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