Famoso—Dental variation in Oligocene equids
tooth positions in equids (Famoso and Davis, 2014). As such, this study assumes that hypostyle morphology is serially homologous between tooth positions at the same state of wear. Hypostyle condition type 1 should be less developed than type 2. Likewise, type 2 appears less developed than type 3, and type 3 appears less developed than the final stage designated as none. If hypostyle condition is related to wear stage, then the hypo- style condition should be ordered and each ordered pair should be significant. However, if hypostyle condition is not related to tooth wear, then there would be no significant relationship between the ordered hypostyle condition pairs. R code is provided in Supplementary Data Set 2.
Repositories and Institutional Abbreviations.—AMNH FM: American Museum of Natural History, Division of Paleonto- logy, New York, NY, USA; JODA: John Day Fossil Beds National Monument, United States National Park Service, Kimberly, OR, USA;UCMP: University of California Museum of Paleontology, Berkeley, CA, USA; UOMNH F-: University of Oregon Museum of Natural and Cultural History, Condon Fossil Collection, Eugene, OR, USA; UWBM: Burke Museum of Natural History and Culture, University of Washington, Seattle, WA, USA; YPM VP: Yale Peabody Museum of Natural History, Division of Vertebrate Paleontology, New Haven, CT, USA.
Results
None of the datasets violated the assumption of Gaussian dis- tribution. The V values and t statistics for the APL andTWof the M1 of the John Day Miohippus, M. equinanus, Mesohippus bairdii, Equus quagga, and Tapirus terrestris are presented in Table 2. Interestingly, when all of the John Day equid material was lumped, the t test was significant, suggesting more than one species was present. However, when specimens that were identified as Mesohippus, those from the Kimberly Member, and one specimen with uncertain locality and lithologic infor- mation were excluded, the t test was not significant, suggesting only one species is present among those remaining specimens. The remaining specimens are only from the Turtle Cove Member. Each group, as defined earlier, was removed one at a time. Interestingly, when M. bardii was compared to the two modern taxa, E. quagga and T. terrestris, it was significantly different in both the APL and TW, suggesting that more than one species is present (Supplementary Data Set 1). The V values and t statistics for the APL andTWof the m1 of the Turtle Cove Miohippus and T. terrestris are also presented in Table 2. The calculations of V and the t statistic are provided in Supplemen- tary Data Set 1. Only the calculated t statistics for the m1 TW between the Turtle Cove Miohippus and T. terrestris was significant. There is no significant difference between the V in the Turtle Cove Miohippus and M. bairdii, M. equinanus, E. quagga, and T. terrestris, with the exception of theTWof the m1, as noted above (Table 2). The ordered logistic regression was significant for all
hypostyle condition pairs (Table 3). The ordered logistic regression was also significant when the M3 was removed. Each hypostyle condition was significantly different from the next
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Table 2. Summary statistics for t test.V=Coefficient of Variation, V'=Coefficient of Variation (small sample size correction), n = sample size, SD = standard deviation, APL=anterior-posterior length, TW = transverse width, N/A=Not available. Mean values are in mm.
V Turtle Cove Miohippus V Mesohippus bairdii V' Miohippus equinanus V Tapirus terrestris Mean Equus quagga
V Equus quagga
M1 APL M1 TW m1 APL m1 TW 5.827 4.805 N/A N/A
Mean Turtle Cove Miohippus Mean Mesohippus bairdii Mean Miohippus equinanus Mean Tapirus terrestris SD Equus quagga
n Turtle Cove Miohippus n Mesohippus bairdii n Miohippus equinanus n Tapirus terrestris
SD Turtle Cove Miohippus SD Mesohippus bairdii SD Miohippus equinanus SD Tapirus terrestris n Equus quagga
p value: Turtle Cove Miohippus vs. Tapirus terrestris
p value: Turtle Cove Miohippus vs. Miohippus equinanus
p value: Turtle Cove Miohippus vs. Mesohippus bairdii
p value: Turtle Cove Miohippus vs. Equus quagga
6.425 4.326 6.156 12.305 9.821 8.262 N/A N/A 5.856 4.012 N/A N/A 5.520 3.891 6.553 3.931 22.140 23.310 N/A N/A 14.183 16.598 13.280 9.681 11.200 14.040 N/A N/A 11.100 13.500 N/A N/A N/A N/A N/A N/A 1.290 1.120 N/A N/A 0.911 0.718 0.818 1.191 1.100 1.160 N/A N/A 0.600 0.500 N/A N/A N/A N/A N/A N/A 42 11 27 3
42 N/A N/A 8
5
0.345 0.636 N/A N/A 0.921 0.955 N/A N/A 0.436 0.448 N/A N/A 0.276 0.359 0.563 <0.001
29 28
N/A N/A 29
29 5
24 N/A N/A 3
Table 3. Summary statistics for ordered logistic regression. Wear = Wear stage as approximated by Hypsodonty Index and represents the overall rela- tionship between wear stage and hypostyle condidtion, 1 | 2 = ordered test of hypostyle condition 1 and 2, 2 | 3 = ordered test of hypostyle condition 2 and 3, 3 | none = ordered test of hypostyle condition 3 and none, * = dependent variable. Hypostyle condition and wear stage appear as independent variables because the logistic regression tests both for a relationship with wear stage and for differences among ordered pairs.
Value 3 | none *
Wear 1 |2* 2 |3*
−16.197 −9.717 −7.192 −4.436
Std. Error 2.903
1.407 1.192 1.004
−6.907 −6.031 −4.417
t-value −5.580
<0.0001 <0.0001 <0.0001
advanced stage, indicating that hypostyle condition is dependent on wear stage.
Discussion
The V values found in the sample of the of Turtle Cove Miohippus population are not significantly different from the V of any other sample of comparative taxa investigated in this study. Because the samples of comparative taxa used in this study are considered to be from populations containing a single species, my results suggest that the length and width of teeth observed in the Turtle Cove sample is what would be expected from a single horse population, or species concept. However, M. bardii may represent multiple species, given that its t tests with modern analogs were significant. Species level diagnoses for Miohippus and Mesohippus use the APL as an important character (Prothero and Shubin, 1989), and it is clear from my results that more than one species of Turtle Cove Miohippus cannot be distinguished based upon the APL or TW.
p-value <0.0001
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