Читать книгу Hadrosaurs - David A. Eberth - Страница 71

Nine cervical vertebrae, 17 dorsal vertebrae, and 6 sacral vertebrae are preserved. The atlas and axis are not present, indicating that there were originally 11 cervical vertebrae in Equijubus. The first two dorsal vertebrae are transitional, bearing features of both cervical and dorsal vertebrae, but in keeping with You, Luo, et al. (2003) and other descriptions of basal iguanodonts (e.g., Norman, 2004) they are considered part of the dorsal column herein. Eleven cervicals and 17 dorsals are known in other basal hadrosauriforms such as Mantellisaurus, Iguanodon, and Ouranosaurus (Taquet, 1976; Norman, 1980, 1986). Jinzhousaurus also has 11 cervicals (Wang et al., 2010).

Cervical Vertebrae All of the cervicals are strongly transversely crushed, obscuring details such as the shape of the cranial and caudal articular facets. All cervicals are opisthocoelous, with a ball-like, convex cranial articular facet and a strongly concave caudal articular facet. Neural spines are not preserved on any of the vertebrae. Representative cervical vertebrae are shown in Figure 3.11.

The centrum of cervical 3 is strongly transversely crushed (Fig. 3.11A–D). The lateral surfaces of the centrum converge ventrally to form a strong keel extending craniocaudally from the cranial to the caudal articular facet; the prominence of this feature may have been accentuated by crushing. The parapophysis is located cranially and dorsally on the lateral surface of the centrum just caudal to the ball-like cranial articular facet. It is raised relative to the sides of the centrum, and the cervical rib capitulum is preserved in articulation on both sides. In lateral view the neural arch is the same length as the centrum. Diapophyses are positioned on the distal ends of short, stout processes that extend laterally from the neural arch dorsal and slightly caudal to the parapophyses. Prezygapophyses are not preserved. Postzygapophyses are elongate and extend caudal to the caudal articular facet. Although they are crushed together, there is a deep groove extending between the postzygapophyses indicating that they were originally separated. The dorsal surfaces of the postzygapophyses bear a groove that separates distinct epipophyses from the surfaces of the postzygapophyses. Epipophyses are known in a variety of saurischian dinosaurs, some basal ornithischians, and the basal iguanodontian Tenontosaurus (e.g., Ostrom, 1970; Butler et al., 2008), but to our knowledge have not previously been reported in any other dryomorph ornithopod. For example, they are not present in Iguanodon (Norman, 1980), Mantellisaurus (Norman, 1986), or Jinzhousaurus (Wang et al., 2010). The presence of epipophyses on cervical 3 therefore represents an autapomorphy of Equijubus within Dryomorpha.

Cervical 4 is similar in many respects to cervical 3, although ribs are not articulated with the parapophyses, allowing their morphology to be described (Fig. 3.11E). Parapophyses, again present on the cranial part of the lateral centrum, are oval with the long axis horizontal. They are held on small, raised tubercles and angled caudolaterally. A shallow ridge extends horizontally from the caudal part of the parapophyses caudally toward the caudal articular facet. The right prezygapophysis of cervical 4 is also preserved. It projects laterally level with the dorsal margin of the neural canal and its articular surface faces dorsomedially. It is oval in dorsal view with the long axis trending craniomedially. The diapophysis is located on the caudoventral surface of the prezygapophysis on a raised tubercle and is angled caudoventrally. Postzygapophyses are not preserved.

Cervicals 5 and 6 are similar to cervicals 3 and 4, except that in both cases, the double-headed cervical ribs are preserved in articulation on the left side. Diapophyses on these vertebrae are larger, circular, face laterally, and are supported on a tubercle. Postzygapophyses, preserved in cervical 5, are similar to those of cervical 3, although there is no evidence of epipophyses on these or subsequent vertebrae.

Cervicals 7 to 9 are preserved in articulation with each other (Fig. 3.11F). Cervical ribs articulate with the parapophyses of all three vertebrae on the right-hand side and cervical 9 on the left. In general their morphology is similar to the other cervicals, although the diapophyses extend caudolaterally and are positioned on distinct transverse processes that are circular in cross section.

