Old world hipparion evolution, biogeography, climatology and ecology

Raymond Bernor

Earth-Science Reviews 221 (2021) 103784 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Old world hipparion evolution, biogeography, climatology and ecology Raymond L. Bernor a, b, *, Ferhat Kaya c, d, Anu Kaakinen c, **, Juha Saarinen c, Mikael Fortelius c, e a College of Medicine, Department of Anatomy, Laboratory of Evolutionary Biology, Howard University, 520 W St. N.W, Washington D.C 20059, USA Human Origins Program, Department of Anthropology, Smithsonian Institution, 10th St. & Constitution Ave. NE, Washington D.C 20002, USA c Department of Geosciences and Geography, P.O. Box 64, 00014, University of Helsinki, Finland d Department of Archaeology, University of Oulu, Oulu, Finland e Finnish Museum of Natural History (LUOMUS), Helsinki, Finland b A R T I C L E I N F O A B S T R A C T Keywords: Hipparion Paleoecology Paleobiogeography Neogene Quaternary Eurasia Africa Nearly five decades ago Berggren and Van Couvering proposed an Old World “Hipparion Datum” wherein a North American Hipparion extended its range across Eurasia and Africa as an “instantaneous prochoresis” populating the Old World. Four decades ago Woodburne and Bernor examined European and North African hipparion assemblages and proposed a number of distinct hipparion lineages, sharply departing from the mono-generic paradigm of previous work. Through the 1980s until now, hipparion systematic studies have delineated multiple superspecific groups of hipparions. Herein, we define 10 recognizable genus-rank Eurasian and African taxa delineating their chronologic occurrences, geographic extent and where data exists, their body mass and paleodietary preferences. Our study supports the current interpretation that a species of North American Cormohipparion extended its range into the Old World in the early late Miocene. Regional first occurrences of Cormohipparion are recognized in the Potwar Plateau, Pakistan and Sinap Tepe, Turkey 10.8 Ma. The slightly derived lineage Hippotherium is recorded earlier in the Pannonian C of the Vienna Basin, 11.4–11.0 Ma marking the chronologic “Hipparion” Datum at the lower boundary of Mammal Neogene (MN) Unit 9. Within MN 9, 11.2–9.9 Ma, Cormohipparion underwent a minor diversification whereas Hippotherium diversified in Central and Western Europe and China and Sivalhippus (S. nagriensis) originated in the Indian Subcontinent. Whereas Cormohipparion did not survive into the late Vallesian, MN10 (9.9–8.9 Ma), Hippotherium and Sivalhippus did and the Cremohipparion and Hipparion s.s. lineages originated. During the early and middle Turolian (MN11–12, 8.9–6.8 Ma) Hippotherium, Sivalhippus, Cremohipparion and Hipparion persisted and new lineages, Eurygnathohippus, Plesiohipparion, Baryhipparion and Shanxihippus originated. An initial extinction interval occurred at the end of the Miocene, MN13 (6.8–5.3 Ma) wherein all but one endemic species of Hippotherium, H. malpassi (Italy), Hipparion and several species of Cremohipparion became extinct. Lineage and species reduction continued across the MioPliocene boundary so that by the beginning of the Pliocene (MN14, 5.3 Ma) only African species of Eurygnathohippus, Chinese Plesiohipparion houfenense and Proboscidipparion sinense remained. The later Pliocene (MN15–16, ca. 5.0–2.5 Ma) documents the persistence of endemic Chinese Baryhipparion insperatum, modest diversification of African Eurygnatohippus spp. and Chinese Plesiohipparion and Proboscidipparion spp. Eurygnathohippus made a limited geographic extension into the Indian subcontinent during MN16, whereas Pleisohipparion and Proboscidipparion extended their ranges into Eurasia during MN15 and MN16. The latest occurring hipparions are Proboscidipparion sinense at 1.0 Ma in China and Eurygnathohippus cornelianus in Africa <1.0 Ma. Old World hipparion lineages early on increased their body mass in MN9. During the Turolian interval (MN11–13) hipparion lineages diversified their body mass from very small (<100 kg) to heavy forms (>300 kg), with the smaller forms being predominately grass feeders and larger ones being mixed feeders. Decreased hipparion lineage and species diversity in the Pliocene was accompanied by increased average body size and hypsodonty probably in response to more seasonal Eurasian and African environments. There is no evidence that hipparions ever adapted to cold and dry Old World Pleistocene environments. * Corresponding author at: College of Medicine, Howard University, USA. ** Corresponding author at: University of Helsinki, Finland E-mail addresses: rbernor@howard.edu (R.L. Bernor), anu.kaakinen@helsinki.fi (A. Kaakinen). https://doi.org/10.1016/j.earscirev.2021.103784 Received 12 May 2021; Received in revised form 1 August 2021; Accepted 23 August 2021 Available online 28 August 2021 0012-8252/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 (1977) succeeded in illustrating the consistency in DPOF morphology in a large sample of skulls from the late Clarendonian Hans Johnson Quarry, Ash Hollow Formation, to support their newly nominated hipparion genus (1977: pg. 915, Fig. 2). They argued that Cormohipparion is distinct from Hipparion s.s in having a well developed dorsal preorbital fossa (DPOF) anterior rim. MacFadden (1980) further described the facial morphology of Hipparion s.s from Mt. Luberon, France (also known as Mt. Lebéron and Cucuron) and recognized this genus at several localities in the Great Plains and Pacific coastal regions. MacFadden (1980) claimed that North American Hipparion s.s. was not rare but common in North America and that there was a generic continuity of this hipparion throughout Holarctica. Forstén (1982) vigorously refuted Skinner and MacFadden’s (1977) assignment of Cormohipparion replying that the species contents of Cormohipparion as defined by them included taxa with dentally heterogeneous morphology and facial morphology that varies widely in species with dentally homogeneous morphology. A debate between North American and European hipparion specialists erupted for some decades about the validity of a taxonomy that depended on facial morphology alone. This led to researchers seeking additional morphological characters and analytical techniques to investigate Equini evolution. MacFadden (1984) produced an important and enduring monograph on the systematics of North American Hipparion, Neohipparion, Nannippus and Cormohipparion. He recognized three species of North American Hipparion, H. shirleyi, H. tehoense, H. forcei and three Cormohipparion species, C. goorisi, C. sphenodus and C. occidentale. MacFadden (1984) reported peak North American hipparion diversity in North America 12–8.5 Ma, which was earlier than seen in the Old World (herein, 8–6.5 Ma). MacFadden (1984: 177) has defined North American Hipparion s.s characters (pg. 177): “by the shared-derived and diagnostic configuration of the DPOF which is poorly defined anteriorly, is shallower than Cormohipparion, and is moderately well defined with a continuous rim posteriorly”. Bernor et al. (1980, 1990b, 1996b, 2016) and Bernor (1985) have argued that North American “Hipparion s.s.” is a clade convergent on the Hipparion gettyi-Hipparion prostylum-Hipparion campbelli + Hipparion dietrichi + Hipparion hippidiodus series recognized in Eurasia supporting this argument using characters of the skull, mandible, dentition and postcranial anatomy when and where present. Reduction of the preorbital fossa (POF) occurs, and in particular the loss of the POF anterior rim, in multiple Old World hipparion species. Hulbert and MacFadden (1991) provided a comprehensive review of North American equid evolution with a focus on Merychippus taxonomy and the systematic position of hipparion horses. Their historical coverage of North American equid paleontology literature is unparalleled to date. The authors produced a cladistic analysis of 12 merychippine-grade species based on 39 cranial, dental and postcranial characters. Hulbert and MacFadden’s (1991): pg. 46, Fig. 13) phylogeny led them to conclude that North American hipparions include Merychippus insignis, Neohipparion, Pseudhipparion, Hipparion, Nannippus and Cormohipparion. Woodburne undertook a longitudinal study on the phylogeny of Cormohipparion and the taxonomic content of the Old World Hipparion Datum. Woodburne (1996) revised Cormohipparion sphenodus recognizing the new taxon C. quinni and revised the geochronology of C. goorisi (early Barstovian), C. quinni (late Barstovian) and the C. occidentale (Clarendonian) group with an address to the Old World Hipparion Datum. Woodburne (2007) followed with a comprehensive revision of the Cormohipparion occidentale Complex, recognizing 8 species: C. occidentale, C. matthewi, C. johnsoni, C. merriami, C. fricki, C. skinneri, C. sp. and C. quinni. Woodburne further recognized Cormohipparion goorisi as a valid taxon and proposed a phylogeny of Cormohipparion utilizing 3 outgroups, Parahippus leonensis, “Merychippus” primus, Merychippus insignis. Woodburne (2007 and 2009) identified Cormohipparion sp. from the California Punchbowl Fm. as a putative ancestral source for the Old World Hippotherium FAD. The North American Cormohipparion species-group showed increased hypsodonty 1. Introduction Our study concerns the evolution, biogeography, climatology and ecology of Eurasian and African hipparion horses between 11.4 and 1.0 Ma. The Tribe Hipparionini includes the North American genera Cormohipparion, Nannippus, Neohipparion and Pseudhipparion and we remain cautious about accepting the occurrence of Hipparion sensu stricto in North America (sensu MacFadden, 1980, 1984, 1994). We recognize 10 generic-level Eurasian and African hipparionine taxa including: Cormohipparion, Hippotherium, Cremohipparion, Hipparion s.s., Sivalhippus, Plesiohipparion, Proboscidipparion, Eurygnathohippus, Shanxihippus and Baryhipparion. Whereas MacFadden (1980, 1984) and Woodburne et al. (1981) and Woodburne (1989) have proposed two migratory extensions of hipparions from North America into Eurasia, Cormohipparion (ca. 12 Ma) and Hipparion (ca. 10 Ma), we do not recognize more than a single immigration event, the Cormohipparion Datum, ca. 11.4–11.0 Ma (being generic-appropriate and replacing the “Hipparion Datum”; sensu Bernor et al., 2017, following Berggren and Van Couvering, 1974). Studies on the evolution of the Equidae have advanced from the middle part of the 19th century. Marsh (1879) produced an early orthogenetic scheme depicting the evolution of equid limbs and teeth through the Cenozoic whereby the cheek teeth became progressively higher crowned, limbs more elongate and feet underwent digit reduction. Gidley (1907) was amongst the first equid researchers to realize that equid phylogeny formed a more complicated branching pattern rather than an orthogenetic sequence and separated hipparions from Protohippus and Pliohippus. Simpson (1951) has credited Osborn for directing the assembly of the best series of fossil horses anywhere in the world at the American Museum of Natural History, New York. This collection enabled Osborn (1918) to prepare an unparalleled monograph on Oligocene, Miocene and Pliocene Equidae of North America. Osborn (1918) chose not to produce a phylogeny in this work, but one soon followed by his colleague W.D. Matthew (1926). Matthew’s, 1926 study served as a foundation for the subsequent study of the Equidae wherein he viewed the family’s evolution as a series of 10 stages or morphological grades, ascending from Eohippus to Equus. His horizontal classification reflected the stratigraphic succession in North America, so it was logical that horse evolution also proceeded step-by-step. Stirton (1940) produced a phylogeny of North American Equidae that has arguably been the most widely accepted one until relatively recently. Like Matthew, Stirton (1940) believed that Merychippus gave rise to hipparions. Stirton recognized three genera of North American hipparion: Neohipparion, Nannippus and Hipparion. Stirton’s fossil horse systematics was principally based on dental, and