EQUUS STEHLINI AZZAROLI, 1964 (PERISSODACTYLA, EQUIDAE). A REVISION OF THE MOST ENIGMATIC HORSE FROM THE EARLY PLEISTOCENE OF EUROPE, WITH NEW INSIGHTS ON THE EVOLUTIONARY HISTORY OF EUROPEAN MEDIUM- AND SMALL-SIZED HORSES

The present work provides a revision of the complete fossil collection of Equus stehlini from the Italian Early Pleistocene (late Villafranchian) of the Upper Valdarno Basin (Tuscany, Italy). The species was originally described in the second half of the last century, even if recent studies questioned its distinct species status, considering it a subspecies of Equus senezensis (i.e., E. senezensis stehlini). Over the succeeding decades the debate about the taxonomic status of E. stehlini extended to its possible evolutionary origin from two different Equus species from the European Early Pleistocene, Equus stenonis or E. senezensis. The revision undertaken here of E. stehlini, by means of morphological, morphometric and statistical comparisons, provides new insights into the taxonomic status and the evolutionary history of this enigmatic species. It is concluded that E. stehlini is a distinct species derived from the medium-sized E. senezensis. This work further highlights new insights and perspectives in the evolutionary framework and in the paleoecological understanding of the medium to small-sized Equus species from the early to middle Early Pleistocene of Europe

A Late Occurring “Hipparion” from the middle Villafranchian of Monopoly, Italy (early Pleistocene; MN16b; ca. 2.5 Ma)

We report here for the first time the occurrence in the Montopoli large mammal fossil assemblage of a small equid taxon identified as “Hipparion” sp., associated to the monodactyl large horse Equus cf. livenzovensis. This occurrence has been recognised on a specimen that the late De Giuli (1938-1988) identified as Hipparion sp. in unpublished notes available in the archives of the Vertebrate Palaeontology Laboratory of the Dipartimento di Scienze della Terra at the Università degli Studi di Firenze. Although fragmentary, the specimen documents the occurrence of “Hipparion” at the middle Villafranchian (early Pleistocene, ca. 2.5 Ma) site of Montopoli, one of the latest occurrences of an hipparionine horse in western Europe. The western Eurasian “Hipparion” evolutionary history is summarised herein

Testing Equid Body Mass Estimate Equations on Modern Zebras-With Implications to Understanding the Relationship of Body Size, Diet, and Habitats of Equus in the Pleistocene of Europe

The monodactyl horses of the genus Equus originated in North America during the Pliocene, and from the beginning of the Pleistocene, they have been an essential part of the large ungulate communities of Europe, North America and Africa. Understanding how body size of Equus species evolved and varied in relation to changes in environments and diet thus forms an important part of understanding the dynamics of ungulate body size variation in relation to Pleistocene paleoenvironmental changes. Here we test previously published body mass estimation equations for the family Equidae by investigating how accurately different skeletal and dental measurements estimate the mean body mass (and body mass range) reported for extant Grevy's zebra (Equus grevyi) and Burchell's zebra (Equus quagga). Based on these tests and information on how frequently skeletal elements occur in the fossil record, we construct a hierarchy of best practices for the selection of body mass estimation equations in Equus. As a case study, we explore body size variation in Pleistocene European Equus paleopopulations in relation to diet and vegetation structure in their paleoenvironments. We show a relationship between diet and body size in Equus: very large-sized species tend to have more browse-dominated diets than small and medium-sized species, and paleovegetation proxies indicate on average more open and grass-rich paleoenvironments for small-sized, grazing species of Equus. When more than one species of Equus co-occur sympatrically, the larger species tend to be less abundant and have more browse-dominated diets than the smaller species. We suggest that body size variation in Pleistocene Equus was driven by a combined effect of resource quality and availability, partitioning of habitats and resources between species, and the effect of environmental openness and group size on the body size of individuals.Peer reviewe

