Abstract The appearance of monodactyl equids in Eurasia and their dispersal towards South Europe is a significant event that marks the beginning of the Quaternary period. During the Pleistocene, horses were a common element in most European large mammal faunal assemblages, providing important palaeoecological clues, but their taxonomy, nomenclature and phylogeny, as well as their actual biochronological significance have been widely debated by scholars. The evolutionary history of horses here regarded as “stenonoid” (Equus livenzovensis, Equus stenonis, Equus ex gr. E. senezensis, Equus altidens, Equus hydruntinus, the large-sized horses Equus major and Equus suessenbornensis, sometimes referred to as the so-called “Equus bressanus-Equus suessenbornensis group”), is one of the most intriguing. This study, mainly focusing on the South European representatives of this group, aims to highlight and discuss the major debated issues, re-examining the taxonomy of some Early and early Middle Pleistocene horses. Alternative phylogenetic hypotheses will also be discussed with the aim of providing new clues on the chronology and palaeoecology of some key species/samples.

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Assuming that the Quaternary has to be recognized as a formal chronostratigraphic/geochronological unit (having a Sub-Erathem/Sub-Era rank, as recommended by the International Commission on Stratigraphy (ICS), or better a System/ Period rank, as suggested by several scientists), what boundary should be chosen? Should the lower boundary of the Quaternary coincide with the base of the Gelasian Stage (2.6 Ma) as proposed by the ICS? If so, should the Quaternary and Pleistocene lower boundaries be the same or should be different? To contribute to the debate, the Villafranchian large mammal fossil record from the NorthWestern Mediterranean region have been revised in order to correlate the diversity and structural dynamics of reconstructed faunal complexes with the changes in environmental conditions occurring from the Middle Pliocene to the Early Pleistocene. According to the results obtained, two major faunal renewals are detectable at the transition from the early to middle Villafranchian [~2.7-2.5 Ma, about at the time of the transition from the Middle (Zanclean Stage) to the Late Pliocene (Gelasian Stage)], and from the middle to late Villafranchian (~2.0-~1.9 Ma, shortly before the official Plio-Pleistocene boundary). The faunal renewal from the early (V1) to the middle (V2) Villafranchian faunal complexes (that means from the MN16a "zone"/Triversa FU to MN16b "zone"/Montopoli FU) is linked to the Middle Pliocene climate worsening, in turn related to the onset of bipolar glaciations followed by glacial-interglacial cycles of moderate amplitude (orbital periodicity of 41 ka). The resulting increase in aridity and more intense seasonality caused the disappearance of several forest-dwelling taxa, especially small carnivores and arboreal-scansorial taxa, whereas new large grazers, mixed feeders or even browsers appeared. This renewal (already called the "Equus-elephant event") can be regarded as a true turnover phase, due to the high percentage of last and new appearances, and to the important ecological structural changes in faunal complexes, involving mainly the herbivore guild. These faunal changes indicate that forests or woodlands gradually gave way to more open environments (including Artemisia steppe) alternating with warm-temperate deciduous forests. Moreover, this event can be considered as the starting point for a dispersal phase leading to a progressive standing richness increase during the following Pliocene (middle Villafranchian, V3). Around 2.0-1.8 Ma (late Villafranchian, V4), despite the extinction of some small browsing and grazing ruminants, diversity notably increased due to the progressive appearance of a number of carnivores. Indeed, the so-called "wolf-event" involved several large and small Carnivora, such as the powerful scavenger Pachycrocuta brevirostris, the jaguar-like Panthera gombazsoegensis,andcooperative foraging canids. On the other hand, minor phyletic adjustments and some new appearances (especially grazers) affected herbivore guild. This relatively long dispersal phase, and corre...

The microstructural features of the tissue of long bones subjected to different biomechanical stresses could be a helpful tool for a better understanding of locomotor behavior in extant and extinct mammals, including equids. However, few researches have attempted to describe the bone tissue of extinct horses. In our study, we analyze and compare the histomorphometric features of the bone tissue in extant modern horses, Equus caballus, and Equus namadicus, a Pleistocene Indian extinct wild horse. The number, position, and size of the osteons and Haversian canals of the bone tissue, classifiable as dense Haversian tissue, were considered for the comparison. The results obtained highlight some differences between the analyzed species, E. caballus having fewer and bigger osteons than E. namadicus. The microstructural differences may depend on the different lifestyles and environmental conditions characterizing the two species. The results obtained suggest that comparing the biomechanical properties of extinct and modern horse species may provide indirect information on their paleoenvironment.