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Journal of Paleontology 91(5):1060–1068
However, specimens of Mesohippus that were included in this study are significantly smaller than what is identified as Mesohippus, suggesting that a second species is present. The Kimberly Member specimens also represent a different species from the other Miohippus specimens. The m1 TW of John Day Miohippus has a significantly
different and relatively higher V than that of Tapirus terrestris, the only other taxon in this study with V for the lower dentition (Table 2). Both the TW and APL are measured at the base of the crown, thus eliminating wear as a confounding factor. MacFadden (1989) found that the TW and APL of the M1 were not consistently related and concluded that the variation of fossil equids is similar in a majority of cases. Therefore, it is not unusual to have the TW be more variable than the APL. The data suggest that there are at least two species of
horse present in the Turtle Cove Member—a larger morph, Miohippus, and a smaller morph, Mesohippus. There is not enough material to justify a species-level identification for the Mesohippus material. The species-level identification of Miohippus is likely to be M. annectens. Provinciality and taxonomic priority can be invoked to support this identification. Additionally, the type specimen of M. annectens was included in the analysis, further justifying this taxonomic assignment. Miohippus annectens has priority in this situation because it is the genotype species of Miohippus and was the first species described from the John Day Formation. The ordered logistic regression demonstrated that the
hypostyle condition was dependent on wear stage for John Day Miohippus. The spread in the variance may be the result of including all tooth positions in the analysis; however, when M3 is removed, the result is unchanged. Further analysis with a larger sample size should consider tooth position because it has been shown to be a significant factor in occlusal morphology of ungulates (Famoso et al., 2013). This is an interesting result because the hypostyle condition has been used as a character to differentiate among species of Miohippus, and even between Miohippus and Mesohippus in the Great Plains (Prothero and Shubin, 1989). As a result, the validity of the hypostyle as a character is called into doubt. Future work to resolve the number of species in the “Anchitheriinae” will need to take wear into account because wear stage is an important factor when dealing with ungulate dentition. The identification of equid teeth is often confounded by the
wear stage of the tooth. For example, the occlusal morphology of hypsodont equid teeth is often used to diagnose tribes, genera, and species. The two major tribes of hypsodont equids, Hipparionini and Equini, are often diagnosed by whether the protocone is connected (Equini) or isolated (Hipparionini). The hipparionine genus Pseudhipparion has a protocone that is isolated in early wear and connected in late stages of wear, and in the equine genus Protohippus, the protocone is isolated in relatively early wear and
connected in later wear (MacFadden, 1998). In this case, identi- fication at the tribal level is far from trivial without considering wear stage. Attention to tooth wear is paramount when identifying teeth from these taxa because they are coeval in the Miocene of North America (MacFadden, 1998; Famoso and Pagnac, 2011). It is not surprising that wear stage is also important to the identifi- cation of dentalmaterial from lower-crowned anchitherine equids like Miohippus. Consideration of wear in these low-crowned taxa
should be adopted when considering the ubiquitous dental char- acters used to diagnose equids of Miohippus-grade. Some species of Miohippus are distinguished based on
non-dental characters, which were not present in this analysis. It may be possible to analyze the John Day Miohippus population more completely once sufficient non-dental material is acquired and appropriate methods are developed. However, these characters may prove to be less useful in light of statistical methods. Future work should focus on non-dental morphology if sufficient material is collected. Statistical methods will better inform analyses that address
the continent-wide issue of distinguishing Mesohippus from Miohippus. These two genera are difficult to distinguish (Stirton, 1940), but are considered distinct based on the presence and condition of the articular facet on the third metatarsal, which articulates with the cuboid; larger hypostyles; a longer face; and a deeper facial fossa (Prothero and Shubin, 1989; MacFadden, 1998). The paleopopulation of John Day Miohippus is not adequate in addressing this issue because there are only five occurrences of Mesohippus in the entire assemblage. Very few specimens from the Turtle Cove assemblage were identified as Mesohippus, and those that were identified as such were determined to be statistically different from the specimens of Miohippus. In the end, a larger sample size of all equid taxa from the John Day Formation would be necessary to get at more nuanced differences among these genera and species. In the geographically limited area of this study where many species have previously been reported, there is no statistically sig- nificant evidence for the presence of more than a single species of Miohippus. Applying these techniques to the Miohippus recovered from the Great Plains would likely lead to reduction in the number of species. The results from the t tests between M. bardii of the Great Plains do suggest that multiple species are present within that population, warranting further analyses. Statistical analyses of dental variation have shown great utility in assessing species-level diversity in the small, geographically constrained, paleopopulation of the Turtle Cove assemblage, therefore the application of these methods to the continent-wide issue seems promising. The dental variation of Miohippus considered herein from
the Turtle Cove Member is not statistically different from other populations of similar perissodactyls, both extinct and extant. The hypostyle condition cannot be used to differentiate species in this population because it is dependent on wear. It is clear that the confounding effects of wear on dental morphology need to be considered even in low-crowned equids like Miohippus. Two species of equid in the Turtle Cove Member of the John Day Formation can be recognized based on analysis of dental char- acters, which have been used to diagnose species of Miohippus and Mesohippus in the Great Plains. Based on the available dental material, there is no statistical or morphological variation in the sample of Turtle Cove Miohippus in excess of the varia- tion seen in a single analog species. Specimens of Mesohippus do represent a different species, as do the specimens from the Kimberly Member, which include Archaeohippus and Miohippus longiceps. As a result, only Miohippus annectens, the genotype and first species recognized in the Turtle Cove Member, can be recognized as the sole species of equid in the Turtle Cove assemblage (Marsh, 1874).
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