CLADISTIC ANALYSIS OF PRIMITIVE EQUIDS, WITH NOTES ON OTHER PERISSODACTYLS

The following hypotheses are developed by cladistic methodology, using only synapomorphic characters to assess phylogenetic relationships. The sister group of perissodactyls is formed by phenacodont condylarths, as evidenced in both groups by reduction (or absence) or metapodials and reduction (or absence) of the paraconid-paralophid complex on the lower cheek teeth. The taxon Perissodactyla is monophyletic, as defined by the presence of a saddle-shaped, or concave, navicular facet on the astragalus. Within perissodactyls, Hyracotherium and other equids form a monophyletic group, distinct from all non-equal perissodactyls, as evidenced by confluence of the foramen ovale and middle lacerate foramen in the ventral basicranium, and by the optic foramen that has migrated close to, or has become confluent with, a group of posteroventral foramina: anterior lacerate, rotundum, and anterior opening of the alisphenoid canal. The Palaeotheriidae are included in the Equidae at a lesser taxonomic rank. Within equids, Orohippus has apomorphic dental characters (greater degree of molarization of the premolars) relative to Hyracotherium. Orohippus and Haplohippus are morphologically similar. The premolars of Epihippus have a degree of molarization greater than those of Orohippus and Haplohippus. The subfamily Hyracotheriinae is a paraphyletic taxon and should be abandoned.