AN EVOLUTIONARY AND ECOLOGICAL OVERVIEW OF EQUIDAE FROM THE KOOBI FORA FORMATION EARLY PLEISTOCENE RECORD

The family Equidae have been prominent members of African large herbivore communities since the expansion of open habitats in the Late Miocene. The family’s diversity has been fed primarily by migration from Eurasia, with secondary speciation occurring on the African continent in two lineages: Eurygnathohippus (Hipparionini, Equinae) and Equus (Equini, Equinae). Though Eurasia’s hipparionin horses were largely replaced soon after the arrival of Equus, Africa hosted both tribes through most of the Pleistocene. In this study, we use the fossil collection from the Early Pleistocene of the Koobi Fora Formation to analyze diversity within Eurygnathohippus and Equus. We do so through employment of metric and non-metric skeletal and dental characteristics, along with ecomorphological analyses of hypsodonty, mesowear, and cursoriality. Our findings support previous claims of multiple equid species occupying the eastern African landscape contemporaneously from 2.0-1.4 Ma, representing much higher taxonomic and morphological variation than known from more recent periods. This variation is primarily expressed in body size and cursoriality, rather than in dietary differences. Our data support a close relationship of the abundant Eq. koobiforensis lineage with Eq. grevyi, the presence of Eq. tabeti outside northern Africa, and the existence of a giant species comparable in size to some of the largest Eq. capensis (possibly synonymous with Eq. oldowayensis). Furthermore, we attribute most Eurygnathohippus specimens to Eu. cornelianus and a minority to late surviving Eu. hasumense and an unnamed species half the size of the smallest modern wild equids. The niches these species occupied did not likely disappear; rather they are now most likely occupied by the family Bovidae. The decline in equid morphological and taxonomic diversity coincides with the decline in other non-ruminant ungulates in Africa, including Suidae, Proboscidea, and Hippopotamidae.