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Journal of Paleontology 91(5):1060–1068
Table 1. Sample size of equid teeth for each stratigraphic unit in this study. Big
Tooth Position
Upper (Indeterminate) P2 P3 P4
M1 M2 M3
John Day Formation
27 12 11 14 16 19 20
Lower (Indeterminate) 19 p1 p2 p3 p4
0 8 5 6
m1 m2 m3
Total 202
12 15 18
Member Member 1
Basin
0 0 0 0 0 0 1 0 0 0 0 0 0 0 2
Turtle Cove
21 6 5 8 9
12 12 16 1 5 1 3 8
131
12 12
Kimberly Member
26
3 2 2 2 3 3 4 0 0 1 2 0 1 1 2
1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 2
The hypostyle is present in all unbroken upper teeth, unlike
most other dental characters used to diagnose species. Further- more, hypostyle morphology has been emphasized when diag- nosing Miohippus species (Prothero and Shubin, 1989). Hypostyle condition follows the terminology of Prothero and Shubin (1989), but I have added the additional category “none,” defined here as the lack of a hypostyle in heavily worn teeth (Fig. 3). Type 1 hypostyles are thin ridges that have no cusps or spurs projecting anteriorly. Type 2 hypostyles exhibit a small spur, which projects anteriorly from the hypostyle ridge. Type 3 hypostyles are characterized by a small distinct ovoid or trian- gular pocket between a posterior ridge and an anterior spur. Wear stage was approximated by using the mesostyle crown height and APL in a similar way to Hyposodonty Index (HI) of Van Valen (1960) for upper cheek teeth. All data are presented in Supplementary Data 1. All other
characters that have been used to differentiate genera and species (e.g., the articulation between the cuboid and third metatarsal, a longer face, and a deeper facial fossa) are not present in a majority of equid specimens from the John Day Formation. The taphonomy of the John Day Formation has resulted in a faunal assemblage where the most diagnostic skeletal elements for generic and specific identification are generally not preserved. A Shapiro-WilkWtest (Shapiro and Wilk, 1965) was used
to determine if APL and TW values were normally distributed (Gaussian distribution), which is an assumption of the para- metric tests used in this study. Violations of this assumption would increase the possibility of Type II error. The coefficient of variation (V), expressed as a percentage, is a metric used to test whether there is more variance than expected for a single species in fossil communities (Simpson and Roe, 1939; Cope and Lacey, 1992, 1995). V was calculated for APL andTWof upper and lower teeth to determine whether the amount of variation present was greater than expected for a single-species popula- tion. Miller (1991) developed a method for comparing the fractional coefficients of variation (CV) of two samples using the t statistic. The upper first molar (M1) of the sample of John Day
Miohippus was compared to the published values for the modern equid Equus quagga Boddaert, 1785; the well-studied and relatively common Oligocene equid Mesohippus bairdii
Haystack Valley Member
Leidy, 1850 (MacFadden 1989) of the Great Plains; the coeval equid Miohippus equinanus (Prothero and Shubin, 1989) from Oregon; and the modern South American tapir Tapirus terrestris Linnaeus, 1758 (Colbert, 2006), which is a taxon also classified as a browser. It is important to note that M. bairdii could actually represent more than one species. The inclusion of M. bairdii in this study will help to not only to better understand the equid population in the Pacific Northwest, but will also begin to shed light on variation in the Great Plains species. Additionally, the lower first molar (m1) of the John Day Miohippus was compared to Tapirus terrestris. This study is constrained to the use of the M1 and m1 because of the availability of comparative data in the literature. The modern equid E. quagga was chosen as a comparative species because it is a decedent of John Day Miohippus, although from different subfamilies. Equus quagga allows for modern species variation of a closely related species to be directly compared to that of fossil assemblages. Mesohippus bairdii and M. equinanus are both extinct horses that would have been coeval with John Day Miohippus. Both species look fairly similar to John Day Miohippus, suggesting similar ecologies, and are also closely related. The modern South American tapir, T. terrestris, was chosen because it is a modern ecological analog to John Day Miohippus. Both are browsers and perissodactyls. They may not be as closely related as the other analog horse species, but they do share a relatively close ancestor compared to other modern browsers. Like E. quagga, T. terrestris allows for direct comparison of modern species variation to that of a fossil assemblage, although this is an ecological analog rather than direct descendent. When calculating Vs on populations with small sample
sizes (n<5), it is necessary to correct for small samples (Sokal and Braumann, 1980; MacFadden, 1989). If the calculated t statistic is smaller than the critical t statistic, I cannot reject the null hypothesis that the two Vs are the same, equating to a single-species null hypothesis. If I can reject the null hypothesis, then the observed V value for the John Day Miohippus is in excess of the values of the analog taxa and is evidence for the presence of more than one species of Miohippus in the sample. The p-value was also calculated for the t tests, and if any p-value is >0.05, I cannot reject the null hypothesis. Hypostyle morphology was investigated using an ordered
logistic regression with the ordinal package 2014.11-14 (Christensen, 2014) in R (R Core Team, 2013). The ordered logistic regression is a type of logistic regression where the order of the dependent variable matters. A logistic regression uses a nominal dependent variable and a continuous indepen- dent variable to determine if the variation in the continuous variable is responsible for the variation in the nominal variable, unlike a linear regression where the dependent variable is continuous. The ordered logistic regression is slightly different in that it uses an ordinal dependent variable and tests for differences between ordered pairs of the dependent variable. HI (a proxy for wear stage) was used as the independent vari- able, and hypostyle condition as the ordinal dependent variable. For this analysis, all tooth positions in the upper tooth row were used. Previous work has shown that occlusal enamel complex- ity, and by extension the occlusal enamel morphology, like the hypostyle morphology, is statistically similar in all molariform
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