Looking Inside (and Under) Theropod Dinosaur Locomotion

Striding bipedalism evolved over 230 million years ago in the ancestors of dinosaurs. Predatory dinosaurs (Theropoda) gave rise to tyrannosaurs and velociraptors, but also to birds, which survived the end-Cretaceous extinction. Fossilized skeletons and trackways offer unique, if static, evidence of ancient species. We seek to integrate data from living avians with the fossil record to understand theropods as living, moving organisms, as well as broader patterns of locomotor evolution along this lineage. I will first present a brief overview of X-ray Reconstruction of Moving Morphology, a 3-D method of skeletal motion analysis that we developed at Brown. I’ll then present animations, data, and questions from two studies using an XROMM approach. First, a six degree of freedom description of joint kinematics in guineafowl reveals a surprising amount of long-axis rotation at the hip and knee. Despite the limb’s superficially planar appearance, rotations about long bone axes are critical to maneuvering and steady locomotion in modern birds. When and why did this mechanism of 3-D limb control evolve? Second, we combine XROMM-based foot motion of birds walking through deformable substrates with Discrete Element Method (DEM) simulation to explore footprint formation. Modeling results from ‘virtual bedding planes’ show dramatic changes in track shape with depth, which could be easily misinterpreted if exposed as fossils. Linking DEM and XROMM techniques fosters a new perspective on the ‘birth’ of track morphology, the origin of footprint diversity, and inferences of trackmaker anatomy and behavior.