Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla)

Characters of cranial and postcranial osteology provide important data for examining the interordinal relationships of mammals. Understanding variation in the cranial and postcranial skeleton is necessary for adequately representing ancestral character states for each mammalian order. This paper provides a comprehensive description and discussion of cranial and postcranial osteology in one of the major perissodactyl lineages, the Tapiromorpha, the lineage including the extant rhinoceroses and tapirs and their fossil relatives. Tapiromorph skeletal morphology is described, and tapiromorphs are compared to each other, to other perissodactyls, and to a number of other eutherians. Characters of potential phylogenetic significance are emphasized. A phylogenetic analysis of tapiromorphs using cranial and postcranial osteological characters provides a number of interesting results. The data do not support placing Homogalax and Chalicotherioidea within Tapiromorpha. The data support several novel hypotheses regarding the phylogeny of rhinocerotoids, including a close relationship between rhinocerotids and amynodontids, relative to hyracodontids. Finally, examples of the relevance of tapiromorph osteology to understanding perissodactyl origins are discussed, particularly new interpretations of certain primitive character states for perissodactyls.