Forelimb motion and orientation in the ornithischian dinosaurs Styracosaurus and Thescelosaurus, and its implications for locomotion and other behavior

Abstract

The range of motion (ROM) in the forelimb has previously been studied in many saurischian dinosaur species but only a few ornithischian dinosaur species. Here, we fill in some of the blanks in current knowledge of ornithischian forelimb function by investigating the range of shoulder motion and the orientation of the humerus, radius, and ulna in the centrosaurine ceratopsid Styracosaurus albertensis and the basal orni-thopod Thescelosaurus sp. Manual manipulation of forelimb bones, using the margins of bony articular surfaces to delimit the range of motion, shows that humeral ROM and forearm orientation in S. albertensis resemble those previously found in chasmosau-rine ceratopsians. Locomotion occurred with the elbows tucked in at the sides and with the radius anterior to the ulna, without pronation. The animal was also capable of splaying its forelimbs with the elbows strongly everted, so that elbow flexion and exten-sion produced side-to-side or up-and-down movements of the torso and head. Thesce-losaurus sp. had limited humeral ROM and could not swing its humerus forward through the parasagittal plane as far as a vertical orientation. While being swung upward through the transverse plane, the humerus could not move as high as a hori-zontal position. Skeletal proportions and spinal curvature indicate that the forelimbs of Thescelosaurus could contact the ground while the animal stood. However, the animal is unlikely to have used quadrupedal locomotion, because its palms faced medially, and its fingers would have flexed through the transverse plane and therefore would not have provided forward propulsion.