First appearance of Grévy’s zebra (Equus grevyi), from the Middle Pleistocene Kapthurin Formation, Kenya, sheds light on the evolution and paleoecology of large zebras

Abstract

Grévy’s zebra (Equus grevyi) has been reported from fossil sites spanning the past 2.3 Myr and covering a wide geographic range. However, no currently published reports dating to >200 ka can be confidently attributed to E. grevyi, with most specimens better allocated to another taxon or lacking diagnostic characteristics aligning them with E. grevyi to the exclusion of other large Equus species such as E. oldowayensis. This leaves the origin of Grévy’s zebra unresolved. Here, we describe a largely complete cranium of a large mare from the Kapthurin Formation in Kenya’s Baringo Basin that represents the first definitive appearance of E. grevyi at 547.0–392.6 ka. This cranium falls within the range of variation for recent E. grevyi and is morphologically distinct from all other fossil and extant zebras. The new Kapthurin specimen has implications for the complex evolutionary history of large zebras. Furthermore, it provides insights into the mechanisms underpinning the expanded range of Grévy’s zebra during the Pleistocene. Based on species distribution modeling and a multivariate analysis of its climate niche, we argue that the wide fossil distribution of E. grevyi may have had little to do with Pleistocene aridity as previously argued. Instead, the range contraction of Grévy’s zebra may have been driven by competition with plains zebra (E. quagga) after the northward expansion of the latter species.