Abstract
The attainment of high crowned teeth (hypsodonty) has often been thought to be associated with grazing because many abrasive opal particles are known in grass that are presumed to wear down mammalian teeth. Equids have often been discussed regarding tooth morphological change due to the evolution of highly hypsodont teeth over time, the hyper-grazing habits of modern horses, and the originally-held view that the acquisition of hypsodonty and the widespread appearance of grasslands were synchronous. Recent studies, however, have reported asynchrony in the origin of hypsodonty and the widespread appearance of grasslands and have considered exposure to exogenous grit as important evolutionary drivers of hypsodonty in ungulates. We tracked changes in crown height (hypsodonty index), relative abrasion (mesowear) and food and grit scar topography on dental enamel (microwear) to examine the relative contributions of grass versus grit as a driving force in ungulate tooth changes during the evolution of North American Equidae compared to four North American ruminant artiodactyl families (Camelidae, Antilocapridae, Dromomerycidae, and Merycoidodontidae). We found that the overall pattern of the timing of the attainment of hypsodonty is inconsistent with grazing as the main impetus for the “Great Transition” within equids nor within the artiodactyl families as highly hypsodont ungulates post-date the spread of widespread grasslands. Mesowear closely mirrored hypsodonty trends in all families. Microwear patterns, particularly high degrees of enamel pitting (particularly large pits) and unusually coarse scratch textures in all five families, are consistent with exposure to exogenous grit as the main driver of hypsodonty acquisition prior to the consumption of significant levels of grass. Equidae exhibited a wider array of dietary behavior than the other families through most of their evolutionary history. Even so, grass was a much more common dietary item for equids than for the other families, and when combined with exogenous grit, which was more accelerated from the early Miocene onward based on more pitting and coarser scratch textures, may explain the more extreme acquisition of hypsodonty in equids compared to the artiodactyl families studied and set the stage for the Equidae alone to become hypergrazers in the Recent.
Highlights
Horse evolution has often been presented as a standard example of evolution
Extant ungulate hypsodonty indices are represented by circles (for extant browsers: open circles represent high-level browsers, whereas closed circles represent regular browsers; for extant grazers: open circles represent fresh-grass grazers, whereas closed circles represent regular grazers and closed circles with a black outline represent equid grazers; for extant mixed feeders: open circles represent closed habitat mixed feeders, whereas closed circles represent open habitat mixed feeders)
The timing of hypsodonty acquisition is not consistent with grazing as the main driver for crown height changes in the equids or the artiodactyl families studied, as highly hypsodont ungulates post-date the appearance of widespread grasslands
Summary
Horse evolution has often been presented as a standard example of evolution This proclivity is not surprising given: (1) welldocumented evolutionary changes, in the late early Miocene, (i.e., greater body size, somewhat higher-crowned cheek teeth, and more cursorially-adapted limbs) that have been perceived for a long time to echo higher-latitude environmental shifts through time (i.e., adaptations to shifts from living in tropical forests to eventual occupation of open grasslands) (Janis, 2007) and (2) the expansive fossil record of North American horses (especially in the Miocene) where the bulk of horse evolution and diversification (Equidae) occurred despite several successive dispersals to the Old World (MacFadden, 1992; Janis, 2007). Hypsodonty was in the past mostly associated with grass consumption because of the large number of silica-rich phytoliths found in grasses which were presumed to impose accelerated wear of mammalian teeth
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