On the Presence and Shape of Anterolateral Scars in the Ontogenetic Series of Femora for Two Early Sauropodomorph Dinosaurs from the Upper Triassic of Brazil

Abstract

The anterolateral scar is a raised and generally rounded ossification on the anterolateral face of the femoral head of certain archosaurs, located anterior to the ‘greater trochanter’ and posterior to the femoral head. The presence and shape of this structure in early dinosaurs is still poorly understood. In the present study, the shape and variation of the anterolateral scar is preserved in two early sauropodomorph dinosaurs Pampadromaeus barberenai and Buriolestes schultzi from the Upper Triassic of Brazil. These sauropodomorphs are particularly significant because of their phylogenetic position as basal members of the clade and because they were coeval with the oldest of dinosaurs. Upon analysis, the specimens revealed distinct surface morphologies related to the anterolateral scar throughout their femoral series. Small specimens (putative juvenile individuals) of each species lack any sign of an anterolateral scar. On the other hand, the structure is well-developed in larger specimens (putatively adult individuals). Thus, an absence/presence of the anterolateral scar on the femora of early dinosaurs is affected by ontogeny, as observed in silesaurids. The presence of the anterolateral scar seems more widely distributed in ornithodirans than previously thought. Further, the absence of a raised anterolateral scar in massopodan sauropodomorphs appears to be a neotenic trait, since putatively immature early sauropodomorph individuals share this absence, while full adults develop a remarkable scar.