The deep time origin of African Zebras through the Eurasian “Equus stenonine” lineage.

The extant African zebras include Equus grevyi (Ethiopia and Kenya), Equus quagga (Ethiopia to southern Africa) and Equus zebra (Southern Africa), E. grevyi being the largest. In order to understand the evolution of the zebra clade during the Plio-Pleistocene, we have compared the cranial and postcranial features of E. grevyi with the Old World fossil Equus from North America Asia, Europe and Africa. We carried out an analysis of cranial and postcranial elements of the following fossil species Equus simplicidens from North America, Equus sivalensis from the Indian Sub-Continent, Equus stenonis and Equus stehlini from Italy, two yet to be named species of Equus from the paleoanthropological site of Dmanisi (Georgia), and Equus koobiforensis and Equus oldowayensis from Kenya. Herein, we provide new hypotheses for the deep-time origin of African zebras. Remarkable similarities are seen in skulls, mandibles, upper and lower dentitions of the species cited above. Statistical analysis (Log10 ratio diagrams on the third metacarpal and third metatarsal) show similar proportions between E. simplicidens, a small species of Equus from Dmanisi and E. grevyi, suggesting a genuine deep-time origin of E. grevyi from a population derived from E. simplicidens that dispersed to Eurasia at the base of the Pleistocene, and evolved from a species of Eurasian stenonine horse. This study shows that E. simplicidens is a plausible ancestor for Old World stenonine horses, and that the extant living zebras are descendants of this lineage. These data allow us to conclude that the basic bauplan of E. grevyi was present in the Old World stenonine horses, confirming the close correlation between the extant African living zebras and the Plio-Pleistocene horses.