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 phylogenetic 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 P. montanum Wortman and P. 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. © 2002 The Linnean Society of London. Zoological Journal of the Linnean Society , 134 , 141–256.     

Cladogenesis and replacement in the fossil record of Microsyopidae (?Primates) from the southern Bighorn Basin,Wyoming

The early Eocene of the southern Bighorn Basin, Wyoming, is notable for its nearly continuous record of mammalian fossils. Microsyopinae (?Primates) is one of several lineages that shows evidence of evolutionary change associated with an interval referred to as Biohorizon A. Arctodontomys wilsoni is replaced by a larger species, Arctodontomys nuptus, during the biohorizon interval in what is likely an immigration/emigration or immigration/local extinction event. The latter is then superseded by Microsyops angustidens after the end of the Biohorizon A interval. Although this pattern has been understood for some time, denser sampling has led to the identification of a specimen intermediate in morphology between A. nuptus and M. angustidens, located stratigraphically as the latter is appearing. Because specimens of A. nuptus have been recovered approximately 60 m above the appearance of M. angustidens, it is clear that A. nuptus did not suffer pseudoextinction. Instead, evidence suggests that M. angustidens branched off from a population of A. nuptus, but the latter species persisted. This represents possible evidence of cladogenesis, which has rarely been directly documented in the fossil record. The improved understanding of both evolutionary transitions with better sampling highlights the problem of interpreting gaps in the fossil record as punctuations.