| The Oligocene-Miocene three-toed horse Archaeohippus (Mammalia; Equidae) is a brachydont browser that exhibits phyletic body size reduction and parahippine pedal adaptations. This study provides a summary of previous work regarding Archaeohippus, documents a new species of Archaeohippus, includes a life table analysis of Archaeohippus blackbergi from Thomas Farm, presents an analysis of stable isotopes from tooth enamel of three fossil horse taxa, and demonstrates phylogenetic relationships of Archaeohippus in a cladistic analysis.; Previous students of middle Cenozoic equids have been polarized in the phylogenetic placement of Archaeohippus. Some, emphasizing the importance of the primitive low-crowned teeth, have aligned this genus with the anchitheres sensu stricto such as Anchitherium . Those who focused on the shared derived condition of the manus and pes linked Archaeohippus with Parahippus. A new species from the middle Arikareean is the oldest and smallest known member of the genus. Its cheek teeth have relatively advanced characters such as crochets, additional enamel plications, and triangular hypostyles enclosing postfossettes, and its manus has reduced side digits and elongated medial phalanges; these characters link it with primitive parahippines.; The life table analysis of the population of Archaeohippus blackbergi from the early Hemingfordian Thomas Farm fossil site displays an age-dependent mortality spike in the young adult cohort that may be due to male combat, and which differs from the mortality profile of sympatric Parahippus leonensis and a population of Miohippus obliquidens , a close outgroup from the Oligocene of Wyoming. Analysis of stable isotopes of carbon from tooth enamel failed to elucidate dietary differentiation of the Thomas Farm equids. Oxygen isotopes revealed reduced ontogeny in A. blackbergi relative to P. leonensis but not M. obliquidens. A cladistic analysis of anchithere grade equids supported the monophyly of the genus Archaeohippus and placed it within the primitive parahippines.; This study provides insight into the paleobiology of Archaeohippus , the small browsing ecomorph of the first great equid adaptive radiation. This radiation produced a variety of equid forms in response to intensive global climate change that resulted in the fragmentation of forests and the appearance of extensive savanna ecosystems across North America. |
