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.]