A review on Bison schoetensacki and its closest relatives through the early-Middle Pleistocene transition: Insights from the Vallparadís Section (NE Iberian Peninsula) and other European localities

The evolutionary history of Bison is a matter of debate due to the scarcity of fossil remains from the earliest members of this clade and the close morphological similarities among species. To clarify the taxonomic status of the earliest stouter bison and their relationships to their putative ancestor, Leptobos, as well as other primitive forms traditionally referred to subgenus Bison (Eobison), we carry out a complete revision of the available European fossil record, with a focus on the forms occurring during the Early-Middle Pleistocene Transition. Emphasis is put on the description of the unpublished Bison remains from the Vallparadís Composite Section (VCS), including the sites of Cal Guardiola and Vallparadís Estació (Terrassa, NE Iberian Peninsula). VCS fossiliferous layers yielded one of the richest faunal assemblages from the European latest Early Pleistocene and one of the few European fossil sites covering almost entirely the Early-Middle Pleistocene Transition (1.2–0.6 Ma). The collection comprises thousands of ungulates remains, especially abundant fossils of a large Bison species. The morphology of the postcranial sample from VCS fits that of Bison (Bison) schoetensacki, i.e., the earliest stout bison (Bison s.s.) recorded in Europe. We studied more than 200 cranial and postcranial elements with a focus on the metapodial remains. Comparisons were performed with all the available fossil record of Pleistocene Eurasian fossil Bison species. We confirm the taxonomic validity of B. schoetensacki and recognize distinct eco-morphotypes of European bison between the late Early Pleistocene and the beginning of the Holocene based on the size and proportions of the metapodials. Although the appendicular skeleton shows reliable characters for the diagnosis of different species, the great morphological homogeneity recognized within the genus requires a cautious approach in systematic studies based on postcranial material.