Theoretical Aspects of Cursorial Adaptations in Dinosaurs

Abstract

A theoretical review of the physical constraints on cursorial animals provides a list of the morphological correlates of superior running ability, with emphasis on osteological features. This list includes the following adaptations: relatively long limbs; small forelimbs (bipeds only); freely rotatin scapula (quadrupeds only); hinge-like joints; short and massive proximal limb elements; long and slender distal limb elements; radius-ulna and tibia-fibula which are reduced to single elements; manus and pes with pronounced median symmetery; digitigrade to unguligrade stance; interlocked or fused metapodials; reduced or lost inner and outer digits, and snap ligaments sometimes present. These adaptations are ubiquitous among phylogenetically diverse animals which run and may be regarded as inevitable in any cursor. Theoretical arguments predict a lower speed potential for very large and very small animals, and this conclusion is supported by empirical data which point to an optimum body mass of about 50 kg for a cursor. By utilizing these findings and a system of 4 levels of running ability (graviportal, mediportal, subcursoiral, cursorial), it is possible to evaluate the running potential of dinosaurs. Quadrupedal dinosaurs had fewer cursorial features than bipeds, and large bipeds had fewer than small bipeds. Sauropods and stegosaurs were graviportal; ankylosaurs and large ceratopsians were low to intermediate grade mediportal; prosauropods were high-grade mediportal; large ornithopods were subcursorial; and large theropods were subcursorial to cursorial. Small ceratopsians were dynamic bipeds and were high-grade subcursorial. Small bipedal ornithischians and small theropods were cursorial and were the fastest of the dinosaurs, but were probably not as fast as the best modern runners.