The systematics of basal perissodactyls and the status of North American early Eocene equids

The relationships among basal perissodactyls and among those taxa historically included in "Hyracotherium" are complicated. It is important because it represents the organisms that evolved into all the living perissodactyls, including the evolution of the horse, a standard example of an evolutionary sequence. To better understand these relationships, cladistic techniques were used to generate a matrix of 49 taxa including the 21 distinguishable early Eocene equids and 125 characters. In this analysis there are five non-perissodactyl outgroup taxa and a series of basal tapiromorphs, paleotheres, basal titanotheres and all North American early Eocene perissodactyl taxa previously identified as equids. A maximum parsimony analysis of this matrix using PAUP 3.0s generated two most parsimonious trees (length 637, CI 0.32). This analysis supports a monophyletic perissodactyl clade composed of the Tapiromorpha (the tapirs, rhinoceroses and all taxa more closely related to them than the equids), and the Hippomorpha (the equids and all fossil taxa more closely related to them than the tapiromorphs). The tapiromorph clade consists of a number of basal "tapiroids", a paraphyletic "isectolophid" group, the ceratomorphs (tapirs and rhinoceroses and extinct relatives) and the ancylopods (chalicotheres and relatives). The hippomorph clade is composed of two distinct clades, the European paleothere clade and the equids. Hallensia is identified as the most basal paleothere, sister taxon to Hyracotherium leporinum and all more derived paleotheres. The equid clade is composed of: a stratigraphically consistent sequence of paraphyletic equid taxa, a basal clade of small-sized equids, and a clade composed of "Hyracotherium" craspedotum and Xenicohippus. The small-sized clade possesses several independently derived equid characteristics which may imply that this clade is an artifact of the diminutive size of these organisms. The majority of the pectinately arranged equid taxa in my analysis lack autapomorphies which suggests that these taxa may represent lineage segments from a single anagenetically evolving lineage. The results also suggest that Orohippus is paraphyletic but not polyphyletic as proposed by Kitts (1957). Furthermore, "Hyracotherium" is not representative of the basal morphology of the perissodactyls and no currently identified fossil provides a good candidate for that morphology. Finally, neither the name "Hyracotherium" nor "Eohippus" is appropriate for the majority of the North American equids. Valid names for early Eocene equids include Protorohippus and Xenicohippus.