Hadrosaurs - David A. Eberth Life of the Past

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longer than it is tall and shorter than the large neural spine; its maximum length is 96 mm and it is 92 mm tall. The centrum is strongly opisthocoelous with a pronounced odontoid process fused to the anterior articular surface and buttressed by a broad ventral lip (Fig. 5.4). This lip terminates in a subrectangular parapophysis on each side of the centrum. The odontoid process is similar in form to that of Zalmoxes, Camptosaurus, Bactrosaurus, and Telmatosaurus, although it is proportionally larger than in Camptosaurus and it lacks the dorsal deflection of Bactrosaurus (Weishampel et al., 1993, 2003; Godefroit et al., 1998; Dalla Vecchia, 2006; Carpenter and Wilson, 2008). The fused odontoid process and ventral buttress are features typical of iguanodontians, and are absent in most hadrosaurids (although present in Telmatosaurus [Weishampel et al., 1993; Horner et al., 2004]). The prezygapophyses are oblong structures that sit flush with the sides of the neural spine. Small, rounded transverse processes occur caudal to the prezygapophyses; each terminates in a diapophysis. The neural arch and spine are robust, extending dorsally 156 mm above the centrum. The vertebral canal is wider than it is tall. The craniodorsal margin of the neural spine is blade like in lateral view and narrows dorsally. It extends cranially over the centrum as a pointed process but does not extend beyond the odontoid process. In lateral view the neural spine has a gentle slope (~60°) that rises caudally. Caudally, the neural arch bifurcates into separate, broad postzygapophyses with ventrally flattened articular facets. A plate from each postzygapophysis extends dorsally and meet at the caudal apex of the neural spine, creating an arch surrounded by a deep, caudally oriented fossa. The caudal articular surface of the centrum is deeply concave and slightly reniform in shape. In caudal view, the outline of the axis is roughly hourglass-shaped. The overall morphology of the axis is similar to the iguanodontians Iguanodon bernissartensis, Camptosaurus, and Bactrosaurus with some affinities to the primitive hadrosaur Telmatosaurus (Norman, 1980; Weishampel et al., 1993; Godefroit et al., 1998; Dalla Vecchia, 2006; Carpenter and Wilson, 2008).

      The postaxial cervical vertebrae are roughly equal in length with strongly opisthocoelous centra, a character typical of hadrosauroids (Horner et al., 2004). The centra are broad mediolaterally and elongate axially, with a slight constriction in the middle of the centrum. Both in cranial and caudal view, the centra appear reniform and possess a ventral keel (Fig. 5.5A–C). The neural arch is vertically short and gently slopes caudodorsally. The neural spine is weakly developed or absent on the more cranial cervicals (Fig. 5.5A–C, F). The neural arch and spine broadens caudally approaching the dorsals (although the most caudal cervicals are incomplete). The prezygapophysis is elongate laterally and extends cranially over the centrum with an ovoid articular facet (Fig. 5.5D). The diapophysis is a rounded process that is deflected ventrocaudally from the transverse process. The postzygapophyses are elongate, wedge-shaped, and sweep caudolaterally at a wide angle. The postzygapophyses possess broad, ellipsoidal articular surfaces that are flattened and extend sharply outward.

      5.5. Postaxial cervical vertebrae of UTA-AASO-2003. (A) cranial cervical vertebra in anterior view; (B) right lateral view of same; (C) posterior view of same; (D) caudal cervical vertebra in anterior view; (E) isolated cervical neural arch in dorsal view; (F) left lateral view of same. Note the cranial cervical in (A–C) is deformed, with the neural arch deflecting laterally to the right.

      Dorsal Vertebrae The dorsal vertebrae consist of at least eight relatively complete specimens – more if miscellaneous fragments are included. Due to the fragmentary and disarticulated nature of the vertebrae, the complete number of vertebrae in the dorsal series is unknown. The cervicodorsal transition is seen in the proximal most dorsals, as they are low with small centra and dorsolaterally directed transverse processes (Fig. 5.6A–C). The neural arch and spines thicken along the dorsal series, with the neural spines increasing in height and straightening dorsoventrally. In one well-preserved cranial dorsal vertebra, the centrum is slightly opisthocoelous and subrounded, broadening posteriorly in lateral view. Above the neurocentral suture, two thin ridges of bone form an arch rising dorsally to the transverse process; within this arch is the parapophysis, which is small and sits at mid-height laterally beneath the transverse process. The transverse process rises dorsolaterally from the neural arch at a sharp angle. It is rectangular in lateral view, ending in a thick, subrounded diapophysis. The vertically arched transverse process is typical of the cranial dorsal series in hadrosauroids. A partial vertebra from the middle dorsal series is composed of a complete centrum with neural arch, a partial neural spine and partial transverse processes (Fig. 5.6D–F). The centrum is amphiplatyan, rounded in cranial and caudal view, and broadens caudally. The transverse processes project laterally at a near 90° angle to the neural spines, a feature that is typical of the middle to sacral dorsal vertebrae. The exact lengths of the transverse processes are unknown as the specimen is incomplete. The neural spine is rectangular in lateral view and projects dorsally. The prezygapophyses are subrounded with lateroventrally facing ellipsoidal facets and project dorsomedially from the base of the neural arch, but do not extend far over the centra. The postzygapophyses are not preserved. The remainder of the dorsal vertebrae are represented by centra only. All specimens are amphiplatyan and relatively rounded in cranial and caudal view.

      5.6. Dorsal vertebrae of UTA-AASO-2003. (A) cranial dorsal vertebra in anterior view; (B) right lateral view of same; (C) posterior view of same; (D) caudal dorsal vertebra in anterior view; (E) right lateral view of same; (F) posterior view of same. Arrow notes slickensides on surface of bone.

      Dorsal Ribs The ribs are represented by partial, disarticulated elements from the middle to the caudal end of the series (Fig. 5.7). The ribs are similar to those of Tethyshadros and Iguanodon, with the most diagnostic characters residing in the tuberculum and capitulum (Norman, 1980; Dalla Vecchia, 2009). The tuberculum is shorter than the capitulum and the shaft of the most caudal ribs is long and ventrally, rather than caudolaterally, situated. The capitulum is rod like; however, the tuberculum is not preserved. The mid-dorsal ribs (Fig. 5.7D, E) are curved ventrolaterally and appear triangular in cross section. The capitulum and tuberculum are much larger and better defined in the mid-dorsal series. The tuberculum is a small, rounded process that projects vertically away from the surface of the rib. The capitulum is a thin, rectangular projection that extends away from the tuberculum at a 30° angle, ending in a flattened articular surface.

      5.7. Dorsal ribs of UTA-AASO-2003. (A) cranial dorsal rib; (B) cranial dorsal rib; (C) caudal dorsal rib; (D) middle dorsal rib; (E) middle dorsal rib.

      5.8. Caudal vertebrae of UTA-AASO-2003. (A) first or second caudal vertebra in anterior view; (B) left lateral view of

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