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The Vertebral Column (Plates 130 and 131)
Leiopelma shares with Ascaphus the distinction of having nine presacral vertebrae, one more than any other Anuran. The vertebrae of these two genera are also unique among frogs and toads through the fact that each is amphicoelous, the centrum being biconcave and the intervertebral cartilage undivided. As McBride (1931) has pointed out in connection with the urodeles Amphiuma and Menobranchus, the term “amphicoelous” used for these amphibians is not strictly the same as in the herring, where the notochord is expanded between successive vertebrae. In Amphibia having biconcave centra, cartilaginous intervertebral pads are present uniting successive vertebrae, while the original notochord running through the intervertebral region becomes crushed and virtually obliterated.
Neural spines are not sharply defined on any of the vertebrae. The two facets on the “atlas” for the articulations of the occipital condyles are oval and relatively shallow (Plate 131, fig. 1). Ventrally, a process of which the most anterior portion remains cartilaginous, projects forward between the occipital condyles, the space between its tapering extremity and the condyles being filled with fibrous connective tissue (Text-fig. 1A). This anterior process does not appear to be identical with the odontoid process of urodeles.
The atlas vertebra of Leiopelma has no transverse processes. Noble's drawing of the atlas of Ascaphus (Noble, 1931, p. 488) shows a small but unmistakable pair of forwardly directed diapophyses. I have examined X-ray photographs of a male and female Ascaphus, however, and found no trace of these structures. Furthermore, it should be noted that the fossil form Protobatrachus (Piveteau, 1937) lacks transverse processes on the first vertebra.
The transverse processes of the vertebra next behind the atlas, the first of the thoraco-lumbar series, are usually fairly short and are directed slightly forward. The third and fourth vertebrae (second and third in trunk region) invariably bear ribs which have constant and characteristic shapes (Plate 130, fig. 1). The possession of ribs in certain members of the Anura is undoubtedly a primitive feature, although the ribs themselves may show certain specialisations. Only Leiopelma, Ascaphus and the Discoglossidae bear ribs in the adult. Members of the Pipidae possess them in larval stages, but in adult life the ribs become indistinguishably fused with the diapophyses (Noble, 1931). Mookerjee (1936) has found ribs in the embryos of Rana temporaria and Bufo melanostictus. In Leiopelma each rib is approximately the same length as the diapophysis with which it articulates, is somewhat flattened distally, and is narrowest at the middle of its length. Distally each rib divides to form a process which is characteristic for the vertebra to which the rib belongs. The rib attached to the third vertebra has a narrow, sharp, posteriorly directed uncinate process, while that of the fourth vertebra usually ends in two more or less equal flattened expansions. Both of these ribs and their processes agree quite closely with those of Ascaphus (Noble, 1931)
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Text-Fig. 1—Contour Reconstructions of Anterior Vertebral Region of L. hochstetteri, approximately one year old.
A. Cranio-occipital joint, anterior view. The most anterior section passes through the occipital condyles.
B. Cranio-occipital joint, posterior view. The most posterior section passes through the atlas vertebra.
C. Part of atlas vertebra and the most anterior region of the second vertebra, anterior view.
nz2, anterior zgyapophysis of second vertebra; d2, diapophysis of second vertebra; fet, fibrous connective tissue; ive, intervertebral cartilage; n, notochord; nar1, neural arch of first vertebra; occ, occipital condyle; odp, anterior process; pz1, posterior zygapophysis of first vertebra; sp1. first spinal nerve; sp2d, dorsal root of second spinal nerve; sp2v, ventral root of second spinal nerve; ur, urostyle; v1, atlas vertebra; IX+X, combined ninth and tenth cranial nerves
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except that the more posterior rib of Leiopelma is developed equally as well if not better than that of the third vertebra.
Piveteau's interesting comparison (op. cit.) between the anterior vertebrae of Protobatrachus and Ascaphus can equally well include those of Leiopelma. In all three genera the rib belonging to the third vertebra articulates by a single head with the corresponding transverse process, so that in all three cases specialisation has occurred in comparison with the double-headed ribs of Amphibia such as Miobatrachus (Watson, 1939). The rib bears an uncinate process of uniform type. Pivetean pointed out that the enlarged size of this rib in Protobatrachus was repeated in Ascaphus and that in Rana also the transverse diapophysis of the third vertebra was greater than the rest. He regarded the phenomenon as being correlated with the presence of lymph hearts immediately behind the third vertebra. In view of Piveteau's explanation it is interesting to find that in Leiopelma no distinction occurs between the relative development of the diapophyses and ribs of the third vertebra and those of the fourth. In Leiopelma also it is by no means rare to find a third rib or pair of ribs attached to the fifth vertebra (Plate 130, fig. 1). When these occur, they bear no processes and are approximately the same diameter as the corresponding transverse processes.
