New insights on the tarsus of Chanaresuchus bonapartei Romer, 1971 (Archosauriformes) and its evolutionary implications.

Chanaresuchus bonapartei is a species of proterochampsid archosauriform found in the late Landinian-early Carnian aged Chañares Formation of Triassic Argentina. In spite of the fact that its anatomy is relatively well-known, the tarsus remains poorly understood. Here, we describe and compare in detail the astragalus and calcaneum of newly recovered Chanaresuchus specimens. The new material shows a combination of characters previously unknown in other basal archosauriforms including a reduced anterior hollow, contiguous crural facets, a concave anterior surface and convex posterior surface on the astragalus, an anteromedially acute corner, and a mediolaterally constricted calcaneal tuber which is posterolaterally oriented by about 45°. In addition, it possesses a convex articular surface for the fibula in the calcaneum and a subvertically oriented perforating astragalocalcaneal canal, features currently unknown in other proterochampsids, and which may be autapomorphic for the species. Furthermore, we found a functional mesotarsal tarsus, as in the closely related Tropidosuchus. The presence in the astragalus of a reduced anterior hollow located under the fibular facet, an acute anteromedial corner, added to the presence of an ascending process, a dorsally concave medial surface for the tibia, a concave anterior surface and convex posterior surface on the astragalus, an astragalus anteroposteriorly thicker medially than laterally, and a mediolaterally constricted calcaneal tuber are features shared with Ornithodira.