Edmontosaurus Lambe 1917

CASE STUDY I: SKULL OF EDMONTOSAURUS Edmontosaurus is a relatively well-known genus of large hadrosaurid from the Upper Cretaceous of North America. One of the most striking synapomorphies of this genus is the presence of a large posterior excavation in the postorbital bone, termed the postorbital pocket, which is best developed in Edmontosaurus regalis (Campione & Evans 2011). E. regalis is a commonly occurring fossil in Alberta, known from numerous monodominant bonebeds (Evans et al. 2015). One of us (HS) recently described cranial material recovered from one such site, the Danek Bonebed in southwestern Edmonton, Canada (Bell & Campione 2014), for an undergraduate class project. Part of this description involved drawing anterior, lateral, and dorsal orthographic views of a skull reconstruction of E. regalis to show the life positions of different skull elements recovered from the Danek Bonebed (Fig. 4). These reconstructions were based on 3D scans of the mostly-disarticulated paratype specimen (CMN 2289), the only E. regalis skull for which 3D scans were available (Rybczynski et al. 2008). The scans of individual bones were assembled in Autodesk Maya 2018 to produce a 3D rendering of the nearly complete skull (Fig. 4A, B), using complete E. regalis skulls (CMN 2288, ROM 801) for guidance in articulating elements and correcting taphonomic distortion. Drawings (Fig. 4C, D) were made by tracing rendered images of this re-articulated skull in Adobe Photoshop 2022. It was quickly noticed upon illustrating the orbits in anterior view that the postorbital pockets were not only deeply concave posteriorly, but also protruded laterally (Fig. 4D). This caused the posterior margin of the orbit to be positioned much further laterally than the anterior margin, suggesting a large degree of binocular overlap in this species (Sharpe et al. work in progress). Re-examination of the 3D scans confirmed the first-order inference regarding the orientation of the orbits, and by extension the second-order inference regarding the presence of binocular overlap: the orbits of E. regalis are shifted laterally, and attempting to distort the postorbitals to minimise this feature resulted in a 3D skull that did not fit together, falsifying the alternative hypothesis that the posterior margins of the orbits were not especially laterally prominent. The simple act of illustrating a skull in anterior view permitted important new structural and functional inferences about a species described over a century ago (Lambe 1917).

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