Shape variability in tridactyl dinosaur footprints: the significance of size and function

The functional anatomy of the hindlimb of bipedal dinosaurs has been intensively studied. Yet, surprisingly little work has been done concerning functional adaptations in the digits for terrestrial locomotion. While complete and articulated pes skeletons are scarce, pes shape is abundantly recorded by fossil footprints. Here we aim to elucidate the significance of footprint shape and size for locomotion using a large sample (n = 303) of tridactyl dinosaur footprints from a broad range of geographic localities and time slots. Size and shape variation is characterised separately for theropods and ornithischians, the two principal trackmaker taxa. At smaller sizes, theropod footprints are best discriminated from ornithischian footprints based on their smaller interdigital angle and larger projection of digit III, while at larger sizes digital widths are effective discriminants. Ornithischian footprints are shown to increase in size from the Early Jurassic to the Late Cretaceous, a trend no...