Abstract
Four groups of equids, “Anchitheriinae,” Merychippine-grade Equinae, Hipparionini, and Equini, coexisted in the middle Miocene, but only the Equini remains after 16 Myr of evolution and extinction. Each group is distinct in its occlusal enamel pattern. These patterns have been compared qualitatively but rarely quantitatively. The processes influencing the evolution of these occlusal patterns have not been thoroughly investigated with respect to phylogeny, tooth position, and climate through geologic time. We investigated Occlusal Enamel Index, a quantitative method for the analysis of the complexity of occlusal patterns. We used analyses of variance and an analysis of co-variance to test whether equid teeth increase resistive cutting area for food processing during mastication, as expressed in occlusal enamel complexity, in response to increased abrasion in their diet. Results suggest that occlusal enamel complexity was influenced by climate, phylogeny, and tooth position through time. Occlusal enamel complexity in middle Miocene to Modern horses increased as the animals experienced increased tooth abrasion and a cooling climate.
Highlights
Horses have long been used as a primary example of evolution through adaptation to a changing environment [1,2,3]
The Chi Square approximation of the Wilcoxon was not significant (p = 0.0757), in contrast to the one-way analysis of variance (ANOVA) with Occlusal Enamel Index (OEI) as the dependent variable and tooth position, which was significant at p = 0.0124
The Hipparionini ANOVA was not significant (p = 0.0687) and the Tukey test showed no significant differences among tooth positions
Summary
Horses have long been used as a primary example of evolution through adaptation to a changing environment [1,2,3]. Increased tooth height provides more resistive enamel over an animal’s lifetime These changes have been interpreted as an adaptation to feeding in open habitats as cooling and drying climates changed woodlands to grasslands, requiring horses to adapt to increased rates of tooth wear created by environmental grit and the phytoliths of grasses [2,8,12]. The shape of the occlusal pattern was shown to be an important character in equin and hipparionin phylogeny [5,24,28] This qualitative difference leads us to ask whether complexity of occlusal enamel evolved differently because of phylogenetic constraint and/or climatic pressures between Equini and Hipparionini
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