This is because +of inadequate preservation and because the elements are not fully +ossified in many immature specimens.</p> + +<p>For the purpose of this review there is no special value in a comparison +of the tarsi of frogs and salamanders, since the leaping adaptation of +the former leaves very little common pattern between them. Even in +<i>Protobatrachus</i>, where the legs were not yet conspicuously lengthened, +the tibiale and fibulare ("astragalus" and "calcaneum" respectively) +were already considerably elongated. The carpus and tarsus of +<i>Amphibamus</i> are as yet undecipherable.</p> + + + +<hr style="width: 65%;" /> +<h3>THE LARVA</h3> + + +<p>Considering the postembryonic developmental stages of modern Amphibia, +there can be no doubt that a gill-bearing, four-legged larva of a +salamander, in which lateral line pores and a gular fold are present, +represents much more closely the type of larva found in labyrinthodonts +than does the limbless, plant-nibbling tadpole of the Anura. +Salamander-like larvae of labyrinthodonts are well known, especially +those formerly supposed to comprise the order Branchiosauria. Many, +perhaps the majority of, labyrinthodonts show some features associated +with aquatic life even when full<span class='pagenum'><a name="Page_175" id="Page_175">[Pg 175]</a></span>-grown, as do the lepospondyls. These +features may include impressions of sensory canals on the dermal bones +of the skull, persistence of visceral arches, reduction in size of +appendages, and failure of tarsal and carpal elements to ossify. In +fact, it appears that very few of the Paleozoic Amphibia were successful +in establishing themselves as terrestrial animals even as adults.</p> + +<p>Nevertheless, in the ancestry of Anura, and that of at least the +Hynobiid, Ambystomid, Salamandrid and Plethodontid salamanders, there +must certainly have been a terrestrial adult, transforming from an +aquatic larva. The leaping mechanism of Anura, shown in so many features +of their anatomy, is perhaps to be explained as a device for sudden +escape from land into the water, but it was not yet perfected in the +Triassic <i>Protobatrachus</i> or the Jurassic <i>Notobatrachus</i>.</p> + +<p>The middle ear, its sound-transmitting mechanism, and the tympanum, well +developed in most Anura, are readily derived from those of early +labyrinthodonts, and are presumably effective for hearing airborne +sounds whether on land or while floating in the water. Reduction of +these organs in Urodela may be correlated with their customary +restriction to subsurface habitats and inability to maintain a floating +position while in water.</p> + +<p>Some light may be shed on the significance of the tadpole of Anura by +considering the early stages of the ribbed frogs, Liopelmidae. +<i>Leiopelma</i> and <i>Ascaphus</i> are so closely similar in the adult that +there is no doubt that they belong in one family, primitive in some +respects (including articulated ribs; pyriformis and +caudalipuboischiotibialis muscles) but not in others (absence of +tympanum and middle ear). In both genera the eggs are large, 5 mm. in +<i>Leiopelma</i>, 4.5 mm. in <i>Ascaphus</i>, and unpigmented; but at this point +the resemblance ends.</p> + +<div class="figright" style="width: 330px;"> +<img src="images/fig10.png" width="330" height="419" alt="Fig. 10." title="Fig. 10." /> +<span class="caption">Fig. 10. Leiopelma hochstetteri larva, lateral and +ventral (after Stephenson, 1955), ×4.</span> +</div> + +<p>Stephenson (1955) showed that embryos of <i>L. hochstetteri</i> develop +equally well on land (in damp places) or in the water, and that embryos +prematurely released from egg capsules develop successfully in the +water. The larvae possess both pairs of legs (Fig. 10) and a broad gular +fold<span class='pagenum'><a name="Page_176" id="Page_176">[Pg 176]</a></span> similar to that of larval salamanders. In <i>L. hochstetteri</i> the +fold grows back over the forelegs temporarily, but remains free from the +body and presently the legs reappear, whereas in <i>L. archeyi</i> the +forelegs are not covered at any time. No branchial chamber or spiracle +is formed. Of course direct development, without a tadpole, occurs in +several other groups of Anura, but in each case terrestrial adaptations +are obvious. This is not true of <i>Leiopelma</i>, which Stephenson regards +as more nearly comparable with Urodela in its development than with +other Anura, and he sees in it a "primary and amphibious" mode instead +of a terrestrial specialization.</p> + +<p>The <i>Ascaphus</i> tadpole bears no outward resemblance to the larva of +<i>Leiopelma</i>, but is a normal tadpole in form, although sluggish in +activity. Its greatly expanded labial folds bear numerous rows of horny +epidermal "teeth," which, with the lips, serve to anchor the tadpole to +stones in the swift water of mountain brooks. Noble (1927) noticed that +particles of food were taken in through the external nares, and that a +current of water passed through these openings and out by way of the +median spiracle. It appears that any action by the teeth and jaws in +scraping algae from the rocks (which were bare in the stream where I +have collected <i>Ascaphus</i>) would be quite incidental, and that the lips +and teeth must be primarily a clinging mechanism. Certain other mountain +brook tadpoles (for example, <i>Borborocoetes</i>) show similar devices, but +these are developed independently, as specializations from the usual +sort of tadpole.</p> + +<p>May it not be that closure of the gill-chamber by the opercular (=gular) +fold, retardation of limb development, expansion of the lips, growth of +parallel rows of horny teeth, and other correlated features that make a +tadpole, were brought about as an adaptation of the primitive Anuran +larva to a swift-stream habitat, and that this "basic patent" then later +served to admit the tadpoles of descendant types to an alga-scraping +habit in quiet water as well? The tadpole, as a unique larval type among +vertebrates, bears the hallmarks of an abrupt adaptive shift, such as +might have occurred within the limits of a single family, and it seems +difficult to imagine the enclosed branchial chamber, the tooth-rows, and +lips of a familiar tadpole as having evolved without some kind of +suctorial function along the way.<span class='pagenum'><a name="Page_177" id="Page_177">[Pg 177]</a></span></p> + + + +<hr style="width: 65%;" /> +<h3>SUMMARY</h3> + + +<p>The Anura probably originated among temnospondylous labyrinthodonts, +through a line represented approximately by <i>Eugyrinus</i>, <i>Amphibamus</i>, +and the Triassic frog <i>Protobatrachus</i>, as shown by Watson, Piveteau and +others. The known Paleozoic lepospondyls do not show clear indications +of a relationship with Urodela, but <i>Lysorophus</i> may well be on the +ancestral stem of the Apoda.</p> + +<p>Between Urodela and Anura there are numerous resemblances which seem to +indicate direct relationship through a common stock: (1) a similar +reduction of dermal bones of the skull and expansion of palatal +vacuities; (2) movable basipterygoid articulation in primitive members +of both orders; (3) an operculum formed in the otic capsule, with +opercularis muscle; (4) many details of cranial development, cranial +muscles, and thigh muscles, especially between <i>Ascaphus</i> and the +Urodela, as shown by Pusey and Noble; (5) essentially similar manner of +vertebral development, quite consistent with derivation of both orders +from Temnospondyli; (6) presence in the larva of <i>Leiopelma</i> of a +salamander-like gular fold, four limbs, and no suggestion of +modification from a tadpole (Stephenson).</p> + + + +<hr style="width: 65%;" /> +<h3>LITERATURE CITED</h3> + + +<p><span class="smcap">Broom, R.</span></p> + +<div class="blockquot"><p>1918. Observations on the genus <i>Lysorophus</i> Cope. Ann. Mag. Nat. +Hist., (9)2:232-239.</p></div> + +<p><span class="smcap">Bystrow, A. P.</span></p> + +<div class="blockquot"><p>1938. Dvinosaurus als neotenische Form der Stegocephalen. Acta +Zool., 19:209-295.</p></div> + +<p><span class="smcap">Case, E. C.