In cervicals 10 and 11 (Fig. 3.11G–I) the transverse processes are more elongate and robust. They project dorsolaterally and are supported by a lamina that extends from the caudal articular facet craniodorsally and extends along the ventral margin of the transverse process in a position equivalent to the posterior centrodiapophyseal lamina of saurischians (Wilson, 1999). Prezygapophyses are located on the transverse process and do not extend farther cranially than the cranial margin of the process. The lateral surface of the centrum of cervical 11 bears several nutrient foramina.

With the exception of the presence of epipophyses on cervical 3, the cervical column is similar in most respects to those of other basal hadrosauriforms (e.g., Iguanodon, Norman, 1980; Mantellisaurus, Norman, 1986; Jinzhousaurus, Wang et al., 2010).

Cervical Ribs A small portion of a Y-shaped left cervical rib is preserved (Fig. 3.12). It has a relatively long capitulum and a much shorter tuberculum. Both processes are transversely compressed and slightly concave on their medial surfaces. Both articular facets are round in cross section. A ridge extends from between the tuberculum and capitulum caudally and disappears farther down the rib shaft. The dorsal margin of the shaft of the rib is gently convex upward. Medially the shaft is concave; it is broken distally. The rib is very similar in morphology to the eighth cervical rib of Iguanodon bernissartensis (Norman, 1980:fig. 32). Several other rib fragments are also preserved (see above), but offer little additional anatomical information.

Dorsal Vertebrae Dorsals 1 and 2 are preserved in articulation with each other and show several features indicating that they are transitional in morphology between cervicals and dorsals (Fig. 3.13A–C). Two dorsal vertebrae exhibiting transitional features are also present in the basal hadrosauriforms Mantellisaurus (Norman, 1986) and Iguanodon (Norman, 1980), and the hadrosauroid Probactrosaurus (Norman, 2002). Centra are similar in morphology to the cervicals: they are transversely crushed and bear a prominent ventral keel that has probably been accentuated by crushing. Both vertebrae are opisthocoelous. This contrasts with the condition in Mantellisaurus, in which the cranial articular facets are not as strongly convex as they are in the cervicals (Norman, 1986), but is similar to the condition in Iguanodon bernissartensis, in which the transitional vertebrae remain opisthocoelous (Norman, 1980). Parapophyses are oval in shape, with the long axis angled cranioventrally, and are situated partially on the centrum and partially on the neural arch, bridging the neurocentral suture. The neurocentral suture itself is visible on both vertebrae, and on the left side of dorsal two a well-developed concavity is present on the lateral surface of the centrum just ventral to it. This appears to be preservational, however, because it is not present on the right side. Transverse processes extend from a point dorsal to the parapophyses and are broken and crushed, as is much of the neural arch, and few details of the prezygapophyses and postzygapophyses can be determined.

In dorsals 3 and 4, only centra are preserved. These are more amphiplatyan, with very slightly convex cranial articular facets and very slightly concave caudal articular facets. Laterally the sides of the centra are rugose and textured proximal to the articular facets, perhaps due to soft tissue that would have bound the vertebrae together. The sides of the centra are pierced by irregular small foramina, as in Jinzhousaurus (Wang et al., 2010), and a ventral keel is present extending craniocaudally.

The centrum of dorsal 5 is similar to those of 3 and 4, except that the cranial and caudal articular facets are flat (Fig. 3.13D–F). The neural arch is partially preserved and the neurocentral suture is visible on both sides. The parapophyses are located caudal to the prezygapophyses on the neural arch, and are large, rounded, and angled caudolaterally. On the left side, the rib capitulum is preserved in articulation. The right transverse process is broken, but the left is preserved, although it is crushed and has rotated to project caudally. A lamina extends from the prezygapophysis along the cranial margin of the transverse process (in the same position as the prezygodiapophyseal lamina of saurischians [Wilson, 1999]), and the bone surface in this area is rugose, suggestive of soft tissue attachment. The transverse process is roughly triangular in longitudinal cross section with the apex pointing cranioventrally, as in Jinzhousaurus (Wang et al., 2010) and Iguanodon (Norman, 1980). The diapophysis is oval with the long axis trending horizontally and is slightly convex. The postzygapophyses of dorsal 4 are preserved in articulation with the prezygapophyses of dorsal 5, obscuring details of their anatomy.

