The tarsus of erythrosuchid archosaurs, and implications for early diapsid phylogeny

Abstract

Abstract The morphology of the erythrosuchid ankle joint is reassessed. Two specimens, recently thought to have been incorrectly referred toErythrosuchus africanus, are shown without doubt to belong to this taxon. Furthermore, the morphology is essentially similar to that of other early archosaurs. The tarsus ofErythrosuchusis poorly ossified and consists of a calcaneum, astragalus, and two distal tarsals. The calcanea ofErythrosuchus, Vjushkovia triplicostata, andShansisuchus shansisuchusare all similar in being dorsoventrally compressed, possessing a lateral tuber, and lacking a perforating foramen. The astragalus ofV. triplicostatais currently unknown. The astragalus ofShansisuchusis apparently unique in form. The erythrosuchid pes is therefore more derived than has been recently proposed. The tarsal morphology of several other archosauromorph taxa is reviewed and many details are found to be at variance with the literature. The plesiomorphic condition for the Archosauromorpha consists of four distal tarsals and a proximal row of three elements; two of which articulate with the tibia. These proximal elements are interpreted as the astragalus, calcaneum, and a centrale, and the same pattern is retained in the earliest archosaurs. This reassessed tarsal morphology has implications for the homology of the centrale and reconstruction of early diapsid phylogeny.