Ratite Footprints and the Stance and Gait of Mesozoic Theropods

Footprints of the rhea (Rhea ameriama) are identical in several diagnostic features to tridactyl footprints of the Mesozoic Era attributed to theropod dinosaurs. Of particular interest,'(i) the rhea's feet are placed very close to its body midline as it walks, so that it virtually places one foot in front of the other; (ii) its middle toe (digit III), the central weight-bearing axis, is directed slightly inward under normal conditions; and (iii) the feet are very deliberately placed on the substrate, and the toes and claws leave no drag marks. These are all characteristic of Mesozoic theropod (and ornithopod) trackways, and invite extended comparison of fossil and recent theropods. Modern ratites and Mesozoic theropods are essentially identical in bone morphology and in joint structure and articulations. Their trackways are similar because the structure and function of the hindlimbs of the two groups are also essentially identical. These similarities are to be regarded as homologies because birds are descended from Mesozoic theropods, and the ratites merely retain characters plesiomorphic for the group since the Late Triassic. Mesozoic theropods had fully erect stance and parasagittal posture, as both bone structure and articulation, and footprints reveal. Hypotheses of semi-erect posture based on hypothetical muscle reconstructions are not supported by the available evidence.