3.12. Cervical rib of IVPP V 12534, holotype of Equijubus normani. (A) lateral view; (B) medial view. Scale bar equals 5 cm.

3.13. Representative cranial dorsal vertebrae of IVPP V 12534, holotype of Equijubus normani. (A) dorsal 1 in cranial view; (B) dorsal 2 in caudal view; (C) dorsals 1 and 2 in right lateral view; (D–F) dorsal 5 in (D) cranial, (E) caudal and (F) left lateral view. Abbreviations: cap, capitulum of dorsal rib; dia, diapophysis; lam, lamina; ncs, neutrocentral suture; para, parapophysis; tp, transverse process. Scale bar equals 5 cm.

Dorsals 6 and 7 are very similar in morphology to dorsal 5, and the left transverse processes on both are preserved but rotated to project caudally as in the latter. The parapophyses are located slightly dorsal to the prezygapophyses on the neural arch. The postzygapophyses of dorsal 6 are broken distally, but a thin, transversely compressed plate of bone extends from dorsal to the neural canal caudally between the postzygapophyses. This feature appears to be a hyposphene (Fig. 3.14), a feature known in many archosauromorphs and saurischians (Apesteguía, 2005), but that has not – to our knowledge – previously been reported in any ornithischian, and therefore represents an autapomorphy of Equijubus. The capitulum of the rib of dorsal 7 is preserved in articulation on the left side, obscuring some details of the anatomy of the neural arch, but a plate-like hyposphene appears to be present.

3.14. Representative caudal dorsal vertebrate of IVPP V 12534, holotype of Equijubus normani. (A) dorsal 13 in cranial view; (B) dorsal 13 in caudal view; (C) dorsal 13 in right lateral view; (D) dorsal 16 in cranial view; (E) dorsals 16 and 17 in right lateral view; (F) detail of neural spine of dorsal 13 in right lateral view showing hyposphene morphology. Abbreviations: cdd, caudal depression on hyposphene; crd, cranial depression on hyposphene; hsp, hyposphene; para, parapophysis; ri, vertical ridge on hyposphene. Scale bars equal 5 cm.

Only the centrum and lower part of the neural arch are preserved on dorsal 8. The centrum is unchanged from the morphology of the cranial dorsals, and the neurocentral suture is clearly visible. Once again the rib capitulum adheres to the left parapophysis, located caudodorsal to the prezygapophyses, which are broken. The hyposphene is again present, although the postzygapophyses are broken. Only the centra are preserved for dorsals 9 and 10, and the centrum and ventral part of the neural arch in dorsal 11. The morphology of these centra does not differ from that of the other dorsals.

3.15. Sacral vertebrae with associated ilia of IVPP V 12534, holotype of Equijubus normani, in (A) dorsal and (B) ventral view. Abbreviations: gr, groove; li, left ilium; ns, neural spine; ot, ossified tendons; ri, right ilium. Scale bar equals 10 cm.

The postzygapophyses of dorsal 11 are preserved in articulation with the prezygapophyses of dorsal 12. Immediately dorsal to the postzygapophyses, the base of the neural spine of dorsal 11 is preserved. It is rounded in cross section and transversely broad caudally, but extends cranially as a thin, plate-like sheet and is similar in morphology to that of Mantellisaurus (Norman, 1986) and Iguanodon bernissartensis (Norman, 1980). The prezygapophyses of dorsal 12 are separated by a groove ventrally, presumably for the hyposphene of dorsal 11. The parapophyses are oval and situated at the base of the transverse processes, which are broken and not preserved. The long axis of the parapophysis extends cranioventrally, and a ridge arises from the cranial margin and extends cranially to form the craniolateral margin of the neural canal in cranial view. The postzygapophyses of dorsal 12 are preserved in articulation with the prezygapophyses of dorsal 13. As in dorsal 11, the neural spine of dorsal 12 arises immediately dorsal to the postzygapophyses and is transversely relatively thickened in this area. Cranially, the neural spine becomes transversely thinner and plate-like. Cranial to the neural spine a deep prespinal fossa is present caudal to the prezygapophyses. A hyposphene is present.

