Evolution of hypsodonty in equids: testing a hypothesis of adaptation

Abstract

The independent acquisition of high-crowned cheek teeth (hypsodonty) in several un- gulate lineages (e.g., camels, equids, rhinoceroses) in the early to middle Miocene of North America has classically been used as an indication that savanna vegetation spread during this time. Implicit in this interpretation is the untested assumption that hypsodonty was an evolutionary response to feeding in open habitats, either due to a change in food source (from browse to graze) or to in- creased incorporation of airborne grit in the diet. I examined the adaptive explanation for hypso- donty in equids using criteria pertaining to process and pattern of adaptations set up in the com- parative-methods literature. Specifically, I tested whether hypsodonty appeared coincident with or just after the spread of open, grass-dominated habitats in the Great Plains of North America. Phytolith (plant opal) analysis of 99 phytolith assemblages extracted from sediment samples from Montana/Idaho, Nebraska/Wyoming, and Colorado were used to establish the first contin- uous record of middle Eocene-late Miocene vegetation change in the northern to Central Great Plains. This record was compared with the fossil record of equids from the same area in a phy- logenetic framework. The study showed that habitats dominated by C 3 grasses were established in the Central Great Plains by early late Arikareean ($21.9 Ma), at least 4 Myr prior to the emergence of hypsodont equids (Equinae). Nevertheless, the adaptive hypothesis for hypsodonty in equids could not be rejected, because the earliest savanna-woodlands roughly co-occurred with members of the grade constituting the closest outgroups to Equinae (''Parahippus'') showing mesodont dentition. Expla- nations for the slow evolution of full hypsodonty may include weak and changing selection pres- sures and/or phylogenetic inertia. These results suggest that care should be taken when using func- tional morphology alone to reconstruct habitat change.