THE ARCTOMETATARSALIAN PES, AN UNUSUAL STRUCTURE OF THE METATARSUS OF CRETACEOUS THEROPODA (DINOSAURIA, SAURISCHIA)

The Cretaceous theropod families Omithomimidae, Tyrannosauridae, Troodontidae, Elmisauridae, and Avimimidae share an unusual condition of the metatarsus. The central (third) metatarsal is greatly reduced proximally, completely excluded from anterior view and nearly to completely excluded in dorsal aspect. This bone forms a wedge shape distally, triangular in transverse cross section, which is buttressed against the more columnar metatarsals II and IV. This morphology forms a tightly bound structure, here termed the arctometatarsalian condition. Morphometric analysis indicates that the arctometatarsalian structure is significantly more elongate and gracile than underived metatarsi, and this structure is associated with relatively elongate distal hind limbs per unit femoral length. When compared with limb proportions of modern and extinct mammals and flightless birds, these limb proportions are seen to be consistent with a hypothesis of enhanced cursoriality in the derived theropods. In some genera, intrametatarsal mobility in this structure may have served as an energy storage system analogous to the snap-ligaments of modern equids. The wedge-and-buttress morphology would have resulted in a more direct transmission of locomotory forces to the epipodium than in less derived theropods. Biomechanical analysis indicates that this type of relatively gracile pes was not significantly weaker than pedes of underived theropods with regards to bending stresses, due to elongation into the parasagittal plane. To the contrary, these metatarsi were well designed to withstand the forces and stresses associated with enhanced cursorial ability.