An updated review on Equus stenonis (Mammalia, Perissodactyla): New implications for the European early Pleistocene Equus taxonomy and paleoecology, and remarks on the Old World Equus evolution

Equus stenonis is one of the most iconic fossil species of the Early Pleistocene. Although it is known from the 19th century, its taxonomic and evolutionary position has been extensively debated among equid specialists, including multiple interpretations. The current work aims to review the European E. stenonis record, by morphological, morphometrical and statistical analyses combined with body mass and net primary production estimates. Furthermore, we investigate the evolution of E. stenonis cranial anatomy in comparison with the Plio-Pleistocene species from North America, Eurasia and Africa, and extant zebras and asses. Our review of European E. stenonis samples has closely considered the E. stenonis subspecies Equus stenonis vireti, Equus stenonis guthi, Equus stenonis pueblensis, Equus stenonis olivolanus and Equus stenonis stenonis which we recognize as being ecomorphotypes of Equus stenonis, with the exception of Equus stenonis senezensis and Equus stenonis mygdoniensis. These two last subspecies warrant the species ranks of Equus senezensis and Equus altidens, respectively. This has led us to propose a new revised biochronology of European Early Pleistocene Equus. Our evolutionary interpretations based on cranial morphology reveals that the evolutionary lineage of extant zebras can be recognized in deep time from the North American species Equus simplicidens, through the European E. stenonis and the African Equus koobiforensis. European stenonine horses likewise represent the original source for the African Pleistocene fossil species and extant Equus grevyi.