Anatomy and function of the forelimb of Mussaurus patagonicus (Dinosauria, Sauropodomorpha)

ALEJANDRO OTERO; DIEGO POL

San Juan

Congreso; 4° Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Museo de Ciencias Naturales, Universidad Nacional de San Juan

A key point in the transition from basal sauropodomorphs to sauropods is the acquisition of a quadrupedal locomotion. Such dramatic postural change involved, among others, a pronated manus and a semicircular metacarpus. The former of which is achieved by the development of the craniolateral process of the ulna that accommodates the radius in a cranial position with respect to the ulna (Bonnan and Yates, 2007). Mussaurus patagonicus Bonaparte and Vince, 1979 from the Late Triassic of Santa Cruz Province, Argentina has been interpreted as a taxon closer related to derived sauropodomorphs (e.g., melanorosaurids) than to more plesiomorphic forms (e.g., Plateosaurus) (Pol and Powell, 2005). We present the forelimb anatomy of adult specimens of M. patagonicus and attempt to shed light on the functional and evolutionary scenario that involves this dramatic locomotory transition. The forelimb of M. patagonicus displays many plesiomorphic features such as the well expanded humeral ends, aligned metacarpus, and robust metacarpal I with a medial divergent pollex. On the contrary, the development of an incipient craniolateral process of the ulna is a derived feature, shared with other anchisaurian sauropodomorphs (e.g., Aardonyx, Melanorosauurus) and sauropods. Such pattern is congruent with the mosaic evolution of the forelimb, implying that the morphology of the antebrachium and the manus are not temporally linked, and were separate phylogenetic events (Bonnan and Yates, 2007). Taking into account this functional incongruence between the antebrachium and the manus in M. patagonicus and following the terminology of Gould and Vrba (1982), we propose that the morphology of the antebrachium of M. patagonicus and other advanced sauropodomorphs represents a nonaptation (a feature whose origin cannot be ascribed to the direct action of natural selection) but that was subsequently co-opted in eusauropods to the function of body support, together with other traits (e.g., ‘U’-shaped metacarpus), and therefore should be interpreted as an exaptation of Eusauropoda.