The transverse processes of the third, fourth, fifth and sixth vertebrae point either straight out or slightly backwards (Plates 130 and 131). Those of the seventh and eighth turn slightly forward, while those of the ninth usually have a markedly forward position (Plate 130, fig. 1; Plate 131, fig. 7). The post-zygapophyses of the ninth vertebra are usually enlarged compared with those of the rest of the series (Plate 130, fig. 1), overlapping the sacral vertebra dorsally.
The sacral “vertebra”, the tenth of the series, is extremely peculiar, if not unique, consisting as it does of two separate bony halves joined
Text-Fig. 2—Modified Contour Reconstruction of Anterior Region of Urostyle,
the Sacral Vertebra and part of the ninth Vertebra, posterior view.
The most anterior section passes through the urostyle. Cartilage is indicated
by dotted lines.
az10, anterior zygapophysis of tenth vertebra; fnar, fused neural arches; il, ilium; ive, intervertebral cartilage; pz9, posterior zygapophysis of ninth vertebra; sd10, sacral diapophysis of tenth vertebra; sp11–13, eleventh to thirteenth spinal nerves; ur, urostyle.
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dorsally and ventrally by cartilage (Plate 130, fig. 1; Plate 131, figs. 7 and 9; Text-fig. 2). It is narrow antero-posteriorly compared with the other vertebrae. Essentially each bony half consists of a curved portion of the neural arch (representing the basidorsal), a prezygapophysis, and a sacral diapophysis attached to the middle of the basidorsal. The diapophysis, though much larger than those of the other vertebrae, is relatively unspecialised. It has a gradual and slight expansion distally and lies on top of the ilium, being bound to the latter by ligamentous tissue. Dorsally the two bony halves are united by a narrow strip of persistent cartilage forming the top of the neural arch, while ventrally they are joined by the continuous cartilage stretching between the centrum of the ninth vertebra and the urostyle. This cartilage may become calcified in late life, but in doing so always remains quite separate from the bony halves of the neural arch (Plate 130, fig. 1). In young frogs (i.e. during the first few months after hatching) cartilage occurs dorsally between the bony halves of the neural arch in all vertebrae, but this later ossifies in all except the sacral vertebra.
In Ascaphus (Noble, 1931), the sacral vertebra agrees with that of Leiopelma in the shape of the transverse processes, but is relatively wider from front to back and is figured as being entirely bony across the ventral surface. The dorsal surface is not shown. Here, again, further investigation would be useful to find if this is always the case. Evidence from X-rays gives no indication that the structure of the sacrum of Ascaphus agrees with that of Leiopelma, but the sacral vertebra is narrower than those preceding it.
In sections of frogs eight months to one year old, bone formation is already well advanced. As far as can be seen, the formation of “membrane bone arches” (Mookerjee) from connective tissue on either side of the basidorsals, and subsequent fusion of the original neural arch with these slightly increases the width of the arch. Ossification of the centra begins with the formation of perichondral layers of bone in the inner notochordal sheath. The intervening spaces above and below the notochord are then subject to enchondral ossification.
In development, the urostyle is quite clearly formed of a single hypochordal cartilage as Mookerjee (1930, 1931) has shown. To the anterior and dorsal portion of this cartilage are fused the postsacral vertebrae formed in development. Of these, usually not more than three occur. As a rule the neural arches of these vertebrae are fused together and enclose the end of the spinal canal (Plate 131, fig. 8). Almost invariably one finds a pair of small but unmistakable transverse processes on the first (and best developed) postsacral vertebra. A common condition of the anterior part of the urostyle is illustrated in Text-figure 2. The very small, posterior neural arch is open above, housing the extreme posterior part of the spinal canal. The latter is entered in front of the arch by the coccygeal nerve. The more anterior two arches are fused together dorsally, but allow the passage between them of the twelfth pair of spinal nerves. The most anterior arch bears small transverse processes and both the fused arches are joined medially to those of the opposite side by cartilage. The eleventh pair of nerves enter the spinal canal between the urostyle and the sacral arch.