Theodore H. Eaton

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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." ”

“ 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. ”

“ 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. ”

“ 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 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 ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”