New Mexico Geological Society Annual Spring Meeting — Abstracts

Hyracotherium is the earliest (Eocene, Wasatchian) known taxon of Equidae and is well represented by fossils from the San Jose Formation, San Juan Basin, New Mexico. In 2001, Froehlich proposed a cladotaxonomy that subdivided Hyracotherium into nine new genera by using cladistic analysis of 121 morphological characteristics. We analyzed nine supposedly diagnostic characteristics of the lower molars as well as individual tooth sizes of two of the genera named by Froehlich, Sifrhippus (S. sandrae) and Minippus (M. jicarillai). These are two of the smallest and most primitive species of equids. Compared to M. jicarillai, S. sandrae's lower molars have an anterior fovea, which is created by the paracristid curving up the metaconid. Also, S. sandrae has comparatively weak lophids. The specimens involved in this study were the M. jicarrillai holotype (NMMNH [New Mexico Museum of Natural History] 9239), four M. jicarrillai paratypes (AMNH [American Museum of Natural History]: 4637, 16761, 48022, 48069), and a cast of the holotype of S. sandrae (UM [University of Michigan] 83567). These specimens did not show the “diagnostic” characteristics consistently; some characteristics were difficult to measure, and others were quite subjective. Also, characteristics were sometimes unobservable due to wear and weathering. The size of each tooth falls within or close to the size range previously proposed for S. sandrae. It is difficult to distinguish the anterior fovea, and the lophid seems relatively the same on all specimens with only very minor variation. Overall, the holotype and paratypes of M. jicarillai appear no different from S. sandrae. The differences are very minor and arguably due to individual variation. We thus conclude that the supposedly “diagnostic” characteristics show intraspecific variation rather than taxonomically significant variation, so S. sandrae and M. jicarillai are the same species. Past studies show that speciation of Hyracotherium is due to body size, and while tooth characteristics vary, they are rarely consistent within a single species. The other small species of Hyracotherium, H. index, was not analyzed in this study; however, this species is likely not taxonomically different from M. jicarillai or S. sandrae. Sifrhippus, Minippus, and the other genera created by Froehlich are not separate taxa, and all should be classified under the genus Hyracotherium.