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Journal of Paleontology, 91(5), 2017, p. 1060–1068 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.42


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


Nicholas A. Famoso1,2,3


1Department of Earth Sciences, 1272 University of Oregon, Eugene, OR 97403, USA ⟨nfamoso2@uoregon.edu⟩ 2University of Oregon Museum of Natural and Cultural History, 1680 E. 15th Ave., Eugene, OR 97403, USA ⟨nfamoso2@uoregon.edu⟩ 3John Day Fossil Beds National Monument, National Park Service, 32651 Hwy 19, Kimberly, OR 97848, USA ⟨nicholas_famoso@nps.gov


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.”


Introduction


Quantifying variation in extinct species is an important first step in understanding the paleoecology of extinct organisms; the number of species present in an assemblage must be known before testing for changes in paleoecological relationships such as rates of turnover and extinction. Quantifying variation in species of terrestrial fossil vertebrates can be exceptionally difficult because large sample sizes and complete skeletal material are rarely recovered. Utilizing taxa that are phylo- genetically related, ecologically similar, and known from more complete material can improve understanding of population variation.


Historically, eight species of the equid genus Miohippus


Marsh, 1874 have been named from the Turtle Cove Member of the John Day Formation (Osborn, 1918). Recent work has suggested that a smaller genus of equid, Mesohippus Marsh, 1875, is also present in this section (Albright et al., 2008). Most recently, authors have refrained from identifying species, instead identifying genera of equids from this section (Albright et al., 2008). However, little work has been done to examine, quantify, and statistically test the level of variation seen within the equid assemblage in the John Day Basin. This emphasizes the need to determine the number of verifiable species in the Turtle Cove assemblage. If a single species of equid is present in an assemblage, then the variation in linear morphological measurements would be similar to that observed for extant


perissodactyl species. Additionally, morphological dental characters that are phylogenetically informative should be stable within species regardless of the animal’s individual age. Equid dental material in the John Day Formation is


common and easily identifiable, consisting of skulls, partial palates, complete and partial jaws, and isolated teeth. This material was reexamined from the Turtle Cove Member, com- paring the coefficients of variation (V) with a t test to determine if the variation seen in John Day Miohippus is similar to varia- tion seen in extant species of perissodactyls. The hypostyle condition, an important dental character used to distinguish species in Miohippus, was tested using an ordered logistic regression to determine if it varies with wear. The hypostyle, an enamel structure located on the distal end of the second premolar through third molar, has three morphological states, or conditions, including a ridge, an anterior projection, and an enamel lake (Prothero and Shubin, 1989). John Day Miohippus was compared to related extant taxa and well-studied extinct taxa.


Geologic setting


The John Day Formation is distributed throughout eastern and central Oregon (Fig. 1) and is currently subdivided into seven members: Big Basin, Turtle Cove, Kimberly, Haystack Valley, Balm Creek, Johnson Canyon, and Rose Creek (Retallack et al., 2000; Hunt and Stepleton, 2004; Albright et al., 2008; Fig. 2).


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