Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla)

The systematics and taxonomy of early Eocene equids are investigated. A paraphyletic sequence of equid taxa is recovered from a pbylogenetic analysis of 40 taxa and 121 characters. This analysis supports the identification of Hyracotherium as a primitive equoid and its restriction to the genotype, Hyracotherium leporinum. Sifrhippus gen. nov. is erected for the sister taxon of all other equids, Hyracotherium sandrae Gingerich. Minippus gen. nov., the next more-derived equid clade, is erected for two small equids, M. index Cope and M. jicarillai nov. species. Arenahippus gen. nov. is erected for the next three sequentially more-derived equid taxa, A. grangeri Kitts, A. aemulor Gingerich, and A. pernix Marsh. The genus Xenicohippus, which is the next more-derived equid clade, is redefined to include X craspedotum Cope. Eohippus Marsh, the next more-derived equid taxon, is resurrected for E. angustidens Cope. Pliolophus, the only early Eocene equid from Europe, is identified as the sister taxon to Protorohippus, a sequence of successively more-derived equid taxa consisting of R montanum Wortman and R venticolum Cope. Protorohippus venticolum is identified as the sister taxon of Orohippus. Systemodon Cope is resurrected for S. tapirinum Cope. This taxon has historically been placed within Hyracotherium yet this analysis allies it with Cymbalophus near the base of the perissodactyl radiation. (C) 2002 The Linnean Society of London. Zoological Journal of the Linnean Society.