Continental-scale hypsodonty patterns, climatic paleobiogeography, and dispersal of Eurasian Neogene large mammal herbivores

The dispersal of land mammals depends not only on the availability of physical connections but also on the presence of habitats that can support viable populations. Here we use mean molar hypsodonty of large mammal plant-eaters to map environmental conditions in Eurasia during the Neogene in order to establish a framework for discussing the distribution and dispersal of Neogene land mammals. The first Early Miocene centres of hypsodont faunas are seen in Iberia and central Asia. In the Middle Miocene Iberian values are low and a strong East-West contrast is seen within Europe, with another centre of hypsodonty developing in eastern Asia. From the Late Miocene onwards we show a pattern of high values in the central part and low values in the humid areas of western Europe and southern China (no suitable data are now available for southernmost Asia). This pattern has remained relatively stable since the Late Miocene, with only regional changes and a general increase in the overall level of hypsodonty. These results suggest that the hypsodonty pattern is primarily controlled by climatic effects of Himalayan-Tibetan uplift, specifically to the drying and increased seasonality of humidity predicted by climate models, rather than to the cooling that would have been most noticeable in the northern half of the continent. The strong and persistent relationship between position on the continent and relative degree of hypsodonty suggests that adaptation to local conditions by natural selection has been the main determinant of ungulate hypsodonty in the Neogene. A logical consequence of this would be that regional climatic conditions have been a major determinant of the geographic ranges of individual species throughout the Neogene, except perhaps at times of major faunal turnover. For example, an initial dispersal of hipparionine horses across the arid Central Asia appears highly improbable compared with dispersal along a more humid northern route, and the apparently early arrival of hipparions in Spain may well reflect this circumstance.