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.