</span></p> + +<div class="blockquot"><p>1935. Description of a collection of associated skeletons of +<i>Trimerorhachis</i>. Contrib. Mus. Pal. Univ. Michigan, 4:227-274.</p></div> + +<p><span class="smcap">Cope, E. D.</span></p> + +<div class="blockquot"><p>1889. The Batrachia of North America. Bull. U. S. Nat. Mus., +34:1-525.</p></div> + +<p><span class="smcap">de Beer, G. R.</span></p> + +<div class="blockquot"><p>1937. The development of the vertebrate skull. Pp. xxiii + 552. +Oxford, Clarendon Press.</p></div> + +<p><span class="smcap">de Villiers, C. G. S.</span></p> + +<div class="blockquot"><p>1934. Studies of the cranial anatomy of <i>Ascaphus truei</i> +Stejneger, the American "Liopelmid." Bull. Mus. Comp. Zool., +77:1-38.<span class='pagenum'><a name="Page_178" id="Page_178">[Pg 178]</a></span></p></div> + +<p><span class="smcap">Emelianov, S. W.</span></p> + +<div class="blockquot"><p>1936. Die Morphologie der Tetrapodenrippen. Zool. Jahrb. (Anat.), +62:173-274.</p></div> + +<p><span class="smcap">Francis, E. T. B.</span></p> + +<div class="blockquot"><p>1934. The anatomy of the salamander. Pp. xxxi + 381. Oxford, +Clarendon Press.</p></div> + +<p><span class="smcap">Gamble, D. L.</span></p> + +<div class="blockquot"><p>1922. The morphology of the ribs and the transverse processes of +<i>Necturus maculatus</i>. Jour. Morph., 36:537-566.</p></div> + +<p><span class="smcap">Gray, P.</span></p> + +<div class="blockquot"><p>1930. On the attachments of the Urodele rib to the vertebra and +their homologies with the capitulum and tuberculum of the Amniote +rib. Proc. Zool. Soc. London, 1930(1931):907-911.</p></div> + +<p><span class="smcap">Gregory, J. T.</span></p> + +<div class="blockquot"><p>1950. Tetrapods from the Pennsylvanian nodules from Mazon Creek, +Illinois. Am. Jour. Sci., 248:833-873.</p></div> + +<p><span class="smcap">Hecht, M. E.</span></p> + +<div class="blockquot"><p>1957. A case of parallel evolution in salamanders. Proc. Zool. +Soc. Calcutta, Mookerjee Mem.:283-292.</p></div> + +<p><span class="smcap">Holmgren, N.</span></p> + +<div class="blockquot"><p>1933. On the origin of the tetrapod limb. Acta Zool., 14:185-295.</p> + +<p>1939. Contributions to the question of the origin of the tetrapod +limb. Acta Zool., 20:89-124.</p> + +<p>1949a. Contributions to the question of the origin of tetrapods. +Acta Zool., 30:459-484.</p> + +<p>1949b. On the tetrapod limb problem again. Acta Zool., 30:485-508.</p></div> + +<p><span class="smcap">Herre, W.</span></p> + +<div class="blockquot"><p>1935. Die Schwanzlurche der mitteleocänen (oberlutetischen) +Braunkohle des Geiseltales und die Phylogenie der Urodelen unter +Einschluss der fossilen Formen. Zoologica, 33:87, 1-5.</p></div> + +<p><span class="smcap">Jarvik, E.</span></p> + +<div class="blockquot"><p>1942. On the structure of the snout of Crossopterygians and lower +gnathostomes in general. Zool. Bidrag fran Upsala, 21:235-675. +1952. On the fish-like tail in the Ichthyostegid Stegocephalians. +Meddelelser on Grønland, 114(12):1-90.</p></div> + +<p><span class="smcap">Mookerjee, H.K.</span></p> + +<div class="blockquot"><p>1930a. On the development of the vertebral column of the Urodela. +Phil. Trans. Roy. Soc. London, B 218:415-446.</p> + +<p>1930b. On the development of the vertebral column of the Anura. +Philos. Trans. Royal Soc. London, B 219:165-196.</p></div> + +<p><span class="smcap">MacBride, E.W.</span></p> + +<div class="blockquot"><p>1932. Recent work on the development of the vertebral column. +Cambridge, Biol. 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Kranium und Visceralskelett der Stegocephalen und Amphibien. +In: Handbuch der vergleichenden Anatomie der Wirbeltiere, by L. +Bolk <i>et al.</i>, 4:501-698.</p></div> + +<p><span class="smcap">Stephenson, N. G.</span></p> + +<div class="blockquot"><p>1955. On the development of the frog, <i>Leiopelma hochstetteri</i> +Fitzinger. Proc. Zool. Soc. London, 124(4):785-795.</p></div> + +<p><span class="smcap">Stipanicic, P. N.</span> and <span class="smcap">Reig, O. A.</span></p> + +<div class="blockquot"><p>1955. Breve noticia sobre el hallazgo de anuros en el denominado +"Complejo Porfirico de la Patagonia Extraandina," con +consideraciones acerca de la composicion geologica del mismo. +Revista de la Asoc. Geol. Argentina, 10(4):215-233.</p> + +<p>1956. El "complejo porfirico de la Patagonia extraandina" y su +fauna de Anuros. Acta Geol. Lilloana (Univ. Nac. del Tucuman), +1:185-297.</p></div><p><span class='pagenum'><a name="Page_180" id="Page_180">[Pg 180]</a></span></p> + +<p><span class="smcap">Sushkin, P. P.</span></p> + +<div class="blockquot"><p>1936. Notes on the pro-Jurassic Tetrapoda from U. S. S. R. III. +Dvinosaurus Amalitzky, a perennibranchiate stegocephalian from the +Upper Permian of North Dvina. Trav. Inst. Pal. Acad. Sci. URSS, +5:43-91.</p></div> + +<p><span class="smcap">Watson, D. M. S.</span></p> + +<div class="blockquot"><p>1940. The origin of frogs. Trans. Roy. Soc. Edinburgh, +40(7):195-231.</p></div> + +<p><i>Transmitted April 7, 1959.</i></p> + + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of The Ancestry of Modern Amphibia: A +Review of the Evidence, by Theodore H. 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Eaton + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: The Ancestry of Modern Amphibia: A Review of the Evidence + +Author: Theodore H. Eaton + +Release Date: September 8, 2011 [EBook #37350] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK THE ANCESTRY OF MODERN *** + + + + +Produced by Chris Curnow, Charlene Taylor, Joseph Cooper +and the Online Distributed Proofreading Team at +http://www.pgdp.net + + + + + + + + + + University of Kansas Publications + Museum of Natural History + + + Volume 12, No. 2, pp. 155-180, 10 figs. + -----------July 10, 1959--------------- + + + The Ancestry of Modern Amphibia: + A Review of the Evidence + + BY + + THEODORE H. EATON, JR. + + + University of Kansas + Lawrence + 1959 + +University of Kansas Publications, Museum of Natural History + +Editors: E. Raymond Hall, Chairman, Henry S. Fitch, Robert W. Wilson + + + Volume 12, No. 2, pp. 155-180 + Published July 10, 1959 + + + University of Kansas + Lawrence, Kansas + + + PRINTED IN + THE STATE PRINTING PLANT + TOPEKA, KANSAS + 1959 + + [Illustration] + + 27-8362 + + + + +[Transcriber's Notes: Several typos have been regulated. + +One typo of "ancester" for "ancestor" was corrected. + +One instance of "salamanderlike" corrected to "salamaner-like".] + + + + +The Ancestry of Modern Amphibia: A Review of the Evidence + +BY +THEODORE H. EATON, JR. + + + + +INTRODUCTION + + +In trying to determine the ancestral relationships of modern orders of +Amphibia it is not possible to select satisfactory structural ancestors +among a wealth of fossils, since very few of the known fossils could +even be considered possible, and scarcely any are satisfactory, for such +a selection. The nearest approach thus far to a solution of the problem +in this manner has been made with reference to the Anura. Watson's paper +(1940), with certain modifications made necessary by Gregory (1950), +provides the paleontological evidence so far available on the origin of +frogs. It shows that several features of the skeleton of frogs, such as +the enlargement of the interpterygoid spaces and orbits, reduction of +the more posterior dermal bones of the skull, and downward spread of the +neural arches lateral to the notochord, were already apparent in the +Pennsylvanian _Amphibamus_ (Fig. 1), with which Gregory synonymized +_Miobatrachus_ and _Mazonerpeton_. But between the Pennsylvanian and the +Triassic (the age of the earliest known frog, _Protobatrachus_) there +was a great lapse of time, and that which passed between any conceivable +Paleozoic ancestor of Urodela and the earliest satisfactory +representative of this order (in the Cretaceous) was much longer still. +The Apoda, so far as known, have no fossil record. + +Nevertheless it should be possible, first, to survey those characters of +modern Amphibia that might afford some comparison with the early +fossils, and second, to discover among the known Paleozoic kinds those +which are sufficiently unspecialized to permit derivation of the modern +patterns. Further circumstantial evidence may be obtained by examining +some features of Recent Amphibia which could not readily be compared +with anything in the fossils; such are the embryonic development of the +soft structures, including cartilaginous stages of the skeleton, the +development and various specializations of the ear mechanism, adaptive +characters associated with aquatic and terrestrial life, and so on. + + + + +COMPARISON OF MODERN ORDERS WITH THE +LABYRINTHODONTS AND LEPOSPONDYLS + + +[Illustration: Fig. 1. _Saurerpeton_ (x 1/2, after Romer, 1930, fig. 6); +_Amphibamus_, the palatal view x 2-1/4, from Watson, 1940, fig. 4 (as +_Miobatrachus_), the dorsal view x 2-1/2, from Gregory's revised figure +of _Amphibamus_ (1950, Fig. 1); _Protobatrachus_, x 1, from Watson, +1940, fig. 18, 19.] + +In both Anura and Urodela the skull is short, broad, relatively flat, +with reduced pterygoids that diverge laterally from the parasphenoids +leaving large interpterygoid vacuities, and with large orbits. (These +statements do not apply to certain larval or perennibranchiate forms.) +The skull in both orders has lost a number of primitive dermal bones in +the posterior part; these are: basioccipital, supraoccipital, +postparietal, intertemporal, supratemporal, and tabular. The +exoccipitals form the two condyles but there are no foramina for the +11th and 12th nerves, since these are not separate in modern Amphibia. +The opisthotic is missing in all except Proteidae (but see discussion +of the ear). Although the skull is normally autostylic, a movable +basipterygoid articulation is present among Hynobiid salamanders and in +at least the metamorphic stages of primitive frogs, and therefore should +be expected in their ancestors. The vertebrae are, of course, complete; +see discussion in later section. The quadratojugal, lost in salamanders, +is retained in frogs, and conversely the lacrimal, absent in frogs, +occurs in a few primitive salamanders. The situation in Apoda is +different, but postfrontal and jugal should be noted as bones retained +in this order while lost in the others. + +Thus, in spite of minor differences, the above list shows that there are +numerous and detailed similarities between Anura and Urodela with +respect to the features in which they differ from the Paleozoic orders. +Pusey (1943) listed 26 characters which _Ascaphus_ shares with +salamanders but not with more advanced frogs; a few of these might be +coincidental, but most of them are of some complexity and must be taken +to indicate relationship. The main adaptive specializations of Anura, +however, including loss of the adult tail, extreme reduction in number +of vertebrae, formation of urostyle, elongation of the ilium and +lengthening of the hind legs, must have appeared at a later time than +the separation of that order from any possible common stem with Urodela, +although they are only partially developed in the Triassic +_Protobatrachus_. + +Turning to the Paleozoic Amphibia, there are two groups in which some +likelihood of a relationship with modern order exists. In the +Pennsylvanian Trimerorhachoidea (Labyrinthodontia, order Temnospondyli) +some members, such as _Eugyrinus_, _Saurerpeton_, and notably +_Amphibamus_ (Fig. 1) had short, broad heads, an expansion of palatal +and orbital openings, posterior widening of the parasphenoid associated +with divergence of the pterygoids, a movable basipterygoid articulation, +and reduction in size (but not loss) of the more posterior dermal bones +of the skull. In recognition of Watson's (1940) evidence that these +animals make quite suitable structural ancestors of frogs, Romer (1945) +placed _Amphibamus_ in an order, Eoanura, but Gregory (1950) indicated +that it might better be left with the temnospondyls. Association of the +urodele stem with this group does not seem to have been proposed +hitherto. + +The other group of Paleozoic Amphibia that has been considered probably +ancestral to any modern type is the subclass Lepospondyli, containing +three orders, Aistopoda, Nectridia and Microsauria. In these the +vertebrae are complete (holospondylous), the centra presumably formed by +cylindrical ossification around the notochord, and there is no evidence +as to the contributions from embryonic cartilage units. It is important +to note at this point that precisely the same statement can be made +regarding the vertebrae of _adults_ of all three Recent orders, yet for +all of them, as shown in a later section, we have ample evidence of the +part played by cartilage elements in vertebral development. Therefore +(a) we cannot say that there were no such elements in embryonic stages +of lepospondyls, and (b) it would take more than the evidence from adult +vertebrae to relate a particular modern order (for example, Urodela) to +the Lepospondyli. Vague similarities to Urodela have been noted by many +authors in the Nectridia, Aistopoda and Microsauria, but these are not +detailed and refer mainly to the vertebrae. The skulls do not show, +either dorsally or in the palate, any striking resemblance to those of +generalized salamanders, and certainly most known lepospondyls are too +specialized to serve as the source of Urodela. It is true that the +elongate bodies, small limbs, and apparent aquatic habitus of some +lepospondyls accord well with our usual picture of a salamander, but +such a form and way of life have appeared in many early Amphibia, +including the labyrinthodonts. The family Lysorophidae (Fig. 2), usually +placed among microsaurs, is sufficiently close in skull structure to the +Apoda to be a possible ancestor of these, but it probably has nothing to +do with Urodela, by reason of the numerous morphological specializations +that were associated with its snakelike habitus. + +[Illustration: Fig. 2. _Lysorophus tricarinatus_, lateral and posterior +views x 2-1/2, modified after Sollas, 1920, Figs. 8 and 12, +respectively; palatal view after Broom, 1918, x 1-1/2. For explanation +of abbreviations see Fig. 3.] + +McDowell's (1958) suggestion that it would be profitable to look among +the Seymouriamorpha for the ancestors of frogs seems to be based upon a +few details of apparent resemblance rather than a comprehensive view of +the major characters of the animals. In most points which he mentions +(limb girdles, form of ear, pterygoid articulation) the present writer +does not see a closer similarity of frogs to Seymouriamorpha than to +Temnospondyli. + +Still other opinions have been expressed. Herre (1935), for instance, +concludes "on anatomical, biological and paleontological grounds" that +the orders of Urodela, Anura, Apoda and Stegocephali were all +independently evolved from fish, but beyond citing the opinions of a +number of other authors he does not present tangible evidence for this +extreme polyphyletic interpretation. + +More notable are the views of several Scandinavian workers +(Saeve-Soederbergh, 1934; Jarvik, 1942; Holmgren, 1933, 1939, 1949a, b), +of whom Jarvik, in a thorough analysis of the ethmoid region, would +derive the Urodela from Porolepid Crossopterygii, and all other +tetrapods from the Rhipidistia; Saeve-Soederbergh and Holmgren, the latter +using the structure of carpus and tarsus, see a relationship of Urodela +to Dipnoi, but accept the derivation of labyrinthodonts and other +tetrapods from Rhipidistia. All of this work is most detailed and +laborious, and has produced a great quantity of data useful to +morphologists, but the diphyletic theory is not widely adopted; the +evidence adduced for it seems to consist largely of minutiae which, +taken by themselves, are inconclusive, or lend themselves to other +interpretation. For instance Holmgren's numerous figures of embryonic +limbs of salamanders show patterns of cartilage elements that he would +trace to the Dipnoan type of fin, yet it is difficult to see that the +weight of evidence requires this, when the pattern does not differ in +any fundamental manner from those seen in other embryonic tetrapods, and +the differences that do appear may well be taken to have ontogenetic +rather than phylogenetic meaning. Further, the Dipnoan specialization of +dental plates and autostylic jaw suspension, already accomplished early +in the Devonian, would seem to exclude Dipnoi from possible ancestry of +the Urodela, an order unknown prior to the Mesozoic, in which the teeth +are essentially similar to those of late Paleozoic Amphibia, and the jaw +suspension is not yet in all members autostylic. + + + + +THE EAR + + +[Illustration: Fig. 3. Occipital region of skulls of _Megalocephalus +brevicornis_ (x 3/10, after Watson, 1926, as _Orthosaurus_), +_Dvinosaurus_ (x 1/4, modified after Bystrow, 1938; the lower figure +after Sushkin, 1936), and _Necturus maculosus_ (x 3, original, from K. +U., No. 3471). + +Abbreviations Used in Figures + + b'd.c.