to a lesser degree postcranial characters because of their abundance in North American assemblages. Simpson (1951: 114, Fig. 13) provided a more vertical classification of Equidae evolution with address to geographic extensions of paleotheres, Anchitherium, Hypohippus, Hipparion and Equus from North America into the Old World. Simpson (1951) also attempted to address the co-evolution of flora and horses. MacFadden (2005) provided a brief update of Simpson’s, 1951 figure likewise providing an orthogonal view of dietary evolution transitioning from browsing, to mixed feeding and then finally grazing through time. Quinn (1955) made a radical departure from the traditional horizontal classification scheme in his description of Miocene horses from the Texas Gulf Coastal Plain. Quinn’s (1955) phylogeny is rooted in the early Miocene and recognized 17 branches including the hipparion genera Neohipparion, Hipparion and Nannipus. At this time, his work was believed to be too extreme by most equid researchers. Skinner and MacFadden (1977) separated paraphyletic merychippines and hipparions into monophyletic groups and erected a new hipparion genus Cormohipparion based mostly on the morphology of the dorsal preorbital fossa (DPOF). They characterized the genus utilizing morphotypes from three different stratigraphic horizons: Early Barstovian Fleming Formation, Late Valentinian Valentine Formation and Late Clarendonian Ash Hollow Formation. Skinner and MacFadden 2 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 associated with more open country environments with increased grassy components converging on Old World hipparion evolution. Bernor et al. (2017) confirmed Woodburne’s (2007, 2009) assertion that California Cormohipparion sp. had a dental morphology consistent with the oldest Old World hipparions from 11.4–11.0 Ma Pannonian C horizons of the Vienna Basin, Austria. MacFadden (2005) followed Simpson (1951) in a review of Equidae evolution. His unnumbered figure (MacFadden, 2005: pg. 1729) depicts phylogeny, geographic distribution, diet and body sizes of the Family Equidae over the last 55 Ma echoing Simpson’s (1951) earlier horizontal co-evolution of feeding from browsing to mixed feeding to grazing. Little address is made to the origin and/or evolution of Old World hipparions. Suffice it to say, neither Simpson (1951) nor MacFadden (2005) addressed Eurasian and African co-evolution of hipparion lineages and their dietary adaptations which we do herein frame within the context of their body masses. Gromova (1952) is widely credited with employing continuous metric variables in her studies of Eurasian hipparions. Her work included studies of skulls, mandibles, dentitions and postcranial material of important type material. Her work was foundational for all work on Eurasian and African hipparions that has followed. Forstén (1968) contributed a major revision of Old World hipparion reducing the number of taxa drastically. She recognized 7 species of Hipparion from Europe, Africa and Southern Asia, 5 species of Hipparion from China and 1 species of Proboscidipparion, P. sinense from China. She offered a noncharacter based phylogeny, with emphases on size and a few general dental and skeletal characters. Her phylogeny (1968: 83, Fig. 34) is a blend of a chronogram of linked regional taxa all but one taxon held to the genus rank of Hipparion. Zhegallo (1978) provided a comprehensive review of Central Asian hipparions following the joint Soviet-Mongolian Paleontological Expedition and made a comprehensive comparison to Eurasian and African hipparions (Zhegallo, 1978: Fig. 1). He recognized all of his stratigraphically ordered species as belonging to the genus Hipparion. Zhegallo (1978) recognized several species of Hipparion (Hipparion) subgenus as well as Hipparion (Neohipparion) houfenense and Hipparion (Proboscidipparion) sinense.Zhegallo proposed that North American Neohipparion was the ancestor of Hipparion (Neohipparion) houfenense which Bernor et al. (2015) recognized as Plesiohipparion shanxiense. Woodburne and Bernor (1980) undertook a longitudinal study on identifying 4 superspecific groups of Eurasian hipparions: Group 1 (= Hippotherium), Group 2 (= Cremohipparion), Group 3 (= Hipparion s.s.), Group 4 (= small Cremohipparion). Bernor et al. (1980) followed implementing this group classification for a preliminary biochronologic ranking of Eurasian hipparion-bearing localities. Bernor et al. (1990b) developed and illustrated a set of discrete characters of skulls, mandibles and dentitions to further delineate hipparion superspecific groups. Bernor et al. (1996b) proposed a number of Eurasian and African lineages with chronologic ranges for each. Bernor et al. (1997) published a monographic study of the Höwenegg quarry sample of 14 skeletons of Hippotherium including measurements on skulls, mandibles, dentitions and all postcranial bones with statistical variation being analyzed and reported. The Höwenegg assemblage was accumulated over a very short duration and as such is effectively a “natural’ population. Variability in skull, mandible and dental discrete characters were also documented and subsequently used in a series of hipparion systematic and biogeographic manuscripts to distinguish species and superspecific groups. Forstén (1984) provided a brief overview of Old World hipparion superspecific groups. While having been critical overall to the superspecific concept, in this report Forstén recognized the Old World genera Hipparion, Proboscidipparion and Neohipparion, the last including subgenera Neohipparion and Stylohipparion, respectively. Qiu et al. (1987) published a comprehensive monograph on Chinese late Miocene – Pleistocene hipparions recognizing 23 Chinese hipparion species organized into 6 subgenera of Hipparion, including: H. (Hipparion), H. (Hippotherium), H. (Cremohipparion), H. (Plesiohipparion), H. Fig. 1. Map of measured tooth crown heights of the hipparion lineages through MN biochronological zones (MN9 to MQ1) for the Old World late Neogene and Pleistocene (11.4–1.0 Ma) (see also Figs. 2–11). Numbers in the red circles show actual measured crown heights of hipparion species placed next to each species. Hipparion skulls overlay the mean ordinated hypsodonty (meanHYP) maps of each MN zone. The mean ordinated hypsodonty map represents the paleoecological condition grading from most humid (blue) to most arid (red). Black dots indicate the spatial position of the localities meanHYP scores calculated (see Supplementary Information for details). MN9 (11.2–9.9 Ma): The meanHYP patterns indicate an arid gradient extended from East Asia to the Eastern Mediterranean province while Western and Central Europe remained humid during this time interval. Hippotherium, Sivalhippus, and “Hipparion” radiated in Eurasia, but Cormohipparion dispersed rapidly in Eurasia and Africa. Hippotherium diverged rapidly in Europe and is known from China. 3 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 (Proboscidipparion) and H. (Baryhipparion). Bernor et al. (1990a) followed with a revision of the Lectotype series of “Hipparion” s.l. housed in Uppsala, Sweden originally published by Sefve (1927). A subsequent study by Bernor and Tobien (1989) on a small hipparion from Samos, Greece raised Cremohipparion to generic rank in naming Cremohipparion nikosi. Bernor and Lipscomb (1991) reported Plesiohipparion huangheense from the basal Pleistocene locality of Gulyazi, Turkey and Bernor et al. (1996b) recognized the generic ranks of Plesiohipparion and Proboscidipparion at generic rank. Forstén (2002) reported Proboscidipparion sp. from the Pliocene locality of The Red Crag, England. Sun et al. (2018) recognized that H. platyodus and H. ptychodus were members of the genus Sivalhippus and Bernor et al. (2019) assigned Hipparion dermatorhinum to a new genus, Shanxihippus (S. dermatorhinus). Bernor and Sen (2017) reported Plesiohipparion cf. longipes and Proboscidipparion heintzi from the 4.0 Ma. Turkish locality of Calta. Jukar et al. (2019) identified Plesiohipparion huangheense from Pliocene horizons of China, Turkey and India. China has been recognized to have been a major evolutionary center of hipparion evolution and also included immigrant taxa Hippotherium, Hipparion s.s., Cremohipparion and Sivalhippus. Plesiohipparion and Proboscidipparion would appear to have first arisen in Asia and extended their ranges into West Asia and Europe in the Pliocene. Baryhipparion and Shanxihippus were endemic to China. Alberdi (1989) recognized a single genus of Old World hipparion horses, Hipparion. She considered multiple genera at this time to be invalid, but apparently reversed this opinion in her contributions to Cantalapiedra et al. (2017) and Prado and Alberdi (2017). In her 1989 work, Alberdi represented 6 morphogroups within the genus Hipparion: Morphotype 1 including different types of Hippotherium, Morphotype 2 including H. concudense-mediterraneum (Group 2 of Woodburne and Bernor, 1980 and largely Cremohipparion of the current work), Morphotype 3, a series of smaller, less diverse peri-Mediterranean taxa; Morphotype 4, Pliocene aged massive limbed forms like “H”. crassum; Morphotype 5, late Miocene–Pliocene aged forms with strongly developed elongate limbs including taxa we recognize as Plesiohipparion; Morphotype 6, including “H”. rocinantis (Plesiohipparion rocinantis) herein, Proboscidipparion and African hipparions recognized as species of Eurygnathohippus herein. Alberdi (1989) has proposed two hipparion immigration events: 1. at 12.5 Ma “Hipparion” primigenium and 2. at 4 Ma. Neohipparion (Plesiohipparion shanxiense of Bernor and Sun, 2015). Zouhri and Bensalmia (2005) recognized a number superspecific groups overlapping extensively with Bernor et al. (1990b, 1996b) and others). They recognized at the genus rank Hippotherium, Hipparion s.s., Cremohipparion while Proboscidipparion including 3 subgenera: P. (Plesiohipparion), P. (Proboscidipparion) and P. (Eurygnathohippus). Their species inclusions and combinations differ somewhat from Bernor’s, but they are in broad agreement with Bernor’s superspecific group concept and have many of the same species contents in their superspecific taxonomic categories. Cantalapiedra et al. (2017) undertook a comprehensive study on the ecomorphological evolution and diversification of Neogene and Quaternary Equini. The authors produced a supertree utilizing a phylogeny of North Amercian Equinae produced by Maguire and Stigall (2008). Cantalapiedra et al.’s (2017) tree included 138 living and extinct species including Parahippus leonensis as an outgroup. They calibrated their tree using published chronologic ranges of taxa and, in effect have produced a chronogram of American, Eurasian and African equine species. For Old World hipparions, Cantalapiedra et al. (2017) recognized a number of superspecific hipparion taxa, including Hippotherium, Hipparion, Plesiohipparion, Proboscidipparion and Cremohipparion. Their taxonomic group species inclusions follow Bernor et al. (1996b) in part, but depart from ours in many regards including species of Plesiohipparion and Hipparion s.s. Prado and Alberdi’s (2017) work directly follows Cantalapiedra et al. (2017). We document below 63 species belonging to multiple Eurasian and African hipparion genera recognized and refined over 4 decades duration. We also adopt Woodburne’s (2007) phylogeny of North American Cormohipparion including Cormohipparion goorisi, Cormohipparion quinni and species of the Cormohipparion Complex. 