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

Abstract

Abstract.-As many as eight species of the "anchitherine" equid Miohippus have 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 Member Miohippus was compared to that of M. equinanus, Mesohippus bairdii, Equus quagga, and Tapirus terrestris using t tests of their coefficients of variation (V). None of the t tests are significant, indicating that the variation seen in Turtle Cove Miohippus is not significantly different from any of the populations of other perissodactyls examined in this study. Data also indicate that Mesohippus is present in the Turtle Cove Member. Additionally, hypostyle condition, used to diagnose all species of Miohippus, was found to be related to stage of wear using an ordered logistic regression. Only two species of equid, one Miohippus and one Mesohippus, in the Turtle Cove Member can be identified, therefore only Miohippus annectens, the genotype and first species described from the region, can be recognized as the sole Miohippus species known from the Turtle Cove assemblage. There are insufficient data to determine which species of Mesohippus is present. The dependence of hypostyle condition on crown height in Miohippus implies that wear stage must also be considered in investigations of dental morphology in the "Anchitheriinae."

Novel analysis of locality data can inform better inventory and monitoring practices for paleontological resources at John Day Fossil Beds National Monument, Oregon, USA

“…The most extensive of these formations, the John Day Formation (~39.7-18 Ma), historically has played a pivotal role in understanding North American Oligocene ecosystems (Condon, 1910;Cope, 1885;Fremd, 2010;Graham, 2014). Major finds have included nimravids, early canids, early equids, oreodonts, numerous rodents, and the last known primate in North America (Cope, 1885;Korth and Samuels, 2015;Samuels et al, 2015;Famoso, 2017). As the national significance of the John Day Formation and the John Day basin was recognized, the State of Oregon designated three areas of the region as state parks in the 1930s (Liggett et al, 2018).…”

Species Diversity in the Hypertragulid (Mammalia: Artiodactyla) Population of the John Day Basin, Oregon

“…A normal distribution of variation in a single-species population can be split somewhat arbitrarily, and new species can be designated based on only a small subset of the actual total population. The work done in this study reveals a lack of statistical support for these designations, but there are many other ways to designate species besides dentition and postcranial characters, most notably qualitative cranial characters (Famoso, 2017) While a preliminary study of the hypertragulids used a Shapiro-Wilks test on upper first molars to search for multiple means in the population, this step was omitted in this study given that only one mean was found (Famoso, 2018). However, modern mouse-deer, chevrotains, and muntjacs all exhibit sexual dimorphism-tragulid males are smaller, but possess larger canine tusks (Janis, 1984), while muntjak males are larger with a larger upper canine tusk than females (Janis, 1990).…”

“…After measuring teeth in all six tooth positions ( Fig. 3), I conducted an Analysis of Variance (ANOVA) and a post-hoc Tukey Test (Fisher, 1928;Tukey, 1949;Hammer & Harper, 2008;Famoso, 2018) on both the APL and TW data, to determine which specific molars differ significantly between individuals. I then calculated the coefficients of variation (V) for the entire population of hypertragulids as shown in Equation 1, where μ is the mean and σ is the standard deviation.…”

“…I then calculated the coefficients of variation (V) for the entire population of hypertragulids as shown in Equation 1, where μ is the mean and σ is the standard deviation. The V gives a percentage of variation in the entire population for a linear measurement, which is a previously established metric of identifying populations with more than one species present (Pearson, 1896;Simpson & Roe, 1939;Cope & Lacy, 1992;Famoso, 2017).…”

“…Hypertragulid specimens are preserved in the Turtle Cove, Kimberly, and Haystack Valley members of the John Day Formation (Tedford et. al., 2004;Woodburne, 2004;Famoso, 2018).…”