Function and evolution of the hind limb in Triassic Archosaurian reptiles

Archosauria, the clade including pterosaurs, dinosaurs and crocodilians, diversified extensively during the Triassic Period. This radiation involved the evolution of more upright hind limb posture in many archosaurs, including the crocodilian lineage, and numerous innovations in hind limb structure. The functional implications of these innovations remain poorly understood in many respects, partly because of the three-dimensional nature of limb movements in sprawling tetrapods such as the basalmost archosaurs. For this study, a three-dimensional geometric framework based on Cardan angles was developed and applied to the American alligator ( Alligator mississippiensis ), and savannah monitor lizard ( Varanus exanthematicus ). Walking movements were videotaped in each species, and the program Maya was used to rotoscopically track the hind limb bones. This analysis demonstrated that the crus of V. exanthematicus pivoted laterally as the femur retracted in a near-horizontal plane. By contrast, the more erect posture of A. mississippiensis allowed the orientation of the crus to be approximately maintained, as medial hip and knee rotation counteracted the tendency toward lateral pivoting. V. exanthematicus resembles basal archosaurs such as Proterosuchus in possessing a lateral heel-like process on the calcaneum, and a proximally expanded fifth metatarsal. In V. exanthematicus , crural pivoting brought the calcaneal process into a near-vertical position, increasing the propulsive moment arm of the peroneus longus muscle about the ankle by an estimated 26%. A similar propulsive mechanism was probably present in the basalmost archosaurs.

In suchian archosaurs (including crocodilians) and their relatives the ornithosuchids, crural pivoting was diminished owing to the relatively upright hind limb posture. The calcaneal process was directed posteriorly, and could thus contribute to propulsion in the absence of crural rotation. The suchian ankle is traditionally described as a "peg-and-socket" articulation between an astragalar prominence and a calcaneal depression, whereas ornithosuchids have a superficially opposite arrangement with the prominence on the calcaneum. These patterns are frequently designated "crocodile-normal" and "crocodile-reversed", respectively. However, detailed examination of suchian and ornithosuchid ankles demonstrated that the two are not morphological opposites, and that both types evolved by realignment of the basal archosaur calcaneum. Accordingly, the terms "crocodile-normal" and "crocodile-reversed" are misleading, and should be avoided.