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Comparative osteology of tapiromorph perissodactyls and its phylogenetic implications

Luke Thomas Holbrook, University of Massachusetts Amherst


Despite a rich fossil record and much attention from paleontologists, the phylogeny of mammalian order Perissodactyla (including extant horses, rhinoceroses, and tapirs) is poorly understood. There are at present no well-supported phylogenetic hypotheses of the interrelationships of perissodactyls, nor has the position of this order among other eutherian orders been established. The main hindrance to this understanding is a lack of thorough and rigorous analysis of characters and taxa. This study attempts to overcome this problem by analyzing characters from all aspects of the skeleton in a broad diversity of taxa for one major perissodactyl lineage, the Tapiromorpha. Studies of interordinal and intraordinal relationships of the Perissodactyla have previously been effectively incompatible, because these two types of studies utilized different types of characters. Interordinal studies focused on cranial and postcranial characters, whereas intraordinal studies relied heavily on tooth morphology. This study focuses on cranial and postcranial osteology. The cranial and postcranial skeleton of a broad range of tapiromorphs is described. These descriptions have two purposes: (1) to identify potentially useful characters for use in a phylogenetic analysis of tapiromorphs; and (2) to provide a reference on tapiromorph osteology that will facilitate the inference of primitive tapiromorph character states for higher-level analyses. Putative tapiromorphs include "tapiroids," rhinocerotoids, and chalicotherioids. Numerous representatives of these groups are compared with other perissodactyls (equoids and brontotheres) and eutherians thought to be closely related to perissodactyls. Cranial and postcranial characters drawn from the afore-mentioned descriptions are combined with selected dental characters in a phylogenetic analysis of tapiromorph genera. For the sake of clarity, higher-level taxonomic groups are defined a priori. The results of the phylogenetic analysis demonstrate that there is no unequivocal evidence for including chalicotherioids or the traditional primitive "tapiroid" Homogala in the Tapiromorpha (as defined here). The analysis supports the close relationship between tapirs and rhinoceroses relative to horses, as well as clades that fulfill the definitions of Tapiroidea and Rhinocerotoidea. The results of the phylogenetic analysis provide the basis for a revised classification of the Tapiromorpha.

Subject Area

Paleontology|Anatomy & physiology|Zoology

Recommended Citation

Holbrook, Luke Thomas, "Comparative osteology of tapiromorph perissodactyls and its phylogenetic implications" (1997). Doctoral Dissertations Available from Proquest. AAI9737539.