2. Methods and abbreviations Students of Old World hipparions developed consistent measurement criteria for studying fossil horses at the American Museum of Natural History equid workshop in 1981. This yielded a clearly illustrated manual for measuring fossil horses (Eisenmann et al., 1988). Bernor et al. (1997) reported the quarry sample of Hippotherium primigenium from Höwenegg, Germany (Hegau, 10.3 Ma) wherein variability in both metric and non-metric (discrete) characters were reported; their measurement protocol followed Eisenmann et al. (1988) with some augmentation to cheek tooth measurement variables. Traditionally, European and Asian hipparion workers have used continuous variables implementing bivariate, multivariate and log10 ratio plots to recognize and name species and reveal their evolutionary relationships along with citation of discrete variables of the skull, mandible and dentition. There have been few Old World hipparion cladistic studies using only discrete variables and those have been limited to a few related and outgroup taxa (Bernor and Lipscomb, 1991, 1995; Bernor et al., 2018). Whereas numerous Old World hipparion species have been recognized with a number of studies erecting superspecific groups (Woodburne and Bernor, 1980; Bernor and Harris, 2003; Bernor et al., 1990b, 1996b; Qiu et al., 1987; Wolf et al., 2013; Zouhri and Bensalmia, 2005; Cantalapiedra et al., 2017; Prado and Alberdi, 2017), cladistic analyses have been frustrated by the lack of characters distributed across skulls, maxillary and mandibular dentitions and the lack of discrete variables for the postcranial skeleton for species population samples. These studies have also been inhibited by a lack of comparative cladistic analyses. Old World hipparion workers favor statistical analyses of continuous variables. While hipparions vary greatly in metapodial III and 1st phalanx III proportions, these have traditionally been reported by a variety of statistical plots of continuous variables. These factors have dissuaded Old World hipparion workers from applying cladistic methodology (Prado and Alberdi, 2017; Maguire and Stigall, 2008), and further frustrate the calculation of supertrees (Cantalapiedra et al., 2017). Azzaroli made a career-long study of Old World Equus, with investigations of North American origins of Old World Equus (Azzaroli and Voorhies, 1993). In 2003, Azzaroli provided his last interpretation on the evolution of Equus providing a branching tree, by biogeographic area with a chronologic ordering: a chronogram. Herein, we provide a listing of species of our superspecific hipparion taxa as they occur chronologically within and between MN units across Eurasia and Africa from 11.4–1 Ma. This chronogram of Old World Hipparion evolution is developed in parallel to Azzaroli’s work. Supplementary Table 1 lists the taxa we recognize and their chronologic distribution and Supplementary Fig. 2 provides a chronogram of the genera and species we discuss herein. We adopt the New and Old World (NOW) database for recognizing chronologic limits of Neogene and Pleistocene MN Unit temporal boundaries following Hilgen et al. (2012). We implement GIS maps of mean hypsodonty (HYP) with measured horse crown height indicated for the occurring taxa and show mean HYP curves of the entire large herbivore assemblage, with and without horses. We also provide a zoogeographical species presence table and a phylogeny graph. Hypsodonty comparisons relied only on large mammals (Orders Artiodactyla, Perissodactyla, Proboscidea and Primates) from terrestrial vertebrate localities ranging from early MN9 to MQ1 (11.2–0.6 Ma; Hilgen et al., 2012). All NOW localities from Eurasia and Africa were included in the study. Mean hypsodonty scores were downloaded from the NOW database with the chronological limits between MN9–MQ1 of the Old World (https://nowdatabase.org). Age of the African localities follows Werdelin and Sanders (2010) and was correlated to European MN biochronological units according to their published ages. We 4 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 calculated the mean ordinated crown height for each locality following Fortelius et al. (2002) for lists with at least two species with a hypsodonty value. Mean ordinated crown height is a robust proxy for humidity and productivity at the regional scale (Fortelius et al., 2002; Eronen et al., 2010a, 2010b; Liu et al., 2012). We plotted the results onto present-day maps and interpolated between the localities using Quantum GIS 3.12.3. For the interpolations, thematic mapping and grid interpolation was used, with the following settings: 20 km grid size; 800 km search radius; 800 grid borders. The interpolation method employed an inverse distance- weighted algorithm (IDW). Body mass estimates of Old World hipparion equids were calculated from the database of equid molar and postcranial measurements of R. Bernor, based on regression equations between postcranial bone measurements (Scott, 1990) or molar measurements (Janis, 1990) and known body mass of extant equid individuals. For the body mass estimates we chose proximal, distal and mid-shaft widths of metacarpals and metatarsals (Scott, 1990; M5, M3 and M11 of Eisenmann et al., 1988 and Bernor et al., 1997, respectively) and occlusal level antero-posterior crown lengths of maxillary M2, mandibular m1 and m2 (Janis, 1990; M2 is measurement 3 and m1, m2 is measurement 1 of Bernor et al., 1997). These measurements were chosen because of their high statistical significance as body mass estimators in modern equids (Saarinen et al., 2021), and because they are abundant in the fossil record. To minimize the risk of bias in body mass estimate comparisons resulting from a systematic difference in the body mass estimates provided by the different measurements, we calculated a mean body mass estimate for all the species as the mean of the estimates from different bone and tooth measurements. The species were arbitrarily assigned into three body size categories: small (body mass estimates less than 110 kg), mediumsized (body mass estimates between 110 and 210 kg) and large (body mass estimates greater than 210 kg). For an overview of diets of the Old World hipparions, we compiled results of mesowear analyses from the literature, complemented by new mesowear analyses by J. Saarinen for Cremohipparion periafricanum from the Late Miocene of Valdecebro, Spain, and Proboscidipparion sp. from the Pliocene of Red Crag, UK. Univariate mesowear scores (based on separate relief and cusp shape scores) were calculated based on the method described by Saarinen et al. (2016). The species were assigned dietary categories based on their mesowear signals, with mesowear scores (Saarinen et al., 2016) less than 1.3 indicating browse-dominated, 1.3–1.6 mixed-feeding, 1.6–2.0 grass-dominated and more than 2.0 grazing diets. These approximate dietary interpretations are based on the empirical results presented in Saarinen et al. (2016). Based on these data, we present a review of body size and diet of Old World hipparionines and discuss the relationship of diet and body size in Old World hipparionine equids. Eastern Mediterranean and China (Bernor et al., 2011). It is a large hipparion with a long POB with lacrimal extending less than ½ way to POF distal rim, POF is large, dorsoventrally and medially deep; maxillary and mandibular cheek teeth have richly ornamented enamel plications; mandibular cheek tooth metaconids are rounded and metastylids squared; metapodial IIIs are more robustly built than in Cormohipparion and many other Old World lineages. The Hippotherium group is derived from an early Old World Cormohipparion but early-on diverged in its more complex cheek tooth morphology and more heavily built metapodial IIIs. Bernor et al. (1997), Sun et al. (2018) Woodburne (1996, 2007 and 2009), Zouhri and Bensalmia (2005), Cantalapiedra et al. (2017) and Prado and Alberdi (2017) have likewise recognized the genus Hippotherium. Cremohipparion first appeared in the Ukraine and Balkans area (Bernor et al., 1996b; Bernor et al., 2016). It extended its range into China, the Indian Subcontinent and the peri-Mediterranean area. Primitively, Cremohipparion has a short POB with lacrimal suture invading or closely approaching the posterior rim of the POF; POF is primitively dorsoventrally and medially deep with well developed peripheral rim. The Cremohipparion mediterraneum, C. proboscideum, C. forstenae and C. licenti subgroup usually expresses a caninus (= intermediate) fossa between the POF and buccinator fossae, other members of the group do not; cheek teeth with moderately complex enamel plications; mandibular cheek tooth metaconids and metastylids are rounded; metapodial III’s are elongate and slender. The Cremohipparion group would appear to be derived from early Old World Cormohipparion (Bernor et al., 1996b, 2003b, 2020). Qiu et al. (1987) originally named Cremohipparion as a subgenus of Hipparion. Bernor and Tobien (1989) raised Cremohipparion to the genus rank in recognizing two small species of Cremohipparion from Samos, Greece. Bernor et al. (1996b, 2016), Wolf et al. (2013) and Cirilli et al. (2020) continued in recognizing Cremohipparion from Eurasia and North Africa. MacFadden (1994), Zouhri and Bensalmia (2005), Cantalapiedra et al. (2017) and Prado and Alberdi (2017) have likewise recognized the genus Cremohipparion with species from Eurasia and North Africa (C. macedonicum, C. mediterraneum, C. proboscideum, C. matthewi, C. nikosi), C. forstenae and C. licenti from China, and C. periafricanum from Spain (Sondaar, 1961; Cirilli et al., 2020). The Genotype species of Hipparion s.s. is Hipparion prostylum from Mt. Luberon, France (de Christol, 1832; Woodburne and Bernor, 1980; MacFadden, 1980). We follow Bernor (1985) and Bernor et al. (1990b, 1996b) in recognizing only five Old World species of this genus: H. gettyi, H. prostylum, H. dietrichi, H. campbelli and H. hippidiodus ranging from France, through Greece and Iran to China (Supplementary Table 1). The first known occurrence of Hipparion s.s. is H. gettyi from Lower Maragheh, Iran, ca. 9.0 Ma. (Bernor, 1985; Bernor et al., 2016). Primitively, Hipparion. gettyi is medium sized, has a prominent POF, long POB, lacrimal extending no more than ½ way to POF posterior rim; POF prominent, medially deep with posterior pocket, peripheral rim moderately well developed; cheek teeth are moderately complexly ornamented; mandibular dentition and metapodial IIIs unknown. Hipparion prostylum occurs in France, Greece, Turkey and Iran and exhibits a long POB with lacrimal extending ½ the distance to the POF; POF is reduced in length and depth and peripheral rim diminished also; maxillary cheek teeth are moderately well ornamented; lower cheek teeth with mostly rounded metaconids and metastylids; metapodial IIIs are elongate and slender. The Hipparion s.s. group would appear to be derived from early Old World Cormohipparion (Bernor et al., 2003b; Bernor et al., 2016). Qiu et al. (1987) and Bernor et al. (1990a) recognized the Chinese species Hipparion hippidiodus (Sefve, 1927). Zouhri and Bensalmia (2005) have likewise recognized the genus Hipparion including H. prostylum, H. antelopinum, H. platyodus, H. coelophyes and H. hippidiodus. MacFadden (1980, 1984) has argued for the occurrence of Hipparion sensu stricto in North America, recognizing three species: H. shirleyi, H. forcei and H. tehoense based on the morphology of the preorbital fossa. Cantalapiedra et al. (2017) and Prado and Alberdi 3. Old World Superspecific Taxa (SupplementaryTable 1; Supplementary Fig. 1) Cormohipparion is the founding genus for the Old World “Cormohipparion Datum” (Bernor et al., 2017). Cormohipparion sp. (Woodburne, 2007, 2009) from the Punchbowl Fm., California has been proposed as a suitable antecedent morphotype for the Old World hipparion ancestor. Bernor et al. (2003b) recognized Cormohipparion sinapensis from Sinap, Turkey as a a member of the genus based on cranial-dental and postcranial characters, summarized as follows: medium sized hipparion with long preorbital bar (POB), lacrimal extending approximately ½ way to POF distal rim; well developed preorbital fossa (POF), pocketed posteriorly with well developed peripheral rim; maxillary cheek teeth with moderate complexity; mandibular cheek tooth metaconids and metastylids rounded; metapodial IIIs moderately elongate and slender with midshaft cranial-caudal dimension modestly expanded. Cormohipparion occurred in Turkey, Pakistan, Algeria and arguably Ethiopia (Bernor and White, 2009; Bernor et al., 2010). Hippotherium occurred in Central and Western Europe, Italy, the 5 