qingyangensis was included within the genus Plesippus ( P. During the last decades, the Chinese species Equus E. simplicidens as possible common ancestor for species of the genus Equus 20. The validity of North American Plesippus and the European Allohippus was supported by a recent morphological qualitative cladistic analysis 12, whereas another cladistic study supported the hypothesis of E. The morphology of the dentition and postcranial elements was never taken into account. Notably, some studies used ten metric characters to distinguish Plio-Pleistocene species and extant Equus 2, whilst more recent studies used only a single character (size of the cranium, brain-box) to distinguish among Plesippus, Allohippus and Equus 7. The segregation into these three genera was based on cranial morphology and proportion 7. stenonis) older than 1Р’В Ma, and previously recognized species of Equus less than 1Р’В Ma as being the sole members of the genus. simplicidens), Pleistocene Eurasian and African Allohippus 14 (type species E. In fact, EquusРІР‚в„ў traditional taxonomy was upended when some investigators proposed segregating the genus into three genera 7: North American Pliocene Plesippus 13 (type species E. simplicidens as the possible ancestor of the genus Equus 1, 2, 3, 4, 5, 6, 11, 18, 22, there is no current consensus on either the taxonomic content nor phylogeny of Equus. Although most of the authors consider the North American E. In the last century, evolution of the genus Equus was actively debated by biologists and paleontologists alike proposing a multitude of hypotheses 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. Abbreviations: Ma (age in million of years) Mp (Magnetic Polarity Time Scale) РћТ‘ O 18 (Ratio of stable isotope oxygen-18 and oxygen-16). Map generated from row data of QGIS v.3.18.1 ( ) and edited by Adobe Photoshop CC2017. Color points in map indicate the main occurrences of the selected Plio-Pleistocene fossil species (purple, Dinohippus mexicanus white, Equus simplicidens yellow, Chinese fossil species Equus eisenmannae, Equus sanmeniensis and Equus huanghoensis black, Equus livenzovensis orange, Equus stenonis red, Equus koobiforensis). The Old World Equus FAD at the base of the Pleistocene is correlated with the O 18 isotopic global trend, marking a progressive environmental aridity since the beginning of the Pleistocene, within the paleoclimatic pulse recorded in terrestrial and marine strata, related to the initiation of a major glaciation pulse in the northern hemisphere 17, 18, 19, 22, 30. Dinohippus mexicanus localities are referred from recent studies 21, 34. Geographic and age distribution of major events in the Old World Equus Evolution, including the Equus First Appearance Datum (FAD) in North America, 4.1 Ma 19, the Equus FAD in the Old World, 2.58 Ma 17, 18, 19, 20, 22 and the Equus FAD in Africa 19. Our phylogenetic results uphold the most recent genomic outputs which indicate an age of 4.0РІР‚вЂњ4.5Р’В Ma for the origin and monophyly of Equus. The North American Pliocene species Equus simplicidens represents the ancestral stock of Old World Pleistocene Equus and the zebra-ass clade. We define the following evolutionary steps directly relevant to the phylogeny of extant zebras and asses: E. Our cladistic analysis, based on cranial and postcranial elements (30 taxa, 129 characters), supports the monophyly of Equus, denies the recognition of Plesippus and Allohippus and supports the derivation of Equus grevyi and members of the zebra-ass clade from European stenonine horses. In this contribution, we assess the validity of the genera Plesippus, Allohippus and Equus by cladistic analysis combined with morphological and morphometrical comparison of cranial anatomy. simplicidens and European and African Pleistocene Equus. Studies based on skull, mandible and dental morphology suggest an evolutionary relationship between North American Pliocene E. Also, the evolutionary role of European Pleistocene Equus stenonis in the origin of the zebra-ass clade has been debated. Some hypotheses segregate Equus species into three genera, Plesippus, Allohippus and Equus.

Currently, there is no consensus on either the taxonomic content nor phylogeny of Equus. Evolution of the genus Equus has been a matter of long debate with a multitude of hypotheses.

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