A PHYLOGENETIC BASIS FOR THE DIVERGENCE OF DIETARY ECOLOGY IN THE AMYNODONTIDAE SCOTT & OSBORN, 1883 (PERISSODACTYLA, RHINOCEROTOIDEA)

The Amynodontidae Scott & Osborn, 1883 are an extinct family of Rhinocerotoidea Owen, 1845 (Perissodactyla) known from the middle Eocene to the latest Oligocene of Asia, North America, and Europe. Based on their skull morphologies, we know that the two main tribes that divide the family, Cadurcodontini Wall, 1982 and Metamynodontini Kretzoi, 1942, had different dietary ecologies. Cadurcodontini are thought to have had a proboscis that would have had a function of locating and manipulating food. Metamynodontini are thought to have had a prehensile upper lip that would have been adapted to seize food.

We describe two specimens of Amynodontidae, a skull and a mandible of Zaisanamynodon borisovi Belyaeva, 1971 from the late Eocene of the Zaysan Basin (Kazakhstan) and a skull of Metamynodon planifrons Scott & Osborn, 1887 from the early Oligocene of the Big Badlands (United States). We incorporated these specimens into a morpho-anatomical character matrix and a cladistic analysis including 31 rhinocerotoid taxa.

Our new phylogenetic hypothesis allows us to discuss the relationships of the studied specimens within Amynodontidae and those of Amynodontidae within Rhinocerotoidea. Our cladistic analysis supports the monophyly of the genus Zaisanamynodon and clarifies the generic and specific composition of the tribes Metamynodontini and Cadurcodontini.

The phylogenetic dichotomy between these two tribes is notably expressed by the presence of several cranial features such as the “deep nasal notch” and the “well-developed preorbital fossa” in Cadurcodontini, features which are not present in Metamynodontini. These cranial characters corroborate with the previously described skull morphologies that define the dietary ecologies of these taxa and attest to an adaptation of the peri-nasal region to the presence of a proboscis in Cadurcodontini or prehensile upper lip in Metamynodontini. Our results indicate that functional dietary morphology was a contributing factor to the phylogenetic divergence of tribes within Amynodontidae. We also open a discussion on the biogeography of Amynodontidae, their emergence and dispersal in Asia and their subsequent migration to North America, and Eastern Europe. Their presence in Western Europe remains restricted to the Oligocene, after a dispersal related to the “Grande Coupure” event.