SAHABI EURYGNATHOHIPPUS FEIBELI: ITS SYSTEMATIC, STRATIGRAPHIC, CHRONOLOGIC AND BIOGEOGRAPHIC CONTEXTS

Sahabi, Libya is an important latest Miocene locality having yielded an extensive paleobotanical and vertebrate fauna. Amongst the fossil mammals there occurs an extensive, species diverse record of hipparionine horses. We develop here a complete record of Sahabi Eurygnathohippus feibeli now based on dental and postcranial material, in comparison to other equids from late Miocene equids from Europe, West Asia and Africa. We find that E. feibeli is the earliest recognized species of the predominantly African clade Eurygnathohippus, that its biogeographic range was Kenya, Ethiopia, Libya and Morocco, it ranged between 7.0 and 5.7 Ma and that it had deep-time evolutionary roots extending back to first occurring Old World hipparions. We further find that Eurygnathohippus was restricted to Africa until a more advanced member of the clade extended its range into the Indian Subcontinent during the late Pliocene, ca. 3.6-2.6 Ma

Evolution of Early Equus in Italy, Georgia, the Indian Subcontinent, East Africa, and the Origins of African Zebras

We report here ecological and morphological characterization of the main Old World Equus in North America, Asia, Europe, and Africa, by comparing the studied fossil forms with the living Equus grevyi zebra. Equus simplicidens from North America, Equus livenzovenzis, Equus stenonis, and Equus stehlini from Italy, Equus sivalensis from India, Equus cf. stenonis and a small Equus from Georgia (Caucasus), Equus oldowayensis, Equus koobiforensis, and Equus cf. tabeti from Kenya and the extant Equus grevyi are described in their cranial and dental features and are compared in morphological postcranial dimensions by means of log10 ratio analysis. The occurrence of the two horses at the Dmanisi Homo site in Georgia is reported here for the first time. Our comparative analyses allow to confirm the primitive lineage of the ancient zebras as derived from Equus simplicidens, and the successive evolution of the stenonine horses in Asia, South Asia, and Europe during the Plio—Pleistocene. The morphological analysis has reveals a clear trend in third metacarpals and third metatarsals of E. simpicidens, the small Equus from Dmanisi and E. grevyi, suggesting a close relationship between these species. The trend of the stenonine Equus from Europe and Asia confirms the possible derivation from the North America Equus simplicidens. The description of all the Old World Equus is integrated with an overview of their paleoecological context, with a referred section for each locality where these fossils were found. This contribution represents a comprehensive review of the present knowledge of the Old World Equus evolutionary history, with some new important data in deciphering the deep origin and evolution of ancient and living zebras

Vigna Nuova : the first Middle Villafranchian mammal assemblage from the Valdichiana Basin, Perugia (Italy)

The region of Umbria (Central Italy) represents one of the most interesting areas for the study of Mediterranean Plio-Pleistocene mammal faunas due to the occurrence of numerous paleontological deposits. Most of the fossils have been discovered within the Tiberino Basin, which extends across the centre of the territory for ca. 1800 km2, and the small Tavernelle-Pietrafitta Basin, south of Lake Trasimeno. Herein, we provide an additional report on an assemblage within continental deposits from the locality of Vigna Nuova (Piegaro, Perugia) in the Valdichiana Basin, which are observed to crop out in only a few areas of western Umbria. Taphonomic and geological evidence, coupled with rare excavation notes, have allowed the sample to be divided into two assemblages. The younger was collected from the conglomerates (here called "upper layer") which are still visible on the outcrop and is composed only of remains of cf. Leptobos. The data obtained from this fragmentary sample together with some sedimentological-stratigraphic considerations suggest that it can be correlated with other well-known Valdichiana assemblages (e.g., Selvella, Farneta), dated at ca. 1.5 Ma. The older sub-sample comes from a putative paleosol (here called "lower layer") no longer visible in the field. It includes cf. Megantereon, Canis sp., Mustelidae indet., Proboscidea indet., Equus cf. senezensis, cf. Leptobos, Croizetoceros ramosus, Pseudodama sp., Cervidae indet. (large size), and Sus cf. strozzii. This assemblage can be referred to the Coste San Giacomo Faunal Unit (late Middle Villafranchian; ca. 2.2-2.1 Ma). It represents the first wellpreserved sample of this age in Umbria and one of the few recorded in Italy, opening new interesting research perspectives on the distribution of mammals within the Peninsula and also on the stratigraphy and evolution of the Valdichiana Basin. "Aggiungeva che per scavare le poche ossa che mi avrebbe subito inviate aveva dovuto faticare enormemente e lottare per due giorni contro ogni sorta di difficoltà. Ricevuto il graditissimo invio mi affrettai a consolidare e restaurare con ogni cura quelle primizie ed oggi sono lieto di poter annunziare, in così solenne adunanza, la interessante scoperta…" ("He added that in order to excavate the few bones that he would immediately send me he had had to work enormously and struggle for two days against all sorts of difficulties. Having received the very welcome shipment, I hastened to consolidate and restore those fruits of the soil with every care and today I am pleased to be able to announce, in such a solemn meeting, the interesting discovery…"). Giovanni Capellini, 189