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 (2017) recognize Hipparion s.l. from both North America and Eurasia. Bernor (1985) and more recently Bernor et al. (1996b, 2016) argued that similarities in facial morphology between Old World Hipparion s.s. and North American “Hipparion” evolved convergently. We follow here by recognizing Hipparion s.s. as having likely originated from Old World Cormohipparion and having a range limited to Eurasia. Sivalhippus first appeared in the Indian Subcontinent and underwent a provincial evolutionary radiation there (Bernor and Hussain, 1985; Wolf et al., 2013). Sivalhippus nagriensis is the oldest and most primitive member of this group distinguished by a large POF with a long POB (MacFadden and Woodburne, 1982; Bernor and Hussain, 1985; Wolf et al., 2013). Maxillary cheek teeth are large and rhomboidal shape with complex plications and frequent flattening of the lingual wall of the protocone. Mandibular cheek teeth frequently have rounded metaconids and metastylids that are pointed disto-lingually; metapodials are robustly built. Sivalhippus theobaldi is a very large form with a long POB, large POF; maxillary dentition is complexly plicated; mandibular dentition is as in S. nagriensis; metapodial III’s are massively built. Uganda Sivalhippus macrodon (Eisenmann, 1994) is believed to be directly derived from S. theobaldi (Wolf et al., 2013; Sun et al., 2018). Sivalhippus perimensis diverged from S. nagriensis and S. theobaldi in having a very long POB and proximo-distally and dorsoventrally short POF placed dorsally high on the face; maxillary and mandibular cheek teeth are as in other Sivalhippus; metapodial IIIs are elongate but very robustly built. Sivalhippus anwari is distinguished from other Sivalhippus in evolving very great crown height and with metapodial IIIs like S. perimensis but more elongate (Wolf et al., 2013). Sivalhippus has been recognized by Bernor and Hussain (1985), Bernor et al. (1996b), Wolf et al. (2013) and most recently by Sun et al. (2018). Eurygnathohippus first appeared in Africa. Eurygnathohippus feibeli is the oldest and most primitive species occurring in Kenya, Ethiopia, Libya and Morocco (Bernor and Harris, 2003; Bernor et al., 2012; Cirilli et al., 2020; Bernor et al., 2020). Eurygnathohippus feibeli has primitive features of the dentition reflecting its relationship to Vienna Basin Pannonian C Hippotherium sp. and ultimately Cormohipparion (Bernor et al., 2017, 2020), most notably the presence of ectostylids and pli caballinids on the adult dentition. Whereas cranial material is rare, its facial morphology is like that seen in Cormohipparion and Hippotherium: long POB, lacrimal extending ½ the distance from orbit to POF; POF is well developed with moderate depth and distinct peripheral outline; maxillary cheek tooth complex; mandibular cheek teeth with rounded metaconid and squared metastylid; metapodial IIIs and 1st phalanges III elongate and slender. While not observed, it is distinctly likely that first occurring Old World Cormohipparion, like Pannonian C primitive Hippotherium sp., had short and diminuitive ectostylids that were welded to the labial enamel wall and buried in cementum. Eurygnathohippus, with these cheek tooth characters accompanied by elongate and slender metapodial IIIs and 1PhIIIs, was likely derived from a species of Old World Cormohipparion and was confined in its species diversification to Africa, except for its extension, and rare occurrence in the Pliocene of India (Jukar et al., 2019). Zouhri and Bensalmia (2005), Cantalapiedra et al. (2017) and Prado and Alberdi (2017) have likewise recognized the genus with their occurrence to the Plio-Pleistocene of Africa, including E. “libycum”, E. cornelianum and E. afarense). Eisenmann and Geraads (2007) recognized “Hipparion” pomeli from North Africa which Bernor and Kaiser (2006) referred to Eurygnathohippus pomeli while further recognizing E. hooijeri from Langebaanweg, South Africa. Bernor et al. (2010, 2013) have further recognized E. woldegabrieli and E. hasumense and various assemblages with Eurygnathohippus sp. from East Africa while Jukar et al. (2019) recognized the only Eurygnathohippus from outside of Africa in the Pliocene Tatrot horizons of India. Cantalapiedra et al. (2017) and Prado and Alberdi (2017) have recognized Eurygnathohippus including 3 species broadly separated cladistically: E. afarense, E. “libycus” and E. cornelianus. Plesiohipparion appeared in Asia during the latest Miocene. This genus is best known and represented by cranial, dental and postcranial material from the Yushe Basin, China (Qiu et al., 1987). Plesiohipparion houfenense is moderate to large size, has skull lacking a POF; cheek teeth that have sharply increased crown height; maxillary cheek teeth have moderate-to-complex plications of the maxillary cheek teeth, often elongate and lingually flattened protocones; lower cheek teeth with markedly pointed metaconids and metastylids with accompanying very broad and U-shaped linguaflexid; metapodial IIIs are elongate to extremely elongate and are often moderately robustly built (Qiu et al., 1987; Bernor and Sun, 2015; Bernor et al., 2015). Skull and dental characters reflect an evolutionary relationship with Sivalhippus and have been generally united with them within the informal rank of “Sivalhippus Complex”, as have species of Eurygnathohippus (Sun et al., 2018). Western Eurasia also has members of this group designated “Plesiohipparion” such as “P”. longipes recorded from the latest late Miocene of Pavlodar, Kazakhstan, Akkasdagi, Turkey and the Pliocene of Calta, Turkey (Bernor and Sen, 2017). These members of the “Plesiohipparion” group are distinguished by extremely elongate metapodial IIIs with unrecorded Plesiohipparion cheek tooth characters. An advanced member of Plesiohipparion, P. huangheense appears in China, Turkey and India during the Pliocene and occurs with early occurring Equus (ca. 2.6 Ma; Jukar et al., 2018). The latest occurring Plesiohipparion, P. shanxiense is a large form, closely related to P. houfenense that persisted into the early Pleistocene of China (Bernor et al., 2015). We do not believe it is coincidental that as Sivalhippus declined in the stratigraphic record and at the same time Plesiohipparion arises in Asia which reinforces our observation that they are related. Zouhri and Bensalmia (2005) have likewise recognized the genus Plesiohipparion including P. rocinantis (Spain), P. houfenense (China) and P. turkanense (Kenya; we refer to Sivalhippus turkanensis). Cantalapiedra et al. (2017) and Prado and Alberdi (2017) have recognized Plesiohipparion rocinantis and P. houfenense and referred Sivalhippus turkanensis to Plesiohipparion turkanense. Cirilli et al. (2021) have reviewed the Eurasian Plio-Pleistocene occurrences of Plesiohipparion with the last occurrence being at the base of the Pleistocene, 2.6 Ma. Proboscidipparion is a clade that first occurred in the early Pliocene of China and evolved in parallel with Plesiohipparion. Proboscidipparion is distinct for its curious strong retraction of its nasal bones which originally led Sefve (1927) to propose that its premaxilla supported a proboscis. Qiu et al. (1987) supported this view. The oldest and most primitive member of this clade is the earliest Pliocene species Proboscidipparion pater having a skull with a premaxilla that is strongly curved and dorsally convex; sharply retracted nasals to the level of M1 mesostyle; no preorbital fossa; lacrimal bone extending nearly to nasal notch; nasals broadly open anteriorly; infraorbital fossa foramen located high on the maxilla, distal to the nasal notch and close to the orbit. Maxillary cheek teeth with moderately complex plications; protocones are compressed oval with some lingual flattening. Mandibular cheek teeth are not reported (Bernor et al., 2018). The Early Pliocene locality of Calta, Turkey has a close relative of Proboscidipparion pater, P. heintzi represented by a partial juvenile skull (MNHN.F.ACA336; Bernor and Sen, 2017: Fig. 11A&B). The skull has a narial opening and retraction of nasal bones virtually identical to P. pater; cheek teeth are of a juvenile individual and not directly comparable to adult Proboscidipparion but exhibit retention of a large dP1 and extremely elongate dP2, maxillary cheek teeth are large, rhomboidal shape and have complexly complicated fossettes; deciduous protocones are round to oval while M1 is elongate and lingually flattened. Remarkable is Proboscidipparion heintzi’s complete, very short and robust MCIII (MNHN.F.ACA49A, Fig. 13A-C) similar to European “H”. crassum, also a plausible member of the Proboscidipparion clade (Forstén, 2001; Cirilli et al., 2021). The most advanced Proboscidipparion is P. sinense which occured in the Early Pleistocene of China (Sefve, 1927; Qiu et al., 1987; Bernor et al., 1990a; Bernor and Sun, 2015). Proboscidipparion sinense is one of the largest Old World hipparions. As in Proboscidipparion pater, it has a dorsally convex, curved premaxilla with nasals reflected back to the orbits with a broadly opened nasal aperture; 6 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 4. Chronology of Hipparion Species Occurrences no POF; very elongate snout, narrow anteriorly; cheek teeth in the type specimen worn, but M3 exhibits complexity of the pre- and postfossettes; protocones on less worn cheek teeth elongate and flattened lingually. Mandibular cheek teeth exhibit very elongate p2 and m3; metaconids and metastylids may be lingually pointed with broad and deep linguaflexid. Postcrania are not reported. Proboscidipparion exhibits a close morphological relationship in the loss of the POF and cheek tooth morphology to Plesiohipparion and, advanced members of the“Sivalhippus Complex”. In her revision of Old World hipparion Forstén (1968) recognized 11 species of Hipparion and the single species of Proboscidipparion, P. sinense. Qiu et al. (1987), Bernor et al. (1996b) and MacFadden (1994) also recognized the genus Proboscidipparion. Zouhri and Bensalmia (2005) have recognized the two Chinese species of Proboscidipparion, P. pater and P. sinense. Furthermore, they recognized three subgenera of Proboscidipparion, P. (Proboscidipparion). P. (Plesiohipparion) and P. (Eurygnathohippus). We maintain these three subgenera at the genus rank because of their distinct morphological characters. Cantalapiedra et al. (2017) and Prado and Alberdi (2017) recognize only the two Chinese species of Proboscidipparion, P. pater and P. sinense. Proboscidipparion s.s. is morphologically the most divergent Old World hipparion lineage. Shanxihippus is a monospecific genus including Chinese late Miocene S. dermatorhinus. Bernor et al. (2018) recognized Shanxihippus dermatorhinus for its unique combination of cranial characters: a medium to large hipparion with a prominent POF that is large, subtriangularly shaped, medially deep and moderately pocketed posteriorly; premaxilla is elongate with an extremely narrow V-shape anterior extent with a very narrow, U-shaped incisor arcade; nasals are strongly retracted to P4; maxillary cheek teeth are complex; pre- and postfossettes are linked in P2; protocones lenticular shaped; mandibular cheek teeth have rounded metaconids and metastylids vary from rounded to angular; pli caballinids are rudimentary. Postcrania are not known. Shanxihippus exhibits strongly retracted nasals as seen in Cremohipparion proboscideum, Cremohipparion licenti, Proboscidipparion pater and Proboscidipparion sinense but exhibits a unique suite of characters that distinguish it as a genus. Nasal retraction evolved convergently between Shanxihippus and these other genera (Bernor et al., 2018). Baryhipparion tchicoicum and its close relative Baryhipparion insperatum constitute another endemic clade from the late MiocenePleistocene of China (Qiu et al., 1987). In many ways Baryhipparion is an archaic genus with a very elongate POF that is very deep medially; it has a very elongate snout; nasals are not retracted; maxillary cheek teeth have complex enamel plications, protocones are rounded; maxillary cheek tooth pre- and postfossettes often link at their opposing margins; pli caballins are poorly developed; mandibular cheek teeth have ectoflexids that extend deeply into the isthmus of the metaconid-metastylid on p3-m3; metaconids and metastylids are rounded; m3 has a “waisted” hypoconulid. There is no apparent relationship between Baryhipparion and other genera or groups recognized herein; it exhibits an odd mix of primitive and advanced characters. Our study does not attempt to include all Old World hipparion species into superspecific taxa. There are many more Hipparion species than we report herein. We do not report those taxa for which we do not have first-hand knowledge. The diversity that we report here is remarkable and exceeds previous reports on the evolution, biogeography and paleoecology of Old World hipparions. Supplementary Table 1 provides a listing of the 63 hipparion taxa under consideration here and Supplementary Fig. 1 positions them geographically and temporally within the MN unit chronologic framework. The numerical ordering in Supplementary Table 1 is replicated within the skulls of Supplementary Figs. 1 and 2. 