--basidorsal cartilage (neural arch) + b'oc.--basioccipital + ce._{1-4}--centrale_{1-4} + ch.--ceratohyal + clav.--clavicle + clei.--cleithrum + cor.--coracoid + d.c._{1-4}--distal carpal_{1-4} + diap.--diapophysis + exoc.--exoccipital + ep.--episternum + hyost.--hyostapes + i.--intermedium + Mk.--Meckel's cartilage + n.--notochord + om.--omosternum + op.--operculum + opis.--opisthotic + par.--parietal + par. proc.--paroccipital process + peri. cent.--perichordal centrum + p'p.--postparietal + prep.--prepollex + pro.--prootic + p'sp.--parasphenoid + pt.--pterygoid + p.t.f.--post-temporal fossa + postzyg.--postzygapophysis + qj.--quadratojugal + qu.--quadrate + ra.--radiale + r.hy.--hyomandibular ramus of VII + rib-b.--rib-bearer + r.md.--mandibular ramus of VII + sc.--scapula + sc'cor.--scapulocoracoid + s'd.--supradorsal cartilage + s'd.(postzyg.)--supradorsal (postzygapophysis) + soc.--supraoccipital + sp.c.--spinal cord + sq.--squamosal + s'sc.--suprascapula + s't.--supratemporal + sta.--stapes + ster.--sternum + tab.--tabular + uln.--ulnare + v.a.--vertebral artery + xiph.--xiphisternum + I,IV--digits I and IV + V, VII, X, XII--foramina for cranial nerves of these numbers (in + Fig. 4, VII is the facial nerve) +] + +In temnospondylous Amphibia the tympanum generally occupied an otic +notch, at a high level on the skull, bordered dorsomedially by the +tabular and ventrolaterally by the squamosal. In this position the +tympanum could receive airborne sounds whether the animal were entirely +on land or lying nearly submerged with only the upper part of its head +exposed. Among those Anura in which the ear is not reduced the same is +true, except that the tabular is lost. In Temnospondyli (Fig. 3) the +posterior wall of the otic capsule was usually formed by the opisthotic, +which extended up and outward as a buttress from the exoccipital to the +tabular, and sometimes showed a paroccipital process for the insertion, +presumably, of a slip or tendon of the anterior axial musculature. The +stapes, in addition to its foot in the fenestra ovalis and its tympanic +or extrastapedial process to the tympanum, bore a dorsal process (or +ligament) to the tabular, an "internal" process (or ligament) to the +quadrate or an adjacent part of the squamosal, and a ligament to the +ceratohyal. Some of these attachments might be reduced or absent in +special cases, but they seem to have been the ones originally present +both phylogenetically and embryonically in Amphibia. + +Among typical frogs (Fig. 4) the base, or otostapes, is present and +bony, the extrastapedial process (extracolumella, or hyostapes) is +usually cartilaginous, the dorsal process (processus paroticus) is of +cartilage or ligament, but the other two attachments are absent in the +adult. The exoccipital extends laterally, occupying the posterior face +of the otic capsule. Between it and the otostapes is a small disc, +usually ossified, the operculum, which normally fits loosely in a +portion of the fenestral membrane, and is developed from the otic +capsule. The opercularis muscle extends from this disc to the +suprascapula, in many but by no means all families of Anura. + +[Illustration: Fig. 4. Diagram of middle ear structures in _Rana_ (upper +figure, after Stadtmueller, 1936, and lower left after DeBeer, 1937), and +_Ambystoma_ (lower right, after DeBeer, 1937); all x 4. For explanation +of abbreviations see Fig. 3.] + +Among Urodela (Fig. 4) the middle ear cavity and tympanum are lacking, +and the stapes (columella) consists of no more than its footplate and +the stylus, which is attached to the border of the squamosal, thus +corresponding to the "internal" process. In families in which +individuals metamorphose and become terrestrial (Hynobiidae, +Ambystomidae, Salamandridae, Plethodontidae), an operculum and +opercularis muscle appear in the adult, just as in frogs, except that in +Plethodontidae, the most progressive family, the operculum fuses with +the footplate of the stapes. Among neotenous or perennibranchiate +urodeles there is no separate operculum or opercularis. The evidence +given by Reed (1915) for fusion of the operculum with the columella in +_Necturus_ appears inconclusive, in spite of the great care with which +his observations were made. On the other hand, _Necturus_ and _Proteus_ +alone among living salamanders have a distinct opisthotic on the +posterior wall of the otic capsule (Fig. 3), as do the Cretaceous +_Hylaeobatrachus_ and the Eocene _Palaeoproteus_. Probably these +Proteidae should be regarded as primitive in this respect, although many +other features may be attributed to neoteny. + +There is a contrast between Anura and most Urodela in the relative +positions of the stapes and facial nerve, as shown in DeBeer's (1937) +diagrams. In the latter (_Ambystoma_) the nerve is beneath, and in the +former (_Rana_) above, the stapes. Judging by figures of _Neoceratodus_, +_Hypogeophis_, and several types of reptiles and mammals, the Urodela +are exceptional. _Necturus_, however, has the nerve passing above its +stapes, and this may be primitive in the same sense as the persistent +opisthotic. There can be, of course, no question of the nerve having +worked its way through or over the obstructing stapes in order to come +below it in salamanders; rather, the peripheral growth of neuron fibers +in the embryo must simply pursue a slightly different course among the +partially differentiated mesenchyme in the two contrasting patterns. + +Although DeBeer (1937) shows in his figure of _Hypogeophis_ (one of the +Apoda) an operculum, this is apparently a mistake. The stapes has a +large footplate, and its stylus articulates with the quadrate, but no +true operculum or opercularis has been described in the Apoda. The +facial nerve passes above the stapes. It does not seem necessary to +regard the conditions in this order as related directly to those of +either salamanders or frogs, but a reduction of the stapes comparable to +that in salamanders has occurred. + +The presence in both frogs and terrestrial salamanders of a special +mechanism involving the opercularis muscle and an operculum cut out in +identical fashion from the wall of the otic capsule behind the stapes +seems to require some other explanation than that of a chance +convergence or parallelism. Although the stapes and otic region are +readily visible in a number of labyrinthodonts and lepospondyls, no +indication of an operculum seems to be reported among them. But in the +Triassic _Protobatrachus_ (Fig. 1), which is unmistakably a frog in its +skull, pelvis and some other features, Piveteau (1937) has shown, +immediately behind the foot of the stapes, a small bony tubercle, which +he and Watson (1940) designated opisthotic. Very clearly it served for +insertion of a muscle, and it is equally clear that the bone is a +reduced opisthotic, carrying the paroccipital process already mentioned +as characteristic of it in some temnospondyls. Since the remainder of +the posterior wall of the otic capsule consists of cartilage, meeting +the exoccipital, it may be that the opisthotic becomes the operculum in +frogs. _Protobatrachus_ was too far specialized in the Anuran direction, +although it still had a tail, and the forelegs and hind legs were nearly +the same size, to be considered a possible ancestor of the Urodeles. But +at one stage in the general reduction of the skull in the ancestry of +both groups, a condition similar to that in _Protobatrachus_ may have +characterized the otic region, long before the Triassic. + +In