Fossil horses from "Eohippus" (Hyracotherium) to Equus, 2: rates of dental evolution revisited

Rates of dental evolution are calculated for four upper first molar (M¹) characters of 26 ancestral-descendant species pairs of Cenozoic horses from North America. On average, crown height evolved significantly more rapidly (= 0.104 darwins, d) than did occlusal dimensions (length and width; = 0.045 d and 0.047 d, respectively). As might be expected, low-crowned Eocene and Oligocene horses ( Hyracotherium through Mesohippus ) exhibit relatively slow rates of dental evolution. During the major early Miocene adaptive shift from browsers to grazers ( Parahippus to Merychippus ), only crown height evolved rapidly. Advanced Miocene-Pliocene three-toed hipparions and one-toed equines are generally normal, or horotelic, in their rates of dental evolution. The most rapid rates are exhibited in Miocene browsing anchitheres and the dwarf genus Pseudhipparion. Horses do not show the very high rates of dental evolution reported elsewhere for Paleogene mammals. The traditional notion of horses being a prime example of rapid morphological evolution as seen from the fossil record is not corroborated by the data presented here.