Scratch-digging in the Cretaceous basal ornithopod dinosaur Oryctodromeus cubicularis : evidence from morphometric analyses and reconstruction of the forelimb musculature

Abstract

The basal ornithopod dinosaur Oryctodromeus cubicularis was discovered within a burrow structure in the mid-Cretaceous Blackleaf Formation of western Montana. This, and features of the pelvis, skull, and forelimbs also seen in burrowing mammals led to the interpretation of Oryctodromeus as the first formally described burrowing dinosaur. This study further describes the forelimb specialization of Oryctodromeus and analyzes the validity of the interpretation of Oryctodromeus as adapted for digging using morphometric analyses and muscular reconstructions of the humerus, scapula, and coracoid. The morphometric analyses used two methods, including traditional morphometrics using principal component analysis of a series of element length measurements and geometric morphometrics using relative warp analysis of a series of landmarks superimposed on photographs of elements. Both methods reduce the number of variables to explain the greatest amount of variation between specimens. Results indicated that the humerus of Oryctodromeus is slightly more robust than other basal ornithopods, the coracoid exhibits no specialization for digging, and the scapula is strongly specialized, with a long narrow acromion and strongly expanded posteroventral margin of the scapular blade providing greater surface area for muscle attachment. Reconstruction of the forelimb musculature was generated by comparing both the presence or absence of muscles and the locations of attachment sites between birds and crocodilians, the extant phylogenetic bracket of dinosaur, and by examination of specimens of Oryctodromeus and other ornithopods for osteological correlates. Muscle groups used for burrowing in mammals include the deltoideus scapularis, latissimus dorsi, triceps longus, and teres major. The first three are present in Oryctodromeus, but the presence of the teres major is equivocal. The strongly expanded posteroventral scapula indicates an increased surface area for the origin of the deltoideus scapularis and possibly teres major. This expanded area would be advantageous for burrowing. The origin of the triceps longus near the glenoid of the scapula would not provide strong extension of the antebrachium, a motion important in scratch-digging. The osteology and musculature both provide evidence of slight adaptation for scratch-digging in Oryctodromeus. As many vertebrates burrow without morphological specializations, the presence of these features sufficiently supports adaptation for burrowing in Oryctodromeus.