the argument thus far we have considered terrestrial, adult +amphibians, since it is only in these that either the normal middle ear +and tympanum, or the opercular apparatus, is present. But among the +urodeles several neotenic types occur (this term applies also to the +perennibranchs). For most of these there is nothing about the otic +region that would be inconsistent with derivation, by neoteny, from +known families in which adults are terrestrial; for example, +_Cryptobranchus_ could have had a Hynobiid-like ancestor. But this, as +mentioned above, does not hold for the Proteidae, which possess an +opisthotic of relatively large size, distinctly separate from the +exoccipital and prootic. Either this bone is a neomorph, which seems +improbable, or there has not been in the ancestry of this particular +family an episode of reduction comparable to that seen in the +terrestrial families, where there is an operculum instead of a normal +opisthotic. Therefore the Proteidae probably are not derived from the +general stem of other salamanders, but diverged sufficiently long ago +that the bones of the otic region were reduced on a different pattern. +They need not be removed from the order, but, in this respect, +recognized as more primitive than any other existing Urodela or Anura. A +recent paper by Hecht (1957) discusses many features of _Necturus_ and +_Proteus_, and shows that they are remote from each other; his evidence +does not seem to prove, however, that they were of independent origin or +that they need be placed in separate families. + + + + +VERTEBRAE AND RIBS + + +Development of the vertebrae and ribs of Recent Amphibia has been +studied by Gamble (1922), Naef (1929), Mookerjee (1930 a, b), Gray +(1930) and Emelianov (1936), among others. MacBride (1932) and Remane +(1938) provide good summaries. In this section reference will be made to +the embryonic vertebral cartilages by the names used for them in these +studies, although the concept of "arcualia" is currently considered of +little value in comparative anatomy. + +[Illustration: Fig. 5. Development of Anuran vertebrae. Upper left, late +tadpole of _Xenopus laevis_; lower left, same just after metamorphosis; +upper right, diagram of general components of primitive Anuran vertebra. +(After MacBride, 1932, Figs. 35, 38, 47D, respectively.) Lower right, +section through anterior portion of urostyle, immediately posterior to +sacral vertebra, in transforming _Ascaphus truei_ (original, from +specimen collected on Olympic Peninsula, Washington). All x 20 approx. +For explanation of abbreviations see Fig. 3.] + +The centrum in Anura (Fig. 5) is formed in the perichordal sheath +(_Rana_, _Bufo_) or only in the dorsal portion thereof (_Bombinator_, +_Xenopus_). The neural arch develops from the basidorsal cartilages that +rest upon, and at first are entirely distinct from, the perichordal +sheath. Ribs, present as separate cartilages associated with the 2nd, +3rd and 4th vertebrae in the larvae of _Xenopus_ and _Bombinator_, fuse +with lateral processes (diapophyses) of the neural arches at +metamorphosis, but in _Leiopelma_ and _Ascaphus_ the ribs remain freely +articulated in the adult. Basiventral arcualia have been supposed to be +represented by the hypochord, a median rod of cartilage beneath the +shrinking notochord in the postsacral region, which at metamorphosis +ossifies to produce the bulk of the urostyle. Fig. 5, lower right, a +transverse section taken immediately posterior to the sacral ribs in a +transforming specimen of _Ascaphus_, shows that the "hypochord" is a +mass of cartilage formed in the perichordal sheath itself, and very +obviously is derived from the ventral part of postsacral perichordal +centra; there are, then, no basiventral arcualia, and the discrete +hypochord shown in MacBride's diagram (Fig. 5, upper right) of a frog +vertebra does not actually occur below the centrum, but only below the +notochord in the postsacral region. + +[Illustration: Fig. 6. Development of Urodele vertebrae. Upper figures, +_Triton_: at left, larva at 20 mm., at right, diagram of components of +vertebra (from MacBride, 1932, figs. 17, 47C). Middle figures, _Molge +vulgaris_ larva: left, at 18 mm.; middle, at 20-22 mm.; right, at 25 mm. +(from Emelianov, 1936, figs. 33, 36, 38 respectively). Lower figures, +_Necturus maculosus_ larva: left, at 21 mm.; right, at 20 mm. (from +MacBride, 1932, figs. 41.5, 41.3 respectively, after Gamble, 1922). All +x 20 approx. For explanation of abbreviations see Fig. 3.] + +In Urodela (Fig. 6) the pattern of vertebral and rib development is more +complex, and there has been much controversy over its interpretation. +Neural arches and perichordal centra form in the same manner as in +frogs, but with the addition in certain cases (_Triton_) of a median +supradorsal cartilage, which gives rise to the zygapophyses of each +neural arch. Difficulty comes, however, in understanding the +relationship of the ribs to the vertebrae. Each rib, usually +two-headed, articulates with a "transverse process" that in its early +development seems to be separate from both the vertebra and the rib, and +is therefore known, noncommittally, as "rib-bearer." This lies laterally +from the centrum, neural arch, and vertebral artery; upon fusing with +the vertebra it therefore encloses the artery in a foramen separate from +the one between the capitulum and tuberculum of the rib (the usual +location of the vertebral artery). At least four different +interpretations of these structures have been suggested: + +(1) Naef (1929) considered the rib-bearer a derivative of the +basiventral, which, by spreading laterally and dorsally to meet the +neural arch, enclosed the vertebral artery. He then supposed that by +reduction of the rib-bearer in other tetrapods (frogs and amniotes) the +vertebrarterial foramen and costal foramen were brought together in a +single foramen transversarium. The implication is that the Urodele +condition is primitive, but it cannot now be supposed that Urodela are +ancestral to any other group, and the rib-bearer is most probably a +specialization limited to salamanders. This does not, of course, +invalidate the first part of his interpretation. + +(2) Remane (1938), noting that rib insertions of early Amphibia are +essentially as in Amniota, argued that the rib-bearer is not from the +basiventral but is a neomorph which originates directly from the neural +arch and grows ventrally. This he inferred mainly from Gamble's (1922) +observation on _Necturus_, but his assumption that _Necturus_ is more +primitive than other salamanders (such as the Salamandridae), where the +pattern differs from this, is not necessarily correct. Rather, the +perennibranchs are distinguished mainly by their neotenous features, and +their development is likely to show simplifications which are not +necessarily primitive. The suggestion of a "neomorph" ought not to be +made except as a last resort, for it is simply an acknowledgment that +the author does not recognize homology with any structure already known; +sometimes further information will make such recognition possible. + +(3) Gray (1930), using _Molge taeniatus_, concluded that the normal +capitulum of the rib was lost, but that the tuberculum bifurcated to +make the two heads seen in Urodela, thus accounting for the failure of +the costal foramen to coincide with that of the vertebral artery. This +answer, too, seems to entail an unprovable assumption which should not +be made without explicit evidence. + +(4) Finally, Emelianov (1936) regarded the rib-bearer as a rudimentary +_ventral_ rib, on account of its relationship to the vertebral artery, +and considered the actual rib to be a neomorph in the _dorsal_ position +characteristic of tetrapod ribs in general. This argument would fit the +ontogenetic picture satisfactorily, provided that (_a_) there were some +evidence of ventral, rather than dorsal, ribs in early Amphibia, and +(_b_) we accept the invention of another neomorph in modern Amphibia as +an unavoidable necessity. Emelianov's conclusion (p. 258) should be +quoted here (translation): "The ribs of Urodela are shown to be upper +ribs, yet we find besides these in