Systematics of the Neogene Siwalik hipparions (Mammalia, Equidae) based on cranial and dental morphology

Abstract

The systematics of the Neogene Siwalik hipparion horses is based on a review of the classic material from numerous institutions as well as a new biostrati-graphically controlled collection recently made by the Yale–Geological Survey of Pakistan expeditions in the Potwar Plateau region of northern Pakistan. A two-fold taxonomic scheme is presented here. The first, which is based on cranial morphology and associated dentions, indicates that five hipparion species are represented in the Middle Siwalik Group, i.e. Hipparion antelopinum, “Hipparion” feddeni, Cormohipparion theobaldi Cormohipparion cf. nagriensis and “Hipparion” sp. The second scheme, which is based principally on dental measurements, is similar to Hussain's (1971) conclusion that three discrete dental morphologies are represented in the Siwaliks. We prefer to treat these here as Dental Morphs 1,2, and 3 rather than as named species. Characters such as those displayed by dentitions and postcranial remains are here concluded to be less useful in discriminating valid species because of morphological similarities resulting from parallel evolution. However, both cranial morphology and isolated dentitions are biostrati-graphically useful. Most of the Siwalik deposits that yield hipparions are of Turolian and later age; none appear to be Vallesian. As is also seen elsewhere in Holarctica, there is an increase in Siwalik hipparion diversity during the late Miocene and Pliocene. The co-occurence of hipparions and Equus in the Upper Siwaliks elsewhere in the Potwar Plateau is similar to the situation seen in other Old World Pleistocene faunas. In general, Siwalik hipparions are taxonomically derived although there are some phylogenetic similarities to common Turolian species from Europe as well as China.