Patterns of phylogeny and rates of evolution in fossil horses: hipparions from the Miocene and Pliocene of North America

Abstract

In North America, hipparion horses consisted of four distinct genera, Hipparion, Neohipparion, Nannippus, and Cormohipparion, known from the medial Miocene to the late Pliocene, ca. 15.5–2 ma ago. The speciation mode is determined for 10 of the 16 valid hipparion species; 50% (5) seem to have resulted from anagenesis and 50% (5) from cladogenesis. Roughly contemporaneous species of the four hipparion genera seem to proceed through similar evolutionary stages of cranial and dental characters. The mean hipparion species longevity is 3.3 ma (N = 16); the mean generic longevity is 8.4 ma (N = 4). Only two species become extinct in less than 2 ma; extinction occurs at a rate of ca. 470 μmacarthurs for species with durations between 2 and 5 ma. For selected dental characters, mean evolutionary rates (for the 16 species) were between .08 darwins (d) and .03 d with crown height evolving most rapidly. Despite the commonly held notion that fossil horses evolved rapidly, these data suggest normal or average (horotelic) evolution for hipparions, which is not surprising because they were already established in the grazing adaptive zone during their history.