The ontogeny of shape disparity in three species of Otariids (Pinnipedia: Mammalia)

Abstract

We compared skull ontogenies in three otariid species to identify evolutionary novelties and to understand their relationships with diversity. The species studied were Arctocephalus australis, Callorhinus ursinus and Otaria byronia. We analyzed evolutionary changes in three parameters of developmental trajectories of skull shape: shape at the outset of ontogeny, allometric pattern, and the amount of change undergone over the course of ontogeny, which depends on its duration (the length of the ontogenetic vector) and on the rate of development. Initial shapes were always very different among the species and the distances between shapes increased with time, independently from size. Furthermore, when the complete samples were considered, all the ontogenetic trajectories were significantly different concerning the directions of the allometric vectors during ontogeny. Ontogenetic trajectories also differed significantly among almost all compared pairs, except for the trajectories of males of A. australis and C. ursinus. However, these differences are expected by chance (considering the range of angles within each sample). A similar pattern was found when the subadults were compared in pairs of species, as well as adult males of A. australis and O. byronia. The correlation found between ontogenies of juveniles was expected by chance, with exception of C. ursinus and O. byronia. The ontogenetic trajectory of C. ursinus is the shortest and that of O. byronia is the longest, with the latter being near the triple of the former. A. australis has an intermediary length of ontogenetic trajectory. Considering all three species, disparity increased significantly over ontogeny since the disparity of the adults is near double that between juveniles. However, the pattern of disparity did not change considerably during ontogeny. For any ontogenetical stage, O. byronia is the species that most contributed to the disparity of the group, followed by C. ursinus. Finally, ontogenies examined herein are clearly not constrained (almost every developmental parameter of shape that could evolve was observed) and perhaps the differences in patterns have additive effects in the differentiation of the ontogenies.