Statistical analysis of dental variation in the Oligocene equidMiohippus(Mammalia, Perissodactyla) of Oregon

Abstract

AbstractAs many as eight species of the “anchitherine” equidMiohippushave been identified from the John Day Formation of Oregon, but no statistical analysis of variation in these horses has yet been conducted to determine if that level of diversity is warranted. Variation of the anterior-posterior length and transverse width of upper and lower teeth of Turtle Cove MemberMiohippuswas compared to that ofM.equinanus,Mesohippus bairdii,Equus quagga, andTapirus terrestrisusingttests of their coefficients of variation (V). None of thettests are significant, indicating that the variation seen in Turtle CoveMiohippusis not significantly different from any of the populations of other perissodactyls examined in this study. Data also indicate thatMesohippusis present in the Turtle Cove Member. Additionally, hypostyle condition, used to diagnose all species ofMiohippus, was found to be related to stage of wear using an ordered logistic regression. Only two species of equid, oneMiohippusand oneMesohippus, in the Turtle Cove Member can be identified, therefore onlyMiohippus annectens, the genotype and first species described from the region, can be recognized as the soleMiohippusspecies known from the Turtle Cove assemblage. There are insufficient data to determine which species ofMesohippusis present. The dependence of hypostyle condition on crown height inMiohippusimplies that wear stage must also be considered in investigations of dental morphology in the “Anchitheriinae.”