Habitual Locomotor Behavior Inferred from Manual Pathology in Two Late Cretaceous Chasmosaurine Ceratopsid Dinosaurs, Chasmosaurus irvinensis (CMN 41357) and Chasmosaurus belli (ROM 843)

摘要

WE CORRELATE PROGRESSIVE degenerative joint disease of the first digit of the manus with inferred habitual forelimb locomotor behavior of two mature chasmosaurine ceratopsid dinosaurs from the Late Cretaceous of Alberta, Canada. A complete articulated right manus from the holotype of Chasmosaurus irvinensis (CMN 41357) includes a first digit that is markedly abnormal in shape and orientation. The external and joint surfaces of distal metacarpal I and the proximal phalanx are highly roughened and rugose, with pitting and ridges disrupting the normally smooth surfaces. Careful matching of joint surfaces yield's a reconstruction in which the metacarpophalangeal joint is medially deviated, causing the phalanges of the pollex to curve sharply laterally toward the second digit. The first metacarpo-phalangeal articulation deviates approximately 34° from the inferred normal position. Strikingly similar abnormal morphology, also involving the first digit, is exhibited by the right and left manus of Chasmosaurus belli (ROM 843), sister taxon to C. irvinensis. Gross appearance of both hands is consistent with superimposed pathological processes: chronic sub-periosteal inflammation and subsequent degenerative changes in articular bone surfaces and joint capsule of the first digit, the latter due in part to repetitive trauma. Precise etiology of the former is unknown but of less importance than the signal of life history presented by the secondary degenerative changes, which affect the integrity of the joint cartilage and which provide an unusually clear picture of abnormal joint angulation. Given the apparent prevalence of this condition in mature chasmosaurs and aspects of the etiology that are similar to a condition in humans known as hallux valgus, we propose that habitual forced adduction of the pollex led to the observed joint angulation deformity with advanced age. Digital animated reconstruction of three alternative locomotor cycles from scanned skeletal elements of CMN 41357 suggests that humeral eversion varying from 20 to 32° during the locomotor cycle, accompanied by strong humeral rotation resulting in manual proration or rolling during the terminal stance phase, best accounts for the observed changes.