4.1. MN 9–11.2–9.9 Ma (Hilgen et al., 2012) Fig. 1 Cormohipparion sp. (Woodburne, 2007, 2009) entered the Old World at the beginning of this interval with rapid diversification of Hippotherium in Central and Western Europe (Bernor et al., 2017; ca. 11.4–11.0 Ma). Hippotherium primigenium has been extensively documented at Höwenegg (Hegau) and Eppelsheim Germany, 10.3 Ma (Bernor et al., 1997). Hippotherium weihoense occured in MN9 of China (Bernor et al., 2018). Spain records two early MN9 species, Hippotherium koenigswaldi (Woodburne, 2007) and Hippotherium catalaunicum (Woodburne and Bernor, 1980; Bernor et al., 1980). Later in MN9 of Hungary, there was an elongate limbed Hippotherium, H. intrans that occurred at Rudabanya (Bernor et al., 1993, 1996b, 2003a). A medium sized Cormohipparion sp. is found in the Siwaliks at this time and a rare smaller member of the Cormohipparion clade occurred later in this interval (Wolf et al., 2013). Cormohipparion sinapensis first occured at Sinap, Turkey at 10.8 Ma and a proliferation of other “Hipparion” species occurred within MN9 including “Hipparion” ankarynum, “Hipparion” uzungazili, “Hipparion” kecigibi and two other unnamed species (Bernor et al., 2003b). There is a strong contrast between Central Europe with a single species of Hippotherium through most of MN9 and a “punctuated” hipparion evolutionary event in Turkey during MN9 (Bernor et al., 2003b). “Cormohipparion” africanum occurred in Algeria and Ethiopia (Bernor and White, 2009; Bernor et al., 2010; Bernor et al., 2017). Sivalhippus nagriensis first occurred in the Pakistan Siwaliks 10.3 Ma. (Wolf et al., 2013). The most primitive MN9 hipparions are from Sinap Turkey where Cormohipparion sp. has a maximum crown height of 45 mm. (Bernor et al., 2003b). Most Vienna Basin Pannonian C hipparions, Höwenegg Hippotherium primigenium and Rudabanya Hippotherium intrans had a maximum crown height of 50 mm but the Mariathal population spiked to 57 mm. (Bernor et al., 2017). MN9 China Hippotherium weihoense also had a maximum crown height of about 50 mm (Sun et al., 2018). Other members of the Hippotherium clade likewise retained primitive crown heights of about 50 mm (Fig. 1). Crown heights are found to have increased in African “Cormohipparion” africanum (60 mm. in Bou Hanifia and 70 mm in Chorora) and the newly derived Pakistan lineage Sivalhippus nagriensis had a maximum crown height of 65 mm. In Spain, a large Hippotherium, H. koenigswaldi appeared at Nombrevilla with crown height less that 60 mm (Woodburne, 2007). The genus Cormohipparion did not extend its Old World range beyond MN9. Overall, lower crown heights were restrained by the dorsoventrally deep POFs (otherwise roots of the cheek teeth would have penetrated the ventral limit of the well defined POF). 4.2. MN10–9.9–8.9 (Hilgen et al., 2012) Fig. 2 Hippotherium primigenium continued to occur in Central Europe while Hippotherium weihoense continued in China (Bernor et al., 2018). Hippotherium giganteum occurred in Moldova and the Ukraine (Gromova, 1952) and Hippotherium aff. giganteum is reported from Nikiti 1 of Greece (Koufos et al., 2016). In Spain, Hippotherium catalaunicum and Hippotherium sp. are also present (Woodburne and Bernor, 1980; Bernor et al., 1980; Woodburne, 2007) The genus Hipparion, H. gettyi, appears at the base of the Maragheh section ca. 8.9 Ma from Kopran 1 (Bernor, 1985, 1986; Ataabadi et al., 2013; Bernor et al., 2016). The genus Cremohipparion first occurred in the Ukraine (C. moldavicum) and Greece (C. macedonicum) during this interval (Bernor et al., 1996b; Koufos et al., 2016; Wolf et al., 2013). Sivalhippus (nagriensis) persisted and S. theobaldi first occurred during this interval) in Pakistan (Wolf et al., 2013). Sivalhippus is believed to have extended its range into Uganda (S. macrodon; Bernor et al., 2010; Sun et al., 2018; Bernor et al., 2018). Bernor (1985, 1986, Bernor et al., 2016), have reported Hipparion gettyi and Central and Western European Hippotherium had a maximum 7 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 Fig. 2. MN10 (9.9–8.9 Ma): By the end of the MN9, an East to West directed arid gradient extended progressively across a larger area in the Eastern Mediterranean and Eastern Europe provinces, and the aridity pattern reached East Africa towards end of MN10. Hippotherium continued in Europe and China. Cremohipparion first appeared in Greece and the Ukraine. Sivalhippus (2 species) occurred in Pakistan and extended its range into East Africa. crown height of 55–60 mm. Cremohipparion, spanning from the Eastern Mediterranean to the Ukraine had a maximum crown height of 50 mm. In Pakistan, Sivalhippus theobaldi had a maximum crown height of 67 mm. whereas S. nagriensis had a maximum crown height of 65 mm. 1996b) particularly well represented from Dorn Dürkheim, Germany (Bernor and Franzen, 1997). A second species of Hippotherium, H. kammershmittae (Kaiser et al., 2003) has been identified at Dorn Dürkheim. A single metacarpal III of Hippotherium brachypus was collected by Tobien from Middle Maragheh, ca. 8.2–8.0 Ma (Bernor et al., 2016); this specimen compares closely with Pikermi H. brachypus which is significantly less robustly built than later Samos and Akkasdagi H. “brachypus” (Koufos, 2006; Vlachou and Koufos, 2009). Cremohipparion antelopinum 4.3. MN11–8.9–7.6 Ma (Hilgen et al., 2012 and NOW update) Fig. 3 Hippotherium primigenium persisted in Central Europe (Bernor et al., Fig. 3. MN11 (8.9–7.6 Ma): The highest meanHYP values are found in Anatolia and Asia while the values remain around low-mid values in Europe and Africa. Hipparion diversity increased. In addition to Hippotherium, Cremohipparion extended its range into Europe, West Asia and Pakistan. Hipparion sensu stricto first occurred along a narrow latitudinal gradient in Greece and Iran. Sivalhippus diversified into two additional species. 8 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 is reported from the Siwaliks beginning at 8.9 Ma (Wolf et al., 2013). Cremohipparion mediterraneum (Pikermi) and Cremohipparion proboscideum and Cremohipparion aff. matthewi (Samos) are reported from Greece (Bernor et al., 1996b; Koufos et al., 2009). Cremohipparion moldavicum is reported from the Ukraine, Iran and Georgia (Bernor et al., 2016), while Hippotherium brachypus and Cremohipparion macedonicum are reported from Bulgaria during this interval (Spassov et al., 2006). Maragheh, Iran had both Cremohipparion moldavicum and Cremohipparion matthewi at this time. Hipparion prostylum occurred at Middle Maragheh between 8.2 and 8.0 Ma and in Upper Maragheh the derived form Hipparion campbelli occured at 7.89 Ma (Bernor et al., 1996b; Swisher, 1996; Ataabadi et al., 2013; Bernor et al., 2016). Hipparion prostylum occurred at Samos, Greece in Quarry X at 8.33 Ma (Bernor et al., 1996b; Swisher, 1996). Geraads (2013) reported Hipparion cf. prostylum from Corak Yerler, Turkey which he correlates with MN11 (Geraads, 2013; Kaya et al., 2016). The genotype locality for Hipparion prostylum, Mt. Luberon, France is correlated with MN 11 (Woodburne and Bernor, 1980; Bernor et al., 1980; Bernor et al., 2016). Both Sivalhippus theobaldi and S. perimensis occurred in Pakistan (Wolf et al., 2013). Hippotherium retained crown heights of about 55 mm. Cremohipparion moldavicum, Cremohipparion mediterraneum and Cremohipparion proboscideum retain low crown heights of about 50 mm in Greece, the Ukraine and Georgia, while Siwalik Cremohipparion antelopinum had a maximum crown height of 60 mm (Bernor et al., 1996b; Bernor pers. observ.). Hipparion prostylum, and Hipparion campbelli had a crown height of about 60 mm. Sivalhippus theobaldi and S. perimensis retained their maximum crown heights from the previous interval: 67 and 70 mm respectively. brachypus [unnamed large species]; Vlachou and Koufos, 2009; Koufos et al., 2011) and Turkey (H. aff. brachypus [unnamed large species] Akkasdagi; Koufos and Vlachou, 2005). Baccinello also had a small Cremohipparion species (Bernor et al., 2011). Greece records the common occurrence of Cremohipparion macedonicum, Cremohipparion mediterraneum, Hipparion dietrichi at Samos, Greece during MN12 (Bernor et al., 1996b; Vlachou and Koufos, 2009; Koufos et al., 2011). Turkey likewise had Hipparion dietrichi occurring at Akkasdagi (7.1 Ma; Koufos and Vlachou, 2005). Maragheh, Iran had Hipparion campbelli (Bernor, 1985, 1986; Bernor et al., 2016). China had Hipparion hippidiodus occurring at this time (Bernor et al., 1990a, 1996b; Bernor et al., 2016). Cremohipparion persisted in the Ukraine (Cremohipparion moldavicum), Greece (Cremohipparion proboscideum, C. cf. matthewi, Bernor and Tobien, 1989; Bernor et al., 1996b; and C. cf. forstenae of Vlachou and Koufos, 2009), Turkey (Cremohipparion moldavicum from Akkasdagi) and Cremohipparion matthewi from Maragheh. Cremohipparion antelopinum continued to occur in Pakistan, and Cremohipparion forstenae occurred at Baode, China (Bernor et al., 1990a, 1996b). The Siwaliks had Sivalhippus perimensis persisting into this interval, and its successor, Sivalhippus, S. anwari (ca. 7.4–7.2 Ma; Wolf et al., 2013). Sivalhippus turkanensis is identified at Lothagam Kenya and a large Sivalhippus is known from Sahabi, Libya (Bernor et al., 2020). China had two species of Sivalhippus reported from this interval, S. platyodus and S. ptychodus (Sun et al., 2018), Baryhipparion tchicoicum (Qiu et al., 1987) and Shanxihippus dermatorhinus (Bernor et al., 2018). The genus Eurygnathohippus is first known to have occurred at this time with the occurrence of E. feibeli in Chad, Libya and Morocco and possibly Kenya (Bernor and Harris, 2003; Bernor et al., 2008, 2012; Cirilli et al., 2020; Bernor et al., 2020) while Morocco and Libya also record a small species of Cremohipparion sp. and Morocco records cf. Hippotherium (Cirilli et al., 2020). The elongate limbed lineage of Plesiohipparion longipes first occured at Pavlodar, Kazakhstan (Gromova, 1952), Westernmost China (Qiu et al., 1987; Jukar et al., 2018), Akkasdagi Turkey at 7.1 Ma (Koufos and Vlachou, 2005) and correlative deposits of Abu Dhabi ca. 4.4. MN12–7.6–6.8 Ma (Hilgen et al., 2012) Fig. 4 Hippotherium continued its range in Hungary with two species, H. intrans and H. microdon at Baltavar, (Kaiser and Bernor, 2006), and H. malpassi at Baccinello, Italy (Bernor et al., 2011) and Greece (H. aff. Fig. 4. MN12 (7.6–6.8 Ma): The mHYP map shows an interesting pattern during MN12: an arid belt represented by intermediate values in East Africa and the midlatitudes of Asia from East Asia to Eastern Mediterranean. Towards the end of the MN12, meanHYP values continued with high values in the Eastern Mediterranean and Southwest Asia and weakening at mid-latitudes of Asia. Most of Europe remained humid during the middle late Miocene. Hipparion generic diversity reaches its apogee with Hippotherium, Cremohipparion, Sivalhippus and Hipparion sensu stricto continuing their occurrence. In addition, Eurygnathohippus is first known to occur in Africa, Plesiohipparion first occurred in Asia and extended into Turkey and the United Arab Emirites. The endemic Chinese genera Shanxihippus and Baryhipparion are recorded in China. 