Dorsal 13 is similar in morphology to dorsal 12 but the hyposphene is better preserved, allowing more details of the anatomy to be observed (Fig. 3.14). The hyposphene comprises a medially situated, transversely compressed plate of bone that arises dorsal to the neural canal and extends dorsally to the ventromedial margin of the postzygapophyses. Laterally, a depression on the hyposphene defines the caudal margin of the neural arch. A dorsoventral ridge extends up the hyposphene and separates this cranial depression from a second, caudal depression on its transverse surface. The transverse processes of dorsal 13 are broken, but at their bases they appear dorsoventrally compressed. The parapophysis, clearly preserved on the right, is located between the prezygapophysis and transverse process.

The centra of dorsals 14–17 become increasingly larger, moving caudally along the dorsal vertebral column; all bear irregular nutrient foramina on their lateral surfaces. Dorsal 14 is similar to dorsal 13 in morphology. Once again, the region of the hyposphene is well preserved, and features such as the two depressions separated by a dorsoventral ridge can also be observed on this vertebra. The centrum of dorsal 17 is preserved independently of its neural arch, which is articulated with the neural arches of dorsals 16 and 17. The postzygapophyses and hyposphene of dorsal 15 are well preserved and similar in morphology to those of dorsals 12 and 13. A portion of the lower part of the neural spine is preserved. It extends from dorsal to the postzygapophyses cranially, is angled slightly caudally, and is transversely compressed. Transverse processes are broken on this vertebra but they appear to be craniocaudally elongate and dorsoventrally compressed, at least at their bases.

Dorsals 16 and 17 are preserved in articulation, and only a portion of the centrum of dorsal 17 is present (Fig. 3.14E). The parapophyses are still located on the neural arch, and have not migrated onto the transverse process. This contrasts with the condition in Iguanodon bernissartensis, Mantellisaurus, and Jinzhousaurus, in which the parapophysis migrates along the transverse process, eventually forming a conjoined facet with the diapophysis in dorsal 17 (Norman, 1980, 1986; Wang et al., 2010). The transverse process of dorsal 17 is small, and has been crushed so that it now projects dorsally. It is relatively short and dorsoventrally flattened, being unsupported by buttresses or laminae, in contrast to more cranial transverse processes. A change in shape of the transverse processes along the dorsal series is also seen in Iguanodon bernissartensis (Norman, 1980) and Mantellisaurus (Norman, 1986). Small portions of the bases of the neural spines are preserved; these are strongly transversely compressed and angled slightly caudally.

3.16. Fragments of left scapula of IVPP V 12534, holotype of Equijubus normani, in lateral view. (A) proximal plate; (B) blade. Abbreviations: acr, acromion process; gle, glenoid; ri, ridge. Scale bar equals 10 cm.

Sacral Vertebrae Six vertebrae are fused together to form the sacrum. Eight vertebrae usually comprise the sacrum in Iguanodon bernissartensis (Norman, 1980), while seven co-ossified vertebrae form the sacrum in Mantellisaurus (Norman, 1986).

The sacrum is preserved with both ilia in natural articulation, and has been crushed transversely, so that details of the articulation between the sacrum and ilia are obscured (Fig. 3.15). The ventral portions of the first three sacrals and the caudal half of the sixth sacral have broken and are not preserved. Small foramina pierce the lateral surface of the centrum of sacral 4. A deep ventral groove is present on sacral 5, but was not present on sacral 4. The more caudal sacrals of Iguanodon bernissartensis also bear a groove ventrally (Norman, 1980), but this is not the case in Mantellisaurus (Norman, 1986) or Probactrosaurus (Norman, 2002). The neural spines, which are broken dorsal to the ilia, are craniocaudally broad and transversely compressed. Ossified tendons extend along the bases of the neural spines and are better preserved on the left-hand side. The total length of the sacral rod, as preserved, is 460 mm.