Dinosaurian precursors from the Middle Triassic of Argentina:<i>Lagerpeton chanarensis</i>

Abstract

ABSTRACT We describe the holotype and referred material of the unusual Middle Triassic dinosaur precursor, Lagerpeton chanarensis. The autapomorphies of Lagerpeton chanarensis include the convex and fluted puboischial flange, hook-shaped femoral head, elongate femoral fourth trochanter, fused astragalocalcaneum, tall posterior ascending process on the astragalus, and functionally didactyl pes. Among ornithodirans, Lagerpeton chanarensis is more closely related to dinosaurs than are pterosaurs. We list tarsal and pedal synapomorphies shared by Lagerpeton chanarensis and other dinosauromorphs that are absent in pterosaurs and crurotarsal archosaurs. These include an increase in the relative size of the central metatarsals and subparallel orientation of pedal digit V. There is no evidence that Lagerpeton and other small-bodied dinosauromorphs in the Los Chañares fauna, namely “Lagosuchus” lilloensis, comprise a monophyletic group. The digitigrade pes and the elongate and erect hind-limbs of basal pterosaurs and dinosauromorphs suggest that an obligatory bipedal posture had evolved in early ornithodirans. In extant vertebrates, obligatory bipedal posture is often associated with either cursorial or saltatory locomotor habits. Although these two locomotor patterns are difficult to distinguish on skeletal traits alone, the anteriorly inclined dorsal neural spines, small pelvic girdle, fused proximal tarsals, and very narrow didactyl pes in Lagerpeton chanarensis suggest that it may have been a saltator.