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Abstract

The seven species of Pseudhipparion are, from oldest to youngest, as follows: unnamed species; P. retrusum; P. curtivallum, new combination; P. hessei, new species; P. gratum; P. skinneri, new species; and P. simpsoni, new species. They range from late Barstovian through latest Hemphillian in the Great Plains and the Gulf Coastal Plain. Both the phylogenetic relationships among these species and the phylogenetic position of this genus among other hipparionine horses are discussed.

A notable feature of Pseudhipparion evolution is its prevailing tendency toward dwarfing. All crown dimensions except unworn height decrease through time, although analysis is complicated by the fact that sample means are larger in the Great Plains than in contemporaneous Gulf Coast samples, in keeping with Bergmann’s Rule. Rates of change in several dental measurements between various Pseudhipparion species pairs, calculated over intervals of about one million years, have a mean value of 0.11 darwins, which is equal to or greater than in other hipparionine species pairs. In P. simpsoni, the final late Hemphillian species, root formation was delayed ontogenetically, producing extremely high-crowned (incipiently hypsodont) cheek teeth and incisors. Potential crown heights are 85 mm in upper premolars and 110 mm in upper molars; and the enamel patterns are greatly simplified. Such extreme hypsodonty was attained during no more than 1.5 million years within the late Hemphillian at a rate of at least 0.58 darwins, roughly six times the normal rate of crown height increase in hipparionine horses.

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