An assessment of the systematic, phylogenetic and biogeographic relationships of Siwalik hipparionine horses

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Abstract
A study of the major collections of Siwalik hipparionines has resulted in the recognition of two supra-specific groups that we refer to “Cormohipparion” (Sivalhippus) and ?Hipparion s.s. The “Cormohipparion” (Sivalhippus) group is the more abundant one in the Siwaliks, and includes at least three species: “Cormohipparion” (Sivalhippus) theobaldi, “Cormohipparion” (Sivalhippus) sp. and “Cormohipparion” (Sivalhippus) perimense. The ?Hipparion group is monospecific and includes the species ?Hipparion antelopinum. In attempting to ascertain the phylogenetic relationships of these horses we have found no sound basis for falsifying the hypothesis that Old World hipparionine Groups 1–4 are monsophyletic. We derive both Groups 2 and 3 from Group 1. Group “4” is polyphyletic, with part of the original hypodigm most probably referable to Group 2. The “Cormohipparion” (Sivalhippus) group is distantly related to early Vallesian Group 1 hipparionines of probable south Asian origin, and may in turn include the ancestor of the late Miocene East African species “Hipparion” turkanense. ?Hipparion antelopinum probably originated from a west Asian species of Hipparion s.s. The relatively late first occurrence of hipparionines in the Siwalik Province is discussed, and further testing of the “Hipparion” “Datum” concept is recommended.