Kevin Padian (2017)
Structure and evolution of the ankle bones in pterosaurs and other ornithodirans.
Journal of Vertebrate Paleontology Article: e1364651
The ankle bone assembly of pterosaurs has received little attention, even though it is critical for understanding the functional morphology of the leg and the foot and has far-reaching implications for interpretations of stance and gait in ornithodirans in general, as well as for any role the leg may have had in the flight of pterosaurs. Of particular importance are the distal tarsal bones, which are seldom preserved clearly. Their concave proximal facets articulate with the medial and lateral condyles (comprising the astragalus and, at least basally, the calcaneum) of the tibiotarsus. Distally, they articulate with metatarsals II–IV, and the relatively large metatarsal V articulates on the distolateral side of the lateral distal tarsal. The homology of these bones in pterosaurs can be established with reference to other early-branching ornithodirans, and the morphology of the bones implies similar functional roles and ranges of motion. The medial distal tarsal is likely the fusion of distal tarsals 2 + 3, and the lateral distal tarsal is distal tarsal 4, a pattern reflected in ontogeny. The pterosaur ankle was capable of plantarflexion, but adduction and abduction of the feet were greatly limited. A synoptic survey of available tarsal bones of pterosaurs shows that the morphology of these bones remained relatively unchanged from the most basal pterosaurs to the pteranodontids and the azhdarchoids. Comparisons among a variety of ornithodirans show that the basic functional pattern did not vary importantly, although some ornithodiran subgroups evolved unique schemes of development and sequential ossification.