Dinosaurian precursors from the Middle Triassic of Argentina:<i>Marasuchus lilloensis</i>, gen. nov.

ABSTRACT We review the morphology and systematics of Lagosuchus from the Middle Triassic Los Chañares Formation. Two species have been named, Lagosuchus talampayensis and L. lilloensis. The holotype of Lagosuchus talampayensis is a fragmentary skeleton that does not exhibit any autapomorphies to distinguish it from contemporary dinosauromorphs, and the genus and species are regarded as nomina dubia. In contrast, the holotype of Lagosuchus lilloensis is a well-preserved skeleton that exhibits several distinctive features, allowing reference of additional specimens. We, therefore, designate a new genus, Marasuchus, for the species “Lagosuchus” lilloensis. Several synapomorphies unite Marasuchus and dinosaurs (as Dinosauriformes) to the exclusion of the contemporary dinosauromorph Lagerpeton and pterosaurs. These include an antitrochanter on the posterior wall of the acetabulum, an anterior (lesser) trochanter developed as a trochanteric shelf, and a posterior flange on the distal end of the tibia. These dinosauriform synapomorphies in the pelvis and hind-limb persist in dinosaurian descendants with body sizes exceeding that of Marasuchus by more than two orders of magnitude.