9 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 7.0 Ma (Bernor et al., in press). MN 12 was the interval of maximum hipparion diversity and dispersion across Eurasia (Bernor et al., 1996b; Eronen et al., 2009). The genera Hippotherium and Cremohipparion (Eurasia and Africa) retained conservative, low crown height, between 50 and 60 mm and retained dorso-ventrally extensive POFs as a result. Sivalhippus perimensis had a maximum crown height of 70 mm and its stratigraphic successor S. anwari had crown heights approaching 80 mm (Wolf et al., 2013). Hipparion sensu strictu and Baryhipparion likewise had crown heights of circa 60 mm. Shanxihippus dermatorhinus, Sivalhippus platyodus and S. ptychodus had crown heights circa 65 mm. Crown heights for Eurygnathohippus feibeli were circa 60 mm. Crown heights for Plesiohipparion ranged from 65 to 70 mm. (Bernor and Sun, 2015; Sun et al., 2018; Bernor et al., 2018). feibeli co-occurred with Sivalhippus turkanensis at Lothagam, Kenya (6.5–6.0 Ma; Bernor and Harris, 2003 Bernor et al., 2020). Eurygnathohippus feibeli also occurred in the Middle Awash, Ethiopia from 6.0–5.4 Ma (Bernor et al., 2005; Bernor and Haile Selassie, 2009). China had the most diverse community of hipparion lineages at the end of the late Miocene, including Sivalhippus ptychodus and Sivalhippus platyodus (Sun et al., 2018), Hipparion hippidiodus (Qiu et al., 1987; Bernor et al., 1990a), Cremohipparion forstenae, Plesiohipparion houfenense and the endemic lineages, Baryhipparion tchicoicum (Qiu et al., 1987; Bernor and Sun, 2015) and Shanxihippus dermatorhinus (Bernor et al., 2018). Hippotherium malpassi, Sardinia retained a crown height of about 55 mm. In Kenya, Sivalhippus turkanensis had a maximum crown height of about 65 mm. Cremohipparion species in Europe, North Africa and China also retained low crown heights of about 50 mm. China Baryhipparion tchicoicum had relatively low crown heights not exceeding 50 mm while Shanxihippus dermatorhinus had a crown height of 65 mm. The dwarf Cremohipparion species, C. periafricanum and C. nikosi had crown heights less than 50 mm. Sivalhippus turkanensis, S. platyodus and S. ptychodus had maximum crown heights of 65 mm. Eurygnathohippus feibeli had a crown height of about 60 mm (Sahabi, Libya and Ethiopia). Plesiohipparion houfenense had a crown height of 70 mm. The end of the Miocene heralded the extinction of Hippotherium, Sivalhippus and Hipparion s.s. with Cremohipparion being restricted to China (C. licenti). 4.5. MN13–6.8–5.3 Ma (Hilgen et al., 2012) Fig. 5 By 6.8 Ma, Eurasian hipparions underwent marked lineage extinction, with the exception of China. Hippotherium made its last appearance with the occurrence H. malpassi in Sardinia, Italy (6.7 Ma; Rook and Bernor, 2013). Hipparion dietrichi has been reported from this interval in Greece (Vlachou and Koufos, 2009). Cremohipparion antelopinum persisted in the Siwaliks (Wolf et al., 2013). Cremohipparion periafricanum occurred in Spain (Alberdi and Alcalá, 1990) and its close relative C. nikosi occurred in Quarry 5 of Samos, Greece (Bernor and Tobien, 1989; Bernor et al., 1996b; Koufos, 2006). At Sahabi, Libya Eurygnathohippus feibeli, Sivalhippus cf. turkanensis, and Cremohipparion sp. occurred (6.8 Ma, Bernor et al., 2008, 2012, 2020). Cremohipparion nikosi has been reported from Q5 (Samos; Bernor and Tobien, 1989; Vlachou and Koufos, 2009). In fact, there are small hipparions similar to both C. periafricanum and C. nikosi also in Italy (Baccinello), Libya and Morocco (together with Greece and Spain mentioned above) in MN13. Upon revisionary work, the small Cremohipparion species, C. nikosi and C. periafricanum, could be recognized as being synonymous with C. periafricanum having priority. In SubSaharan Africa, Eurygnathohippus 4.6. MN14–5.3–5.0 Ma (Hilgen et al., 2012) Fig. 6 The MN14 interval immediately followed the mass extinction of Eurasian hipparion lineages at the end of MN13. Eurygnathohippus feibeli occurred at Lothagam, Kenya (Bernor and Harris, 2003) and Eurygnathohippus sp. occurred in the Middle Awash, Ethiopia (Bernor and Haile Selassie, 2009). Eurygnathohippus hooijeri occured at Langebaanweg, South Africa ca. 5.0 Ma (Bernor and Kaiser, 2006). Plesiohipparion houfenense continued in China and Proboscidipparion pater first occurred in the Yushe Basin, China during this interval (Qiu et al., Fig. 5. MN13 (6.8–5.3 Ma): In North Africa aridity began to increase in the eastern and western corners, arid areas increased in Western Europe similar to the western corners of North Africa as well. East Africa (Kenya and Uganda) shows more hypsodont elements (arid conditions) in its fauna than the north. The arid areas are located in the Eastern Mediterranean, partially on the northern coast of the Black Sea region, and Central Asia at the end of late Miocene. China’s generic diversity persisted into MN13: Baryhipparion, Cremohipparion, Hipparion s.s., Plesiohipparion, Shanxihippus and Sivalhippus. Cremohipparion occurred in Pakistan, the Eastern Mediterranean, Spain and North Africa. 10 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 Fig. 6. MN14 (5.3–5.0 Ma): In contrast with the previous time interval, aridity has decreased in the Eastern Mediterranean and Southwest Asia, and it was replaced with intermediate or low values by the beginning of the Pliocene. Hypsodonty map patterns show that aridity increased in Western Europe and South Africa, and the aridity pattern switched from south to north in East Africa during MN14. Most arid areas were located in Central Asia, Southeast Asia, and partially in East Asia by the Pliocene. The Mio-Pliocene boundary records a drastic reduction of hipparion diversity. The lineages that survived are Pleisohipparion in China and Eurygnathohippus in Africa. The highly derived genus Proboscidipparion is first recorded in China at the beginning of MN14. 4.8. MN16–3.55–2.5 Ma (Hilgen et al., 2012 and NOW update) Fig. 8 1987). Eurygnathohippus feibeli had a crown height of 60 mm whereas Eurygnathohippus sp. in the Middle Awash has a maximum crown height of about 60 mm. Eurygnathohippus hooijeri had a maximum crown height approaching 80 mm. Proboscidipparion pater and species of Plesiohipparion had crown heights of 70 mm. A late occurring “Hipparion” sp. has been reported as co-occurring with early Equus cf. livenzovensis from Montopoli, Italy, ca. 2.6 Ma by Rook et al. (2017). Baryhipparion insperatum occurred in China during this interval. Eurygnathohippus was broadly dispersed through East Africa during this interval (Eurygnathohippus sp.) with Hadar Eurygnathohippus hasumense being well represented during this interval (Bernor et al., 2010; Armour-Chelu and Bernor, 2011. The Manonga Valley and Laetoli, Tanzania (Upper Laetoli Beds) have a Eurygnathohippus that compares closely with Hadar E. hasumense (Bernor and Armour-Chelu, 1997). Laetoli Ndolanya Beds are dated 2.66 Ma and have yielded an advanced member of the Eurygnathohippus clade, E. aff. cornelianus (Armour-Chelu and Bernor, 2011). Bernor and Lipscomb (1991) reported the Chinese taxon Plesiohipparion huangheense (Qiu et al., 1987) co-occurring with Equus from Gulyazi, Turkey ca. 2.6 Ma. Jukar et al. (2018) reported Plesiohipparion huangheense from the Indian Siwaliks in pre-Equus occurring strata (3.5–2.6 Ma) and in the same horizons Jukar et al. (2019) have reported Eurygnathohippus sp. with an evolutionary grade similar to Ethiopian E. hasumense. The Red Crag, England (Von Koenigswald, 1970; Forstén, 2001; Rook et al., 2017; Cirilli et al., 2021) has an assemblage of Proboscidipparion sp. from the “Nodule bed” that is dated 2.7 Ma, just prior to the Equus Datum by Kahlke et al. (2011); also re: Saarinen and Lister, 2016). Plesiohipparion sp. is recorded from the latest MN16 locality of Roca Neyra, France (Cirilli et al., 2021). Cirilli et al., 2021 also reported the last occurrence of Plesiohipparion from Spain as having occurred circa 2.6 Ma. The crown height of Montopoli “Hipparion” is unknown. Ethiopian, Kenyan and Tanzanian Eurygnathohippus hasumense had a crown height of 70 mm. Ndolanya Beds, Laetoli Eurygnathohippus aff. cornelianus had a 4.7. MN15–5.0–3.55 Ma (Hilgen et al., 2012, NOW update) Fig. 7 Plesiohipparion huangheense is reported as first occurring at 3.9 Ma in Inner Mongolia (Qiu et al., 1987; Jukar et al., 2018). Plesiohipparion houfenense persisted in China and extended its range into North America at this time (Hulbert Jr and Harrington, 1999). During this interval (ca. 4.0 Ma), Plesiohipparion cf. longipes along with Proboscidipparion heintzi occurred in Calta, Turkey (Bernor and Sen, 2017). In the Yushe Basin, China, Proboscidipparion pater and the highly derived species Cremohipparion licenti occurred circa 4 Ma (Qiu et al., 1987; Bernor et al., 1990a, 1996b). Eurygnathohippus woldegabrieli is recorded at Aramis, Ethiopia 4.4 Ma (Bernor et al., 2013) and Eurygnathohippus hasumense is found in Ethiopia, Kenya and Tanzania during this interval (Bernor et al., 2010). Cremohipparion made its last appearance in China during this interval. Cremohipparion licenti had a low crown height of about 50 mm. Plesiohipparion cf. longipes, Proboscidipparion pater and Plesiohipparion houfenense had crown heights of about 70 mm. In that Proboscidipparion heintzi is represented by a juvenile skull and postcrania, crown height is not known. Eurygnathohippus woldegabrieli had a crown height of 65 mm and E. hasumense had crown height of 70 mm. 11 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 Fig. 7. MN15 (5.0–3.5 Ma): Humidity increased in continental Europe and central North Africa during MN15. A mesic environmental condition with an arid gradient became more prominent in the Eastern Mediterranean, Southwest Asia, Central Asia and East Africa. Eurygnathohippus is recorded in East Africa. Plesiohipparion, Proboscidipparion and the last, highly derived Cremohipparion are recorded in China. Proboscidipparion and Plesiohipparion extended their range into the Eastern Mediterranean during this interval. Fig. 8. MN16 (3.5–2.5 Ma): Arid environmental conditions extended throughout much of Eurasia and Africa towards end of the Pliocene, except central and northern parts of Europe and southern Asia. Eurygnathohippus continued to occur in East Africa and extended its range into India at this time. Baryhipparion continued in China. Plesiohipparion occurred in India, Turkey and Spain. Proboscidippairon occurred in England. “Hipparion” incertae sedis occurred in Italy. The Equus Datum occurred at the end of this interval. 12 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 crown height of 80 mm. Material of Plesiohipparion huangheense is rare but likely had a crown height of between 60 and 70 mm. Indian material of Eurygnathohippus sp. is very similar to Hadar E. hasumense with a likely comparable crown height of about 70 mm. Red Crag Proboscidipparion sp. had a crown height of 60 mm. had low crown height of about 50 mm. Eurygnathohippus pomeli had a maximum crown height of 65 mm. East African Eurygnathohippus had crown heights that scaled up to 80 mm (Bernor et al., 2010). 