Evolutionary transformations of the malleus in pinnipeds, with emphasis on Southern Hemisphere taxa

Abstract

The systematic value of the middle-ear ossicles, in particular the malleus, has been long recognized for diverse groups. We present here systematic work on middle-ear ossicle characters of pinnipeds, focusing on until now poorly studied Southern Hemisphere species. Mallei were extracted from specimens representing 16 specimens of pinnipeds (from five austral species of Phocidae and two of Otariidae, and one northern Phocidae) and two extinct species. Many characters used in this study were introduced previously, but some were here modified. Three new characters are here defined and analysed, and all the characters were mapped onto the phylogeny. Our character analysis serves to elucidate the transformations that have occurred in the evolution of middle ear ossicles in pinnipeds and to identify diagnostic features of many of its clades. Beyond the identification of specific changes within the clade, our study of pinniped ossicle evolution documents the occurrence of anatomical convergences with other groups of mammals that live in an aquatic environment, as has occurred in other organ systems.