Urodela rudimentary lower ribs fused +with the vertebral column. The ribs of Apoda are lower ribs. In Anura +ribs fail to develop fully, but as rare exceptions rudiments of upper +ribs appear." + +Of these various interpretations, that of Naef seems to involve the +minimum of novelty, namely, that the rib-bearer is the basiventral, +expanded and external to the vertebral artery. It is not necessary to +take this modification as the ancestral condition in tetrapods, of +course. The basiventral (=intercentrum) would merely have expanded +sufficiently to provide a diapophysis for the tuberculum as well as the +(primitive) facet for the capitulum. No neomorph appears under this +hypothesis, which has the distinct advantage of simplicity. + +Figures of early stages in vertebral development by the authors +mentioned show that the basidorsals chondrify first, as neural arches, +while a separate mass of mesenchyme lies externally and ventrally from +these. This mesenchyme may chondrify either in one piece (on each side) +or in two; in _Molge_ the part adjacent to the centrum is ossified in +the 20-mm. larva, and subsequently unites with the more dorsal and +lateral cartilaginous part, while the rib, appearing farther out, grows +inward to meet this composite "rib-bearer." In _Necturus_ the mesenchyme +below the neural arch differentiates into a cartilage below the +vertebral artery (position proper to a basiventral), a bridge between +this and the neural arch, and a rib, the latter two chondrifying later +than the "basiventral" proper. In the "axolotl" (presumably _Ambystoma +tigrinum_) the rib-bearer grows downward from its first center of +chondrification at the side of the neural arch (Emelianov, 1936). + +Thus it appears that the simplest hypothesis to account for the +rib-bearer is that (_a_) it is the basiventral, (_b_) it is recognizable +just before chondrification as a mass of mesenchyme in contact with both +the notochordal sheath and the basidorsal cartilage, (_c_) it may +chondrify or ossify first in its ventral portion or in its dorsal +portion, the two then joining before it fuses with the rest of the +vertebra, (_d_) the enclosure of the vertebral artery is a consequence +of the extension of the basiventral beyond the position occupied by it +in primitive Amphibia, and (_e_) there is no indication that this took +place in other orders than the Urodela. + +It seems that the vertebrae in Urodela have at least the following +components: perichordal centra, separate basidorsal cartilages, and +basiventrals, which are somewhat specialized in their manner of +development. The vertebrae of Anura develop in the fashion just +described except that basiventrals are lacking. It would seem no more +difficult to accept the derivation of salamander vertebrae from the +temnospondylous type than it is in the case of frogs, if other evidence +points to such an ancestry. + +[Illustration: Fig. 7. Vertebrae of _Eusthenopteron_ (x1) and +_Ichthyostega_ (x2/3, after Jarvik, 1952), _Trimerorhachis_ (x1-1/2, +after Case), and _Amphibamus_ (x10, after Watson, 1940) in lateral and +end views; the two lower right-hand figures are from Watson (1940, as +_Miobatrachus_); the lower left is from a cast of the "_Miobatrachus_" +specimen in Chicago Natural History Museum, No. 2000, in the presacral +region (original, x10).] + +Fig. 7, lower right, is Watson's (1940) illustration of the anterior +trunk vertebrae of _Amphibamus_ (_Miobatrachus_), in which the +intercentrum is shown as a single median piece. Fig. 7, lower left, +shows two of the more posterior trunk vertebrae seen as impressions in a +cast of the type of "_Miobatrachus romeri_;" evidently the inter-centra +were paired at about the level of the 16th vertebra, and relatively +large. Gregory's (1950) figure of the type specimen of "_Mazonerpeton_" +(also equivalent to _Amphibamus_) shows the anterior trunk vertebrae in +relation to the ribs essentially as they appear to me in the cast of +_Miobatrachus_, and rather differently from Watson's figure of the +latter. Gregory is probably right in considering the specimens to +represent various degrees of immaturity. So far as present information +goes, then, the vertebrae of salamanders and frogs show no _clear_ +evidence of derivation from those of any particular group among the +early Amphibia, but their features are not inconsistent with a +simplification of the pattern of Temnospondyli. + +[Illustration: Fig. 8. Pectoral girdles of _Protobatrachus_ (after +Piveteau, 1937), _Notobatrachus_ (after Stipanicic and Reig, 1956), +Ascaphus (after Ritland, 1955 a) and _Rana_ (original); all x2. For +explanation of abbreviations see Fig. 3.] + + + + +PECTORAL GIRDLE + + +Hecht and Ruibal (Copeia, 1928:242) make a strong point of the nature of +the pectoral girdle in _Notobatrachus_, as described recently by +Stipanicic and Reig (1955, 1956) from the Jurassic of Patagonia, and +quite rightly recommend that the significance of the arciferal and +firmisternal types of girdle be restudied. That of _Notobatrachus_ is +said to be firmisternal; in view of the arciferal condition in the +supposedly primitive _Leiopelma_, _Ascaphus_, _Bombinator_, etc., this +comes as a surprise. Is the firmisternal girdle, as seen in _Rana_, +_Bufo_, and others, actually the ancestral type, and has the arciferal +been derived from something like this? + +In the figures given by Stipanicic and Reig the ossified parts of the +girdle are figured in detail (Fig. 8) and Reig's discussion of it is +thorough. The decision to call it firmisternal was taken with some +hesitancy, for no median elements are indicated, and the position and +shape of those seen is closely similar to the ossified parts in +_Ascaphus_ and _Leiopelma_; there is no bony sternum or omosternum. It +is safe to suppose that some cartilage lay in the midline between the +clavicles and coracoids, but there is no evidence as to its extent, +rigidity, or degree of overlapping if any. Apparently, then, there is +not sufficient reason to infer that this Jurassic frog had a pectoral +girdle comparable with the modern firmisternal type. + +Piveteau (1955:261) remarks that the only living Anuran that can be +compared usefully with _Protobatrachus_ (Triassic) with regard to its +pectoral girdle is _Ascaphus_. Again, the extent of cartilage in +_Protobatrachus_ (Fig. 8) can only be inferred, and there are no median +elements. The agreement with _Ascaphus_ includes the presence, in both, +of a separate coracoid ossification situated posterior to the ossified +"scapulocoracoid" (actually scapula). This ossification is evidently +that shown in _Notobatrachus_ as "coracoid." Direct comparison of the +three genera with one another suggests that if we use the term arciferal +for any, we should use it for all. + +In the remote predecessor of Anura, _Amphibamus_ of the Pennsylvanian, +the pectoral girdle was less substantial than in many of its +contemporaries, but it contained the primitive median interclavicle in +addition to the clavicle, cleithrum, and scapulocoracoid. (The figure of +Watson, 1940, and that by Gregory, 1950, are of individuals of different +ages, the latter being older.) It is clear that the paired elements of +such a girdle were held rigid by their attachment to the interclavicle, +_via_ the clavicles. Subsequent elimination of the interclavicle in the +Anuran line of descent, and decrease of ossification, left a girdle like +that of _Protobatrachus_, _Notobatrachus_, _Ascaphus_ and _Leiopelma_. +But in several advanced families a more rigid median "sternum," of one +or two bony pieces plus cartilage, is developed secondarily, possibly +(as Cope, 1889: 247, suggested) in correlation with axillary amplexus. + +Among Urodela no dermal bones occur in the pectoral girdle. There is +usually a scapulocoracoid ossified as a single piece, from which a thin +cartilaginous suprascapula extends dorsally and a broad cartilaginous +coracoid plate extends medially, overlapping the one from the opposite +side; a precoracoid lobe of this reaches forward on either side, and a +median, posterior "sternum" of cartilage may make contact with the +edges of the two coracoids. In _Siren_ and _Amphiuma_ two