4.10. MQ1–1.8–0.6 Ma (Hilgen et al., 2012, NOW update) Fig. 10 Eurygnathohippus was restricted to East and South Africa at this time with a last reported occurrence from the Middle Awash, Ethiopia, Olorgesailie, Kenya and the Orange Free State of South Africa (Brink et al., 2012). Eurygnathohippus cornelianus had an age of about 1.0 Ma (Bernor et al., 2010; personal observation). Proboscidipparion persisted until 1.0 Ma (Liu and Lovlie, 2007). Crown height for Chinese Proboscidipparion sinense is believed to have been between 80 and 90 mm. A specimen reported from Olduvai Gorge IV having an individual with a crown height of 90 mm. (ArmourChelu and Bernor, 2011; Bernor, pers. observation). Bernor’s study of Olorgesailie Eurygnathohippus sp. from Member 1 (0.99 Ma) has revealed that it is a small unnamed species with crown heights not exceeding 50 mm. During the MNQ1 interval hipparion diversity dropped dramatically coincident with the evolutionary radiation of Equus (Azzaroli, 2003; Alberdi and Palombo, 2013; Bernor et al., 2018, 2019). African occurrences of Eurygnathohippus are rare after 1.0 Ma with extinction believed to have occurred soon after 1.0 Ma. 4.9. MN17–2.5–1.8 Ma (Hilgen et al., 2012, NOW update) Fig. 9 Eurasian stenonine horses (Equus spp.) first occurred and diversified throughout Eurasia at this time and extended slightly later into Africa, 2.33 Ma (Bernor et al., 2010, 2018, 2019; Cirilli et al., 2021). The youngest European hipparion, attributed to Plesiohipparion rocinantis (Zhegallo, 1978; Qiu et al., 1987; Bernor and Sun, 2015; Rook et al., 2017), was originally reported from Villaroya, Spain by HernándezPacheco (1921) as Hipparion rocinantis. Pueyo et al. (2016) calibrated the age of the Villaroya assemblage as being correlative with the Reunion chron C2r.1n, 2.128–2.148 Ma. Cirilli et al. (2021) argued that the age of this hipparion occurrence was likely correlative with the Equus Datum at 2.58 Ma, latest MN16. A large species of Plesiohipparion, P. shanxiense reported from China (2.0 Ma; Bernor et al., 2017); this species was demonstrated not to be a species of Neohipparion immigrating from North America (sensu Zhegallo, 1978; MacFadden, 1984). An advanced and large species of Proboscidipparion, P. sinense occurred in China at this time. Baryhipparion insperatum extended its range in China during this interval. Eurygnathohippus pomeli has been reported from the late Pliocene of Ain al Oughlam, Morocco circa 2.5 Ma in age by Eisenmann and Geraads (2007). Eurygnathohippus had a common occurrence in East and South African localities during this interval with the initial occurrence of the Eurygnathohippus cornelianus lineage at Laetoli (Bernor et al., 2010; Armour-Chelu and Bernor, 2011). At this time, hipparions were restricted to the periphery of Eurasia in China and Spain as well as the East-South African corridor. Chinese Proboscidipparion sinense and Plesiohipparion shanxiense both had maximum crown heights of about 80 mm. Baryhipparion insperatum 5. Hipparion chronogram We provide here (Supplementary Fig. 2) a chronogram of the Old World hipparion lineages we have identified herein. The chronogram displays species lineages through time calibrated to the MN time scale following the NOW database standard of recognizing intervals with chronologic boundaries defined in Hilgen et al. (2012). Generic groups are included in colored polygons with the numbers representing the taxonomic ordering in Supplementary Table 1, Supplementary Fig. 1 and Figs. 1–10. Fig. 9. MN17 (2.5–1.8 Ma): Aridity increased in East Africa. Central and East Asia were mostly dry while Europe had wetter conditions. Eurygnathohippus ranged through East and South Africa and has been reported from Algeria. Baryhipparion, Proboscidipparion and Plesiohipparion occurred in China. Plesiohipparion occurred in Spain. The monodactyl horse Equus occurred broadly in Eurasia during this interval. 13 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 Fig. 10. MQ1 (1.8–0.6 Ma): MeanHYP patterns of the Old World mammal communities indicate that while Europe and the Eastern Mediterranean occupied by mid meanHYP values, arid environmental conditions increased drastically in Southwest Asia, Central Asia, India, Africa, except Southeast Asia. Hipparions last occurred during this interval. Proboscidipparion occurred in China and Eurygnathohippus occurred in East and South Africa. We follow Woodburne (1996, 2007, 2009) in identifying North American Cormohipparion sp. as the morphological source for Old World hipparion. Bernor et al. (2003b) identified Sinap Cormohipparion sinapensis as a valid Cormohipparion with primitive features of the skull, mandible, dentition and postcrania and in particular the metapodial III’s, first occurring in early MN 9, 10.8 Ma. Within the MN9 interval, the Sinap hipparion assemblage records a period of more than 0.5 Ma of stasis followed by a “punctuated” evolutionary diversification that included: “Hipparion” ankarynum, “Hipparion” uzungizili and “Hipparion” kecigibi. A rare and smaller Cormohipparion sp. also occurred in MN9 correlative horizons of the Siwaliks (Wolf et al., 2013). “Cormohipparion” africanum, similar in cranial, dental and postcranial anatomy occurred at Bou Hanifia, Algeria during early MN9, ca. 10.5 Ma. (Bernor et al., 1980; Bernor and White, 2009), with a similar form known from Chorora, Ethiopia (Bernor et al., 2010). Cormohipparion did not extend its range in the Old World later than MN9, with the possible exception of Ethiopia. While Cormohipparion sinapensis (black polygon) is the most primitive Old World hipparion, Hippotherium sp. also has several primitive features and is the oldest first occurring Old World hipparion first appearing in Pannonian C horizons of the Vienna Basin, 11.4–11.0 Ma. Hippotherium (red polygon) consistently had more complex enamel plications of the cheek teeth and the Pannonian C hipparion had distinctly primitive features of the upper and lower dentition (Bernor et al., 2017). Hippotherium primigenium is common in MN9–11 horizons of Central Europe. In MN11 a smaller form Hippotherium kammershmittae cooccurred with Hippotherium primigenium at Dorn Dürkheim, Germany (Kaiser et al., 2003). Hippotherium weihoense, a species very similar to Hippotherium primigenium both in size and cranial-dental morphology, occurred during MN9 and 10 in China (Qiu et al., 1987; Sun et al., 2018). Spain records at least two species of Hippotherium during MN 9, H. koenigswaldi and H. catalaunicum, with the latter species having extended its range into MN10. Hungry had an advanced Hippotherium, H. intrans from the late MN9 locality of Rudabanya, distinguished by its elongate metapodials (Bernor et al., 2003a and b). Hippotherium intrans persisted until late MN12 at Baltavar where it co-occurred with the smaller and more gracile species, Hippotherium microdon (Kaiser and Bernor, 2006). Hippotherium brachypus is a species characterized by its short robust metapodial IIIs best known from Pikermi, Greece (Koufos, 1987a, 1987b) but also Middle Maragheh, Iran (Bernor, 1985; Bernor et al., 2016). Koufos and Vlachou (2005) identified a large form of H. brachypus from MN12 of Samos and Akkasdagi, Turkey which we believe is a valid, unnamed larger taxon than the Pikermi form. Another larger form, Hippotherium giganteum occurred during MN10 in the Ukraine (Bernor et al., 1996b). The last occurring Hippotherium occurred in Italy and Sardinia during MN12–13 and exhibits conservative characters of the cranium, dentition and postcranial skeleton. Hippotherium became extinct by the end of the Miocene. Cremohipparion’s (green polygon) first occurrence is with the appearance of Cremohipparion moldavicum in the Ukraine (Gromova, 1952; Bernor et al., 1996b). Cremohipparion moldavicum is similar to Hippotherium except for the sharp reduction of its preorbital bar and resulting reduced posterior pocketing of the preorbital fossa (Woodburne and Bernor, 1980 [their Group 2]; Bernor et al., 1980). Cremohipparion moldavicum extended its range through MN12 of the Ukraine and Iran (Bernor et al., 1996b; Bernor et al., 2016). Also occurring in MN10 is the smaller Greek species Cremohipparion macedonicum which extended its range through MN11 up to MN12 (Koufos, 2016). Bernor et al. (2016) recently provided evidence that the small Cremohipparion, C. matthewi is likely derived from C. moldavicum first occurring in MN 11 and extending its range in MN12 of Maragheh Iran (Bernor et al., 1996b). Cremohipparion nikosi is known from the upper stratigraphic levels (Quarry 5) of Samos, MN13 (Bernor and Tobien, 1989). Cremohipparion mediterraneum was abundant at the MN11 locality of Pikermi, Greece and persisted into MN12 of Greece. Cremohipparion proboscideum appears to have been derived from C. mediterraneum with an increase in size, further dorsoventral and medial deepening of the preorbital fossa, retraction of the nasals and lengthening of the snout having occurred in MN11 and 12 of Greece. The tiny Cremohipparion, C. periafricanum is possibly related closely to C. nikosi and is known from MN 13 of Italy and peri-Mediterranean localities (Bernor et al., 1996b; Cirilli et al., 2020). The Potwar Plateau, Pakistan records the occurrence of Cremohipparion 14 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 antelopinum from MN11–MN13 correlative horizons. China records an advanced member of Cremohipparion forstenae from MN12 and 13 horizons (Qiu et al., 1987; Bernor et al., 1990a). We do not follow Vlachou and Koufos (2009) who refer Samos specimens to C. forstenae because of the lack of recurved nasals in the Samos species (re: Sefve, 1927; Qiu et al., 1987; Bernor et al., 1990a). The last occurring member of the Cremohipparion group is Yushe Basin, China Cremohipparion licenti occurring in MN15 correlative horizons, ca. 4 Ma. (Qiu et al., 1987). Hipparion sensu stricto (magenta polygon) was originally identified and characterized by Woodburne and Bernor (1980; their Group 3 form) based upon the Genotype series of Hipparion prostylum de Christol (1832) from Mt. Luberon. The oldest and most primitive member of the clade is the MN10 species Hipparion gettyi from Lower Maragheh, Iran. Hipparion prostylum occurred in Middle Maragheh MN11 horizons (Bernor, 1985) as well as correlative lower stratigraphic horizons from Samos, Greece. The more derived Hipparion campbelli occurred in MN11 and 12 horizons of Maragheh (Bernor, 1985; Bernor et al., 1996b; Bernor et al., 2016). Hipparion dietrichi occurred in MN12 and 13 horizons of Samos, Greece (Vlachou and Koufos, 2009). The easternmost occurrence of Hipparion s.s. is Hipparion hippidiodus from MN12 and 13 of Baode, China (Sefve, 1927; Qiu et al., 1987; Bernor et al., 1996b; Bernor et al., 2016) Sivalhippus is a lineage that arose in the Indian Subcontinent (yellow polygon; Bernor and Hussain, 1985; Wolf et al., 2013). The most primitive form Sivalhippus nagriensis occurred in MN9 correlative horizons of the Siwaliks, Pakistan and extended its range there into MN10 correlative horizons. The larger, relatively rare Siwalik form Sivalhippus theobaldi first occurred in MN10 equivalent horizons and extended it range locally into MN11 correlative horizons. The more common relative Sivalhippus perimensis first occurred in MN11 and extended its range into MN12 equivalent horizons. The large and hypsodont species Sivalhippus anwari, a probable desendant of S. perimensis, occurred in late MN12 Siwalik horizons. China had two species of Sivalhippus, S. platyodus and S. ptychodus that occurred in MN12 and 13 correlative horizons of China (Qiu et al., 1987; Bernor et al., 1990a; Sun et al., 2018). Uganda had an MN10 equivalent large species of the clade, Sivalhippus macrodon (Eisenmann, 1994; Wolf et al., 2013; Sun et al., 2018), an apparent derivative of Siwalik S. theobaldi. Another member of the Sivalhippus clade is Sivalhippus turkanensis occurred in MN12 and 13 of Kenya, Ethiopia and Libya (Bernor and Harris, 2003; Wolf et al., 2013; Cirilli et al., 2020; Bernor et al., 2012. Bernor et al., 2020). Eurygnathohippus (blue polygon) first occurred in Africa during MN12 equivalent horizons, and extended its range into MN14, as the medium sized, slender-limbed form E. feibeli. Eurygnathohippus feibeli is known to have occurred in Kenya (Bernor and Harris, 2003), Ethiopia (Bernor and Haile Selassie, 2009), Morocco (Cirilli et al., 2020) and Libya (Bernor et al., 2012, 2020). It is a primitive clade with features of the cranium and dentition reflecting its distant relationship to Austrian Pannonian C hipparions. A larger Eurygnathohippus, E. hooijeri occurred in MN 14 equivalent aged horizons of Langebaanweg, South Africa (Bernor and Kaiser, 2006). Eurygnathohippus woldegabrieli occurred in MN15 equivalent horizons of