The Eurasian Equus Datum and the early evolution of the genus in the Old World

The Equus Datum has been traditionally viewed as a single immigration event of a North American Equus species into Eurasia at the base of the Pleistocene (2.58 Ma.) This hypothesis has yet to be rigorously tested taxonomically within a geochronologically well resolved context. Toward a better understanding of the taxonomic content of the Old World Equus Datum, and evolutionary diversification following the datum, we have undertaken the current review. We have recognized a very large Equus, E. livenzovensis together with E. stenonis near the Equus Datum in the Italian record. Equus stehlini appears soon thereafter in Italy. It is not until 1.6 Ma that a new, smaller Equus, E. altidens occurs in Italy. In the Indian Subcontinent, Equus sivalensis is first seen at ~2.58 Ma, and co-occurs with a smaller species of Equus later in the Early Pleistocene. In China, the Early Pleistocene has a robust sample of a large Equus, E. eisenmannae from Longdan, Linixia Basin occurring unchanged from 2.55 to 1.8 Ma. Equus sanmeniensis is another common large Early Pleistocene species from China. Equus qingyangensis and E. teilhardi are two smaller Equus species from Qingyang, Gansu. Equus qingyangensis is particularly remarkable for its elongate slender limbs. Our current understanding is that E. sanmeniensis, E. teilhardi and E. qingyangensis overlapped in their age range in China and are correlative with the Nihowanian mammal age. Equus yunnanensis has been reported from Burma and is similar in its morphology to E. teilhardi. Remarkable is the similarity between E. qingyangensis and E. simplicidens from the 3.3 Ma. Hagerman, Idaho quarry and, in turn their dissimilarity with Italian Villafranchian E. stenonis. It has been further proposed that China E. sanmeniensis and E. stenonis are plausibly descendants of China E. eisenmannae. Equus first appears later in East Africa with the earliest occurrence being the Omo Shungura Member G, 2.2 Ma. Equus is rare in the East African record until Olduvai Bed I, ca. 1.8 Ma, where E. oldowayensis has been identified. Here, using cranial, dental and postcranial elements, we compare and contrast the morphologies between these different Early Pleistocene Old World lineages of Equus. We aim to understand the taxonomic content of the Old World Equus Datum, whether it was likely a single taxon event or multiple taxa emigrated from North America. Ultimately, we would like to discover the origin of modern Equus lineages across Eurasia and Africa