centers of +ossification are found for each scapulocoracoid, and in _Triton_ and +_Salamandra_ three. Probably the more dorsal and lateral of these +represents the primitive scapula and the other one (or two) the +primitive coracoid. + +Comparing the girdle of a salamander with that of a frog, the closest +similarity can be seen between _Ascaphus_ and a salamander in which the +scapula and coracoid ossify separately. Both have the median "sternum" +in contact with the coracoid plates. The major difference, of course, is +the lack of clavicle and cleithrum in the salamander. + + + + +CARPUS AND TARSUS + + +In _Ascaphus_ (Ritland, 1955a; cleared and stained specimens of nearly +grown males) distal carpals 1, 2, 3 and 4 are present and separate, +increasing in size in the order given (Fig. 9). A prepollex rests +against centrale 1; centralia 2 and 3 are fused; the radiale fuses with +centrale 4, and the intermedium fuses with the ulnare; radius and ulna +are fused with each other as in other frogs. The digits (and +metacarpals) are considered by Ritland to be 1-4, in addition to the +prepollex, rather than 2-5. + +[Illustration: Fig. 9. Skeleton of fore foot of _Notobatrachus_ (after +Stipanicic and Reig, 1956, terminology revised) and _Ascaphus_ (after +Ritland, 1955 a); all x5. For explanation of abbreviations see Fig. 3.] + +In the Jurassic _Notobatrachus_ Stipanicic and Reig (1956) have shown +the carpus with surprising clarity (Fig. 9). If their nomenclature of +the parts be revised, we obtain a fairly close resemblance to +_Ascaphus_, except that centralia 2 and 3 are not fused, distal carpals +1 and 2 do not show (which would easily be understood if they were of +the size of those in _Ascaphus_, or not ossified), and the intermedium +remains separate from the ulnare. + +In _Salamandra_ (Francis, 1934; Nauck, 1938) distal carpals 1 and 2 are +fused in both larva and adult, and 3 and 4 are separate; the radiale, +intermedium and ulnare are separate in the larva but the latter two fuse +in the adult; centrale 1 (labelled prepollical cartilage by Francis) and +centrale 2 are separate. Francis considers the digits (and metacarpals) +to be 1-4. Apparently the arrangement here indicated for the larva is +characteristic of other larval salamanders, except where further +reduced, and reduction below the number given for the adult is common in +other terrestrial forms. The radius and ulna are, of course, separate. + +The ossification of carpals is more likely to be complete in adult frogs +than in salamanders, but some ossification of all parts named is found +in several of the latter. A common ancestor of frogs and salamanders +could be expected to have the following elements present and ossified in +the adult: distal carpals 1-4 separate; 3 centralia; radiale, +intermedium and ulnare separate. Comparison with fossils older than +_Notobatrachus_ is fruitless on these points, unless we go back to forms +too distant to have any special value, such as _Eryops_. This is because +of inadequate preservation and because the elements are not fully +ossified in many immature specimens. + +For the purpose of this review there is no special value in a comparison +of the tarsi of frogs and salamanders, since the leaping adaptation of +the former leaves very little common pattern between them. Even in +_Protobatrachus_, where the legs were not yet conspicuously lengthened, +the tibiale and fibulare ("astragalus" and "calcaneum" respectively) +were already considerably elongated. The carpus and tarsus of +_Amphibamus_ are as yet undecipherable. + + + + +THE LARVA + + +Considering the postembryonic developmental stages of modern Amphibia, +there can be no doubt that a gill-bearing, four-legged larva of a +salamander, in which lateral line pores and a gular fold are present, +represents much more closely the type of larva found in labyrinthodonts +than does the limbless, plant-nibbling tadpole of the Anura. +Salamander-like larvae of labyrinthodonts are well known, especially +those formerly supposed to comprise the order Branchiosauria. Many, +perhaps the majority of, labyrinthodonts show some features associated +with aquatic life even when full-grown, as do the lepospondyls. These +features may include impressions of sensory canals on the dermal bones +of the skull, persistence of visceral arches, reduction in size of +appendages, and failure of tarsal and carpal elements to ossify. In +fact, it appears that very few of the Paleozoic Amphibia were successful +in establishing themselves as terrestrial animals even as adults. + +Nevertheless, in the ancestry of Anura, and that of at least the +Hynobiid, Ambystomid, Salamandrid and Plethodontid salamanders, there +must certainly have been a terrestrial adult, transforming from an +aquatic larva. The leaping mechanism of Anura, shown in so many features +of their anatomy, is perhaps to be explained as a device for sudden +escape from land into the water, but it was not yet perfected in the +Triassic _Protobatrachus_ or the Jurassic _Notobatrachus_. + +The middle ear, its sound-transmitting mechanism, and the tympanum, well +developed in most Anura, are readily derived from those of early +labyrinthodonts, and are presumably effective for hearing airborne +sounds whether on land or while floating in the water. Reduction of +these organs in Urodela may be correlated with their customary +restriction to subsurface habitats and inability to maintain a floating +position while in water. + +Some light may be shed on the significance of the tadpole of Anura by +considering the early stages of the ribbed frogs, Liopelmidae. +_Leiopelma_ and _Ascaphus_ are so closely similar in the adult that +there is no doubt that they belong in one family, primitive in some +respects (including articulated ribs; pyriformis and +caudalipuboischiotibialis muscles) but not in others (absence of +tympanum and middle ear). In both genera the eggs are large, 5 mm. in +_Leiopelma_, 4.5 mm. in _Ascaphus_, and unpigmented; but at this point +the resemblance ends. + +[Illustration: Fig. 10. _Leiopelma hochstetteri_ larva, lateral and +ventral (after Stephenson, 1955), x4.] + +Stephenson (1955) showed that embryos of _L. hochstetteri_ develop +equally well on land (in damp places) or in the water, and that embryos +prematurely released from egg capsules develop successfully in the +water. The larvae possess both pairs of legs (Fig. 10) and a broad gular +fold similar to that of larval salamanders. In _L. hochstetteri_ the +fold grows back over the forelegs temporarily, but remains free from the +body and presently the legs reappear, whereas in _L. archeyi_ the +forelegs are not covered at any time. No branchial chamber or spiracle +is formed. Of course direct development, without a tadpole, occurs in +several other groups of Anura, but in each case terrestrial adaptations +are obvious. This is not true of _Leiopelma_, which Stephenson regards +as more nearly comparable with Urodela in its development than with +other Anura, and he sees in it a "primary and amphibious" mode instead +of a terrestrial specialization. + +The _Ascaphus_ tadpole bears no outward resemblance to the larva of +_Leiopelma_, but is a normal tadpole in form, although sluggish in +activity. Its greatly expanded labial folds bear numerous rows of horny +epidermal "teeth," which, with the lips, serve to anchor the tadpole to +stones in the swift water of mountain brooks. Noble (1927) noticed that +particles of food were taken in through the external nares, and that a +current of water passed through these openings and out by way of the +median spiracle. It appears that any action by the teeth and jaws in +scraping algae from the rocks (which were bare in the stream where I +have collected _Ascaphus_) would be quite incidental, and that the lips +and teeth must be primarily a clinging mechanism. Certain other mountain +brook tadpoles (for example, _Borborocoetes_) show similar devices, but +these are developed independently, as specializations from the usual +sort of tadpole. + +May it not be that closure of the gill-chamber by the opercular (=gular) +fold, retardation of limb development, expansion of the lips, growth of +parallel rows