Arguments for a mammalian and reptilian dispersal from Asia to Europe during the Paleocene-Eocene boundary interval

The Paleocene-Eocene mammalian faunal turnover included the first appearance in northern regions of Eurasia and America of artiodactyls, perissodactyls, primates and hyaenodontids. An origin of these groups in Asia seems very probable (with the exception of hyaenodontids, of possible African origin). Dispersal of these groups is highly contentious, with recent assessments suggesting a dispersal from North America to Europe. The fossil evidence concerning these groups, including recent discoveries in eastern Asia, is reviewed. It confirms the presence in Europe of primitive sister-species or sister-genera of most of these North American newcomers, and emphasizes the absence of most of these genera in eastern Asia. More convincing evidence is found for turtles, with Geoemydinae and Carettochelyinae arriving in Europe from Asiatic origins, but not found in North America. On the whole, another scenario appears more likely: equoid perissodactyls, notharctid primates, the stem-artiodactyl Diacodexis, and hyaenodontids probably dispersed from western Asia to Europe first, at the same time as geoemydines, carettochelyines and several lizards. These mammals then dispersed to North America from Europe, whereas some of their relatives dispersed from their unknown province of origin to east Asia (and Isectolophidae probably from Asia to North America). Such a direct Asia to Europe dispersal was long dismissed because the Turgai Strait was believed to have acted as a barrier. However, new geological evidence suggests that the Turgai region allowed the dispersal of land vertebrates, possibly controlled by sea level falls and environmental changes.