Dinosaur monophyly

ABSTRACT

several attempts have recently been made to recognize synapomorphic characters of Dinosauria. However, most of the alleged dinosaur synapomorphies show broader distributions among Archosauria, whereas other features are diagnostic to less inclusive groups than Dinosauria. With the aim to test the hypothesis supporting dinosaur monophyly, a comparative study of thirty-seven derived skeletal features in several archosaur taxa (e.g., Pterosauria, Scleromochlus, Lagerpeton, Marasuchus, Pseudolagosuchus, Saurischia, and Ornithischia) was undertaken. The monophyly of Dinosauriformes (=Marasuchus + (Pseudolagosuchus + Dinosauria)) is supported by 7 postcranial synapomorphies (e.g., proximal femur with fossa trochanterica and trochanteric shelf; cnemial crest present on proximal tibia; quadrangular distal tibia with lateral longitudinal groove). Dinosauria is diagnosed by 17 apomorphic features, such as: deltopectoral crest distally projected; at least three sacral vertebrae; perforated acetabulum; presence of brevis shelf on ilium; astragalar ascending process inserts beneath the tibia; distal tarsal 4 proximodistally depressed.

The analysis suggests that a sustained modification of hindlimb morphology occurred in the early evolution of Ornithodira, since most of the diagnostic features of this and less inclusive groups (e.g., Dinosauromorpha, Dinosauriformes, and Dinosauria) pertain to the femur, tibia, tarsus, and pes. The transformation of the pelvic bones (and presumably in the musculature originating from the ilium, ischium, and pubis) was delayed with respect to the hindlimbs, since most important pelvic modifications occurred at the Dinosauria node, but not before. Forelimb transformation was not as profound as that which occurred in the hindlimbs.