of horny teeth, and other correlated features that make a +tadpole, were brought about as an adaptation of the primitive Anuran +larva to a swift-stream habitat, and that this "basic patent" then later +served to admit the tadpoles of descendant types to an alga-scraping +habit in quiet water as well? The tadpole, as a unique larval type among +vertebrates, bears the hallmarks of an abrupt adaptive shift, such as +might have occurred within the limits of a single family, and it seems +difficult to imagine the enclosed branchial chamber, the tooth-rows, and +lips of a familiar tadpole as having evolved without some kind of +suctorial function along the way. + + + + +SUMMARY + + +The Anura probably originated among temnospondylous labyrinthodonts, +through a line represented approximately by _Eugyrinus_, _Amphibamus_, +and the Triassic frog _Protobatrachus_, as shown by Watson, Piveteau and +others. The known Paleozoic lepospondyls do not show clear indications +of a relationship with Urodela, but _Lysorophus_ may well be on the +ancestral stem of the Apoda. + +Between Urodela and Anura there are numerous resemblances which seem to +indicate direct relationship through a common stock: (1) a similar +reduction of dermal bones of the skull and expansion of palatal +vacuities; (2) movable basipterygoid articulation in primitive members +of both orders; (3) an operculum formed in the otic capsule, with +opercularis muscle; (4) many details of cranial development, cranial +muscles, and thigh muscles, especially between _Ascaphus_ and the +Urodela, as shown by Pusey and Noble; (5) essentially similar manner of +vertebral development, quite consistent with derivation of both orders +from Temnospondyli; (6) presence in the larva of _Leiopelma_ of a +salamander-like gular fold, four limbs, and no suggestion of +modification from a tadpole (Stephenson). + + + + +LITERATURE CITED + + +Broom, R. + + 1918. Observations on the genus _Lysorophus_ Cope. Ann. Mag. Nat. + Hist., (9)2:232-239. + +Bystrow, A. P. + + 1938. Dvinosaurus als neotenische Form der Stegocephalen. Acta + Zool., 19:209-295. + +Case, E. C. + + 1935. Description of a collection of associated skeletons of + _Trimerorhachis_. Contrib. Mus. Pal. Univ. Michigan, 4:227-274. + +Cope, E. D. + + 1889. The Batrachia of North America. Bull. U. S. Nat. Mus., + 34:1-525. + +de Beer, G. R. + + 1937. The development of the vertebrate skull. Pp. xxiii + 552. + Oxford, Clarendon Press. + +de Villiers, C. G. S. + + 1934. Studies of the cranial anatomy of _Ascaphus truei_ + Stejneger, the American "Liopelmid." Bull. Mus. Comp. Zool., + 77:1-38. + +Emelianov, S. W. + + 1936. Die Morphologie der Tetrapodenrippen. Zool. Jahrb. (Anat.), + 62:173-274. + +Francis, E. T. B. + + 1934. The anatomy of the salamander. Pp. xxxi + 381. Oxford, + Clarendon Press. + +Gamble, D. L. + + 1922. The morphology of the ribs and the transverse processes of + _Necturus maculatus_. Jour. Morph., 36:537-566. + +Gray, P. + + 1930. On the attachments of the Urodele rib to the vertebra and + their homologies with the capitulum and tuberculum of the Amniote + rib. Proc. Zool. Soc. London, 1930(1931):907-911. + +Gregory, J. T. + + 1950. Tetrapods from the Pennsylvanian nodules from Mazon Creek, + Illinois. Am. Jour. Sci., 248:833-873. + +Hecht, M. E. + + 1957. A case of parallel evolution in salamanders. Proc. Zool. + Soc. Calcutta, Mookerjee Mem.:283-292. + +Holmgren, N. + + 1933. On the origin of the tetrapod limb. Acta Zool., 14:185-295. + + 1939. Contributions to the question of the origin of the tetrapod + limb. Acta Zool., 20:89-124. + + 1949a. Contributions to the question of the origin of tetrapods. + Acta Zool., 30:459-484. + + 1949b. On the tetrapod limb problem again. Acta Zool., 30:485-508. + +Herre, W. + + 1935. Die Schwanzlurche der mitteleocaenen (oberlutetischen) + Braunkohle des Geiseltales und die Phylogenie der Urodelen unter + Einschluss der fossilen Formen. Zoologica, 33:87, 1-5. + +Jarvik, E. + + 1942. On the structure of the snout of Crossopterygians and lower + gnathostomes in general. Zool. Bidrag fran Upsala, 21:235-675. + 1952. On the fish-like tail in the Ichthyostegid Stegocephalians. + Meddelelser on Gronland, 114(12):1-90. + +Mookerjee, H.K. + + 1930a. On the development of the vertebral column of the Urodela. + Phil. Trans. Roy. Soc. London, B 218:415-446. + + 1930b. On the development of the vertebral column of the Anura. + Philos. Trans. Royal Soc. London, B 219:165-196. + +MacBride, E.W. + + 1932. Recent work on the development of the vertebral column. + Cambridge, Biol. Rev., 7:108-148. + +McDowell, S.B. + + 1958. Are the frogs specialized seymouriamorphs? (Abstract) Anat. + Rec., 132(3):472. + +Naef, A. + + 1929. Notizen zur Morphologie und Stammesgeschichte der + Wirbeltiere. 15. Dreissig Thesen ueber Wirbelsaeule und Rippen, + insbesondere bei den Tetrapoden. Zool. Jahrb., 50:581-600. + +Noble, G. K. + + 1922. The phylogeny of the Salientia; I. The osteology and the + thigh musculature; their bearing on classification and phylogeny. + Bull. Amer. Mus. Nat. Hist., 46:1-87. + + 1927. The value of life-history data in the study of the evolution + of the Amphibia. Annals New York Acad. Sci., 30:31-128. + +Piveteau, J. + + 1937. Un Amphibien du Trias inferieur. Essai sur l'origine et + l'evolution des Amphibiens Anoures. Annales de Paleontologie, + 26:135-176. + + 1955. Anoura. In: Traite de Paleontologie, 5:250-274. J. Piveteau, + Masson et Cie, Paris. + +Pusey, H. K. + + 1943. On the head of the liopelmid frog, _Ascaphus truei_. I. The + chondrocranium, jaws, arches, and muscles of a partly grown larva. + Quart. Jour. Micr. Sci., 84:105-185. + +Reed, H. D. + + 1915. The sound-transmitting apparatus in Necturus. Anat. Rec., + 9:581-590. + +Remane, A. + + 1936. Wirbelsaeule und ihre Abkoemmlinge. In: Handbuch der + vergleichenden Anatomie der Wirbeltiere, L. Bolk _et al._, + 4:1-206. Urban and Schwarzenberg, Berlin, Vienna. + +Ritland, R. M. + + 1955a. Studies on the post-cranial morphology of Ascaphus truei. + I. Skeleton and spinal nerves. Jour. Morph., 97:119-174. + + 1955b. Studies on the post-cranial morphology of Ascaphus truei. + II. Myology. Jour. Morph., 97:215-282. + +Romer, A. S. + + 1945. Vertebrate paleontology. 2nd edition. Pp. viii + 687. Univ. + Chicago Press. + + 1947. Review of the Labyrinthodontia. Bull. Mus. Comp. Zool., + 99:3-368. + +Saeve-Soederbergh, G. + + 1934. Some points of view concerning the evolution of the + vertebrates and the classification of this group. Arkiv foer + Zoologi, 26A:1-20. + +Stadtmueller, F. + + 1936. Kranium und Visceralskelett der Stegocephalen und Amphibien. + In: Handbuch der vergleichenden Anatomie der Wirbeltiere, by L. + Bolk _et al._, 4:501-698. + +Stephenson, N. G. + + 1955. On the development of the frog, _Leiopelma hochstetteri_ + Fitzinger. Proc. Zool. Soc. London, 124(4):785-795. + +Stipanicic, P. N. and Reig, O. A. + + 1955. Breve noticia sobre el hallazgo de anuros en el denominado + "Complejo Porfirico de la Patagonia Extraandina," con + consideraciones acerca de la composicion geologica del mismo. + Revista de la Asoc. Geol. Argentina, 10(4):215-233. + + 1956. El "complejo porfirico de la Patagonia extraandina" y su + fauna de Anuros. Acta Geol. Lilloana (Univ. Nac. del Tucuman), + 1:185-297. + +Sushkin, P. P. + + 1936. Notes on the pro-Jurassic Tetrapoda from U. S. S. R. III. + Dvinosaurus Amalitzky, a perennibranchiate stegocephalian from the + Upper Permian of North Dvina. Trav. Inst. Pal. Acad. Sci. URSS, + 5:43-91. + +Watson, D. M. S. + + 1940. The origin of frogs. Trans. Roy. Soc. Edinburgh, + 40(7):195-231. + + + + +_Transmitted April 7, 1959._ + + + + + +End of the Project Gutenberg EBook of The Ancestry of Modern Amphibia: A +Review of the Evidence, by Theodore H. Eaton + +*** END OF THIS PROJECT GUTENBERG EBOOK THE ANCESTRY OF MODERN *** + +***** This file should be named 37350.txt or 37350.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/3/7/3/5/37350/ + +Produced by Chris Curnow, Charlene Taylor, Joseph Cooper +and the Online Distributed Proofreading Team at +http://www.pgdp.net + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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