Advances in the Reconstruction of Ungulate Ecomorphology with Application to Early Fossil Equids

A new and greatly simplified methodology for the assessment of the dietary adaptations of living and fossil taxa has been developed which allows for microwear scar topography to be accurately analyzed at low magnification (35×) using a standard stereomicroscope. In addition to the traditional scratch and pit numbers, we introduce four qualitative variables: scratch texture, cross scratches, large pits, and gouges, which provide finer subdivisions within the basic dietary categories. A large extant comparative ungulate microwear database (809 individuals; 50 species) is presented and interpreted to elucidate the diets of extant ungulates. We distinguish three major trophic phases in extant ungulates: traditional browsers and grazers, two phases represented by only a few species, and a browsing-grazing transitional phase where most species fall, including all mixed feeders. There are two types of mixed feeders: seasonal or regional mixed feeders and meal-by-meal mixed feeders. Some species have results that separate them from traditional members of their trophic group; i.e., browsers, grazers, and mixed feeders. Duikers are unique in spanning almost the entire dietary spectrum. Okapia, Tapirus, Tragulus, and Moschus species have wear similar to duikers. Proboscideans fall in the browsing-grazing transitional phase, as do the two suids studied. The latter differ from each other by their degree of rooting.Archaic fossil equids spanning the supposed browsing-grazing transition were compared to extant ungulates. Two major clusters are discerned: (1) Hyracotherium has microwear most similar to that of the duiker Cephalopus silvicultor and was a fruit/seed eating browser. (2) Mesohippus spp., M. bairdii, Mesohippus hypostylus, Meso-Miohippus (a transitional form larger than M. bairdii), Parahippus spp., and Merychippus insignis differ from Hyracotherium and are most similar to the extant Cervus canadensis. Group (2) is characterized by fine scratches which are the result of C3 grazing, an initial phase of grazing in equids which most likely did not occur in open habitats. Finer differentiation of group (2) diets shows a dietary change in the expected direction (toward the incorporation of more grass in the diet) and follows the expected evolutionary transition from the Eocene to the Oligocene and early Miocene. Consequently, these equid taxa are reconstructed here as mixed feeders grazing on forest C3 grasses. The finer dietary differentiation shows a progressive decrease in the number of scratches and pits. Mesohippus has the most pits and scratches, followed by Parahippus, and then Merychippus (which has the least). The taxon incorporating the most grass into its dietary regime in this array is Merychippus. In Mesohippus-Parahippus versus Merychippus, differences in tooth morphology are major but microwear differences are slight.