Morphometric analysis of the forelimb and pectoral girdle of the Cretaceous ornithopod dinosaur Oryctodromeus cubicularis and implications for digging

ABSTRACT

The basal ornithopod Oryctodromeus cubicularis was described as burrowing due to its discovery in a burrow structure, and the presence of several morphological features considered consistent with burrowing. Using traditional and geometric morphometric analyses, the morphology of the humerus and scapula of ornithopods, basal ornithischians, and marginocephalians was analyzed to describe and characterize quantitatively the differences between Oryctodromeus and other ornithopods. These differences were then compared with the morphological adaptations for digging in mammals, because there are no adequate analogues for burrowing in extant archosaurs. A canonical variates analysis was also conducted on the geometric morphometric data to determine if phylogeny impacted morphological trends. The humerus of Oryctodromeus is slightly more robust than other basal ornithopods, indicating an adaptation for increased force applied to the humerus. The scapula provides the most compelling morphological support for digging in Oryctodromeus. The large acromion process and prominent scapular spine of Oryctodromeus, as well as the large posteroventrally expanded scapular blade, distinguish the scapula of Oryctodromeus from those of other ornithopods and would have provided surface areas for the attachment of the supracoracoideus, deltoideus clavicularis, and deltoideus scapularis muscles, the latter of which is important in digging in mammals. These features provide evidence for specialization for producing burrows by scratch digging.

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