Hadrosaurs. David A. Eberth
the coronoid process attains its greatest rostrocaudal width ventral to its apex (Fig. 3.3).
Surangular and Angular From the coronoid process, the dorsal margin of the surangular slopes caudoventrally toward the glenoid fossa in which the ventral condyle of the quadrate sits (Fig. 3.9E). Rostroventral to the glenoid fossa, the lateral surface of the surangular is pierced by a small surangular foramen (Fig. 3.9E). Caudal to the glenoid fossa, the surangular curves caudodorsally to form the articular process. The angular is visible in lateral view and contacts the ventral margin of the surangular along a horizontal suture (Fig. 3.3).
Hyoids The left and right hyoids are preserved, but only slivers of them can be observed due to compression of the skull.
Dentition The dentary tooth crowns are mesiodistally broad and oblong, as in more basal iguanodontians such as Altirhinus (Norman, 1998), Iguanodon (Norman, 1980), and Kukufeldia (McDonald, Barrett, and Chapman, 2010). The dentary teeth bear numerous ridges on their lingual surfaces, with a distally offset primary ridge, a similarly prominent secondary ridge mesial to the primary, and multiple less prominent accessory ridges arising from the marginal denticles mesial and distal to the primary and secondary ridges (Fig. 3.10A, B). The marginal denticles are tongue shaped with mammilated edges, and steadily increase in size from the apex of the crown to its base (Fig. 3.10A). There is only one replacement tooth per alveolus, and only one active tooth per alveolus to contribute to the occlusal plane.
3.9. Right dentary and surangular of IVPP V 12534, holotype of Equijubus normani. (A) rostral ramus of right dentary in lateral view; (B) same in medial view; (C) same in dorsal view; (D) right dentary with rostral ramus attached in lateral view; (E) right surangular in lateral view. Abbreviations: ap, articular process; dia, diastema; gf, glenoid fossa; lcb, bulge at base of coronoid process on lateral surface of dentary; Mg, Meckelian groove; pdg, groove for predentary; saf, surangular foramen; sym, symphysis; vcb, bulge ventral to base of coronoid process along ventral margin of dentary; vcq, ventral condyle of quadrate. Scale bars equal 10 cm.
3.10. Dentition of IVPP V 12534, holotype of Equijubus normani. (A) dentary teeth in lingual view; (B) tracing of best-preserved dentary tooth (topmost tooth in [A]) in lingual view; (C) maxillary teeth in labial view; (D) tracing of representative maxillary tooth (fourth from left in [C]) in labial view. Abbreviations: dacr, distal accessory ridge; macr, mesial accessory ridge; md, marginal denticle; prr, primary ridge; sr, secondary ridge. Scale bars equal 1 cm.
Examples of unworn maxillary teeth are not visible in IVPP V 12534; however, the worn crowns visible in labial view are lozenge shaped (Fig. 3.10C). The maxillary teeth bear multiple ridges on their labial surfaces, with a distally offset primary ridge. There are several faint accessory ridges mesial to the primary ridge (Fig. 3.10C, D). As is the case for the dentary teeth, only one maxillary tooth per alveolus forms part of the occlusal plane. The morphology of the marginal denticles cannot be discerned.
The numbers of tooth positions in the dentaries and maxillae cannot be ascertained due to overlap of the caudal ends of these elements by the coronoid processes and jugals (Figs. 3.3, 3.4). There are 18 active dentary teeth exposed in the right dentary; the active teeth of the left dentary are largely hidden under the left active maxillary teeth. There are 18 active teeth visible in the right maxilla, and 20 in the left maxilla.
3.11. Representative cervical vertebrae of IVPP V 12534, holotype of Equijubus normani. (A–D) cervical 3 in (A) cranial view, (B) caudal view, (C) right lateral view, and (D) detail of epipophyses and postzygapophyses in oblique dorsal–right lateral view; (E) cervical 4 in right lateral view; (F) cervicals 7 to 9 in left lateral view; (G–I), cervical 10 in (G) cranial, (H) caudal and (I) right lateral view. Abbreviations: cap, capitulum of cervical rib; dia, diapophysis; epi, epipophysis; lam, lamina; para, parapophysis; pozy, postzygapophysis; przy, prezygapophysis; tp, transverse process. Scale bars equal 5 cm.
Axial Skeleton
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,