North American-Asian aquatic bird dispersal in the Miocene: evidence from a new species of diving duck (Anseriformes: Anatidae) from North America (Nevada) with affinities to Mongolian taxa

Prehistoric intercontinental dispersals are often used to explain the modern geographic distributions of various organisms, including birds. The extant Holarctic avifauna formed largely in the Neogene, and thus dispersals of various taxa during the Miocene likely have had a strong long-lasting effect upon the geographical pattern of the extant avian communities. However, the uneven fossil record of Neogene birds prevents accurate reconstruction of the biogeographic history of many bird clades, and the present evidence on dispersal of birds in the Neogene among continents is very limited. Past dispersals are most likely to be documented by taxa that are well represented in the fossil record, including diving ducks. Although these birds have a rather substantial fossil record in Europe and Asia, they remain very poorly known from the Neogene of North America. Here we document a new species of Miocene diving duck represented by a proximal humerus and a distal tibiotarsus from the Esmeralda Formation in Nevada (USA) and describe it as a new species of the primitive diving duck genus Protomelanitta Zelenkov (Protomelanitta bakeri sp. nov.), previously known only from the middle Miocene of Mongolia. Both species (from Mongolia and Nevada) are from the ca 11-12Ma age range during the warm (though cooling) middle Miocene after the Middle Miocene Climatic Optimum and Middle Miocene Climate Transition. Given their proposed close relationship, it appears that Protomelanitta dispersed between Asia and North America, and this instance is the first clear indication of an aquatic bird dispersal between North America and Eurasia in the middle Miocene. This palaeobiogeographical event predates the famous immigration of Hipparion horses to the Old World and the late Miocene dispersals between continental Eurasian and North American faunas in general, but likely reflects one prolonged faunal interchange related to global climatic conditions and its effects.