The Ancestry of Modern Amphibia: a Review of the Evidence

Part 1

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

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

27-8362

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

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.

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.

Continue reading The Ancestry of Modern Amphibia: a Review of the Evidence free in the Wunder reader →