Mid-Cenozoic rhinocerotid dispersal via the North Atlantic.

The North Atlantic Land Bridge (NALB), which connected Europe to North America, enabled high-latitude dispersal, particularly during globally warm periods such as the Palaeocene-Eocene Thermal Maximum, a period of dramatic faunal reorganization. It has been generally accepted by palaeontologists, based on faunal comparisons between Europe and North America, that terrestrial vertebrates did not disperse via the NALB more recently than the early Eocene. Here we describe a new Early Miocene rhinocerotid species from the Canadian High Arctic with proximity to the NALB and present novel phylogenetic hypotheses for rhinocerotids. The new species, Epiaceratherium itjilik sp. nov., is differentiated from the four other members of the genus by characteristics of the P3, M1-2, mandible and lower premolars. E. itjilik also possesses an enlarged fifth metacarpal and reduced third trochanter of the femur. Global-scale biogeographic analyses reveal a high number of dispersals between Europe and North America, in both directions; cumulatively, they near the number of dispersals within Eurasia. Notably, multiple dispersals occurred in the Oligo-Miocene, suggesting that the NALB may have been crossable for mammals for at least 20 million years longer than previously considered, consistent with emerging geological and palaeoclimatological models. In combination with a NALB that was interrupted by only narrow, shallow waterways until the Miocene, we suggest that the formation of seasonal ice as early as the mid to late Eocene may have facilitated movement of terrestrial organisms between Europe and North America. We thus provide insight into the importance of the NALB as a persistent high-latitude connector of geographically disparate terrestrial faunas, underscoring the pivotal role of the Arctic in mammalian evolution.