THE HIPPARIONINE HORSES (PERISSODACTYLA: MAMMALIA) FROM THE LATE MIOCENE OF TIZI N’TADDERHT (SOUTHERN OUARZAZATE BASIN; CENTRAL HIGH ATLAS; MOROCCO)

The fossiliferous locality of Tizi N’Tadderht, already known in the literature , has yielded a significant vertebrate fossil association as it represents the first documentation of a Late Miocene vertebrate fauna in the western area of North Africa. The group of fossil Equidae here analyzed had been preliminarly studied by previous authors, who identified the following hipparionini species: aff. Cremohipparion periafricanum, Hippotheriini gen. et sp. indet., and cf. Hippotherium primigenium. The sample retrieved from the considered area has been revised through the description of the morphologies and the dimensional measurements analysis. Then, it has been compared with the collection of fossil Equidae of the Libyan fossil site of As Sahabi, where the following species of Equidae hipparionini are represented: Sivalhippus sp., Eurygnathohippus feibeli and Cremohipparion mattewi. The revision of the Tizi N’Tadderht association led to the identification of the following species: Hippotherium sp. (characterized by large size); Eurygnathohippus cf. feibeli (a medium-sized Equidae; for the first time recognized in the Tizi N’Tadderht site); aff. Cremohipparion periafricanum (distinguished for its small size, as previously hypothesized in literature). The discovery of Eurygnathohippus cf. feibeli at Tizi N’Tadderht is of particular importance as it extends the paleogeographic record of this Hipparion species present in other African sites, showing that it is well represented in the fossil record of the Late Miocene

Target Deformation of the Equus stenonis Holotype Skull : A Virtual Reconstruction

Equus stenonis is one of the most prevalent European Pleistocene fossil horses. It is believed to be the possible ancestor of all Old World Early Pleistocene Equus, extant zebras and asses, and as such provides insights into Equus evolution and its biogeography and paleoecology. The Equus stenonis holotype skull (IGF560) was first described by Igino Cocchi in 1867, from the Early Pleistocene locality of Terranuova (Upper Valdarno basin, Italy). IGF560 is a nearly complete, although medio-laterally crushed and badly compressed skull. Here we provide the first application of a new virtual reconstruction protocol, termed Target Deformation, to the Equus stenonis holotype. The protocol extends beyond classic retrodeformation by using target specimens as a guide for the virtual reconstruction. The targets used as a reference are two fragmentary, yet well-preserved E. stenonis skulls, coming from Olivola (Italy; IGF11023) and Dmanisi (Georgia; Dm 5/154.3/4.A4.5), both Early Pleistocene in age. These two specimens do not display any major deformation, but preserve different, only slightly overlapping portions of the skull. The virtual reconstruction protocol we carried out has shown its feasibility, by producing two 3D models whose final morphology is perfectly congruent with the natural variability of a comparative sample of E. stenonis specimens. This study shows the potential of using even broken or otherwise fragmentary specimens to guide retrodeformation in badly distorted and damaged specimens. The application of Target Deformation will allow us to increase the availability of comparative specimens in studies of fossil species morphology and evolution, as well as to the study of taphonomic processes