Aramis, Ethiopia (4.4 Ma; Bernor et al., 2013) and maybe derived directly from E. feibeli. Eurygnathohippus hasumense succeeded E. woldegabrieli being common at Hadar between 3.6 and 2.9 Ma, MN15–16 correlative. Eurygnathohippus sp., closely resembling E. hasumense, made an extension into the Indian Subcontinent during MN16 (Jukar et al., 2019). Eurygnathohippus sp. is common in MN16-MQ1 horizons and may include more than one taxon. Eurygnathohippus cornelianus is earliest known from Olduvai Gorge Beds I and II (Leakey, 1965) and is remarkable for its expanded premaxillary and mandibular incisor dentitions. Eurygnathohippus cornelianus is known to occur at the 1.0 Ma locality of Daka, Ethiopia (Gilbert and Bernor, 2008) and the type locality of Cornelia, Orange Free State (Van Hoepen, 1930; Brink et al., 2012 – age of Cornelia-Uitzoek 990,000 yrs.). Plesiohipparion (orange polygon) arose in Central Asia during MN12 (Gromova, 1952) as a species with extremely elongate metapodial IIIs, Plesiohipparion longipes. Plesiohipparion cf. longipes likewise occurred during MN12 of Akkasdagi, Turkey (Koufos and Vlachou, 2005; Bernor et al., 2017), Abu Dhabi, Arabia (Bernor et al., in press) and MN15 of Calta, Turkey (Eisenmann and Sondaar, 1998; Bernor and Sen, 2017). Plesiohipparion is well represented by cranial, dental and postcranial material from China (Qiu et al., 1987) during MN13–15. Plesiohipparion houfenense first occurred in the Yushe Basin, China during MN13. Plesiohipparion huangheense occurred in China during MN15 (Qiu et al., 1987) and extended its range into Turkey (Bernor and Lipscomb, 1991) and India (Jukar et al., 2018) during MN16. Plesiohipparion had late occurrences including Plesiohipparion sp. and Plesiohipparion rocinatis from Spain in latest MN16/earliest MN17 (Pueyo et al., 2016; Cirilli et al., 2021). Proboscidipparion (lavender polygon) first appeared in China during MN14 with the occurrence of the more primitive form, P. pater (Qiu et al., 1987). Proboscidipparion pater extended its range in China into MN15 and a close relative Proboscidipparion heintzi is known to occur at Calta, Turkey in MN 15 (ca. 4 Ma; Bernor and Sen, 2017). A larger and more advanced form Proboscidipparion sinense occurred during MN 17 (ca. 2 Ma) in China (Sefve, 1927; Qiu et al., 1987; Bernor et al., 1990a) with its last occurrence being 1 Ma (Liu and Lovlie, 2007). Cirilli et al. (2021) have reported Proboscidipparion from the latest Pliocene of the Red Crag, England (late MN 16). China had two endemic lineages, Baryhipparion (violet polygon) with species B. tchicoicum (MN12 and MN13) and B. insperatum (MN16 and 17; Qiu et al., 1987) and Shanxihippus dermatorhinus (white polygon) (Bernor et al., 2018) that occurred at the end of the late Miocene (MN12 and 13). 6. Discussion Body masses for hipparions have been calculated for our sample of Old World hipparions (Fig. 11, Supplementary Table 2) when known. Paleodiet data for our sample is also documented herein (Figs. 12 and 13, Supplementary Table 2). The North American genus Cormohipparion (1a in Fig. 13) has been recorded from the Sinap Formation, Turkey (Bernor et al., 2003b) and Siwaliks, Pakistan (Wolf et al., 2013) where it occurs at 10.8 Ma. A slightly advanced form, “Cormohipparion” africanum is recorded at 10.5 Ma from Bou Hanifia, Algeria (Bernor and White, 2009). The earliest Old World Cormohipparion retained the low maximum molar crown height (MCH) of only 45–50 mm. The earliest records of hipparion horses in the Old World are dated between 11.4 and 11.0 Ma and appear all to represent the early divergent lineage Hippotherium (2 in Fig. 13), with an MCH of 50–55 mm. (Bernor et al., 1997, 2003b, 2017; Wolf et al., 2013). Cormohipparion was a short-lived lineage in the Old World, whereas Hippotherium persisted in Europe up into MN13. Cormohipparion and Hippotherium retained diets ranging from browsing to mixed feeding throughout their range. The Old World Cormohipparion species were medium-sized hipparions with mean body mass estimates between 160 and 170 kg, while species in the genus Hippotherium ranged from medium to comparatively large body sizes (mean body mass estimates from ca. 180 to 260 kg). “Cormohipparion” sp. from Chorora, Ethiopia was a browse-dominated species. The next Old World genus recorded is Sivalhippus (4 in Fig. 13) from the Indian subcontinent (Pakistan) at 10.8 Ma, time correlative with MN9 and represented over time by the clade’s sequence S. nagriensis, S. theobaldi + S. perimensis and terminating in MN13 with S. anwari. Already at its emergence in the record, this genus had a higher MCH than Cormohipparion and Hippotherium and over time it increased crown height in its species from 65 to 80 mm. Paleodiet data have been collected and analyzed but remain unpublished to date. Suffice it here to say that dietary tracking the transition from C3/C4 mixed environments to C4 environments at 8.2 Ma has been observed (Nelson, 2005). The genus Sivalhippus is characterized by the evolution of very large body size for hipparionines already during M10–MN11, with mean body mass estimates ranging from 260 to 350 kg. 15 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 Fig. 11. Body mass estimates (in kg) of Old World hipparion genera through time. MN and MQ units are used as time bins. Hipparion s.s. (3b in Fig. 13) first appeared in MN10 of the Subparatethyan Province (Iran) in late MN 10/earliest MN11 as a plausible branch of a provincial Cormohipparion species. It had a relatively low diversity with a broad geographic range spanning western Europe (France) to eastern Asia (China) but has never been identified in Africa or on the Indian subcontinent. Hipparion spp. persisted into MN13 of China (H. hippidiodus) and was a mixed feeder throughout its range, with an MCH not exceeding 60 mm. The genus Hipparion includes mediumsized hipparionine species, with mean body mass estimates ranging from ca. 135 to ca. 200 kg. The three Maragheh Hipparion species, H. gettyi, H. prostylum and H. campbelli were medium sized mixed feeders. 16 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 increasing from ca. 230 kg of P. longipes and P. houfenense in MN12–15 to over 300 kg in P. rocinantis in MN16 and P. shanxiense in MN17. Proboscidipparion (10 in Fig. 13) was a highly derived lineage of large body size and with highly retracted nasal bones. Proboscidipparion first occurred in the early Pliocene of China but extended its range westward into eastern Mediterranean (Turkey) and western Europe (England) (Qiu et al., 1987; Bernor et al., 1990a; Cirilli et al., 2021). The terminal Chinese species Proboscidipparion sinense achieved an MCH of 80+ mm, with a mean body mass of 319 kg, and appears to have had a mixed feeding adaptation (Bernor et al., 1990a; Bernor and Sun, 2015). Proboscidipparion sinense had highly retracted nasals thought to have supported a proboscis for feeding. Mesowear analysis of Proboscidipparion sp. from the latest Pliocene Red Crag Nodule Bed from England indicates mixed-feeding diet (Rivals and Lister, 2016; Saarinen, present study). However, the heavy abrasion on the type skull maxillary dentition of P. sinense (PMUM3925; Sefve, 1927; Bernor et al., 1990a: Fig. 17) suggests feeding on abrasive plants and possibly points at an alternative functional role of the proboscis in filtering dust inhaled in open environments similarly to the saiga antelope, rather than being a browsing adaptation. Palynological data indicate that the paleoenvironment in the Red Crag Nodule Bed was forested, being dominated by temperate forest elements and including subtropical taxa (Head, 1998), while the Early Pleistocene paleoenvironments of Proboscidipparion in China have been interpreted to have been sparsely wooded shrublands or grasses (Xue et al., 2006). This suggests a wide range of habitats and considerable ecological plasticity for Proboscidipparion. Additionally, there are two endemic lineages known from the late Mio–Pliocene of China, Baryhipparion (6) and Shanxihippus (7). Baryhipparion includes species in the medium size range for hipparions (mean body mass estimates 150–200 kg) and available dietary analyses for B. tchicoicum from the Early Pliocene of Udunga, Russia (Nakaya et al., 2009), indicates mixed feeding. Shanxihippus dermatorhinus was a mixed-feeding (Bernor et al., 2018), relatively large hipparion (mean body mass estimate ca. 240 kg), and with retracted nasals that suggest a specialized snout for selective feeding. We summarize evolutionary trends in taxa, their body mass and diet in Fig. 13. The dietary development of the Old World hipparions shows a shift from browsing and mixed feeding in the early late Miocene, with a gradual increase in the proportion of grazing forms over time. Dietary diversity nevertheless remained, with the last browser recorded at the end of the Miocene and mixed feeders persisting at least well into the Pliocene, even becoming the dominant dietary category in Eurasia again during the Pliocene. Although grazers clearly average higher MCH than browsers or mixed feeders overall, there is also an overlying trend of crown height increase over time regardless of diet, reflecting the known double relationship of hypsodonty with habitat as well as diet. In comparison to other ungulates, hipparions show higher ordinated mean hypsodonty throughout their Old World history, with the greatest difference seen at the beginning of their history. All ungulates show an essentially parallel increase in hypsodonty in the mid- to late Pliocene but with the Pleistocene the equid curve again departs to a much higher level. Despite their specialized dentition, the hipparions show a remarkably wide dietary range from browsing to grazing and a prevalence of mixed-feeding dietary strategies in most lineages. Dedicated grazers emerged at least within small-sized members of the genus Cremohipparion, and within the African lineage of Eurygnathohippus. In the latter case at least, the heavily grazing diet is associated with the spread of C4 grasslands during the Late Neogene. Eventually, equids of the genus Equus replaced the last hipparions in Eurasia during the Early Pleistocene and in Africa during the Middle Pleistocene (Cirilli et al., 2021). Dietarily, most members of the genus Equus were grazers during the Pleistocene, and in this respect they appear dietarily more fixed to the grazing niche than the hipparions (e.g. Mihlbachler et al., 2011; Saarinen et al., 2016; Saarinen et al., 2021). However, even Equus apparently retained the ability to shift to mixed-feeding and sometimes even browse-dominated feeding in the Pleistocene of Europe, indicating Fig. 12. Old World hipparion dietary distribution (presented as univariate mesowear scores calculated using the method of Saarinen et al., 2016) in the small, medium and large body size categories. Color-coded are approximated dietary interpretations based on the mesowear scores. A) All taxa. B) Eurasian taxa (Eurygnathohippus excluded). The genus Cremohipparion (5 in Fig. 13) is known from an extensive range spanning eastern Europe (Greece, Iran and Ukraine) to China. It also had a long temporal range, equivalent to MN10–MN15 but MCH remained low, never exceeding 60 mm. This is evidently related to the persistence of a dorso-ventrally large preorbital fossa, incompatible with tall tooth crowns, but what is cause and what is effect is difficult to glean. Body size distribution within the genus Cremohipparion is bimodal, comprising a group of small-sized species (mean body mass between 50 and 110 kg, such as C. periafricanum, C. matthewi, C. nikosi and C. macedonicum), and a group of medium-sized species (mean body mass between ca. 150 and 200 kg, such as C. moldavicum, C. mediterraneum, C. proboscideum and C. antelopinum). Available paleodietary data indicates that while the medium-sized species of Cremohipparion (e.g. C. moldavicum) were mixed-feeders, the small-sized species such as C. matthewi and C. periafricanum were grazers (Bernor et al., 2014; Saarinen, this article). Eurygnathohippus (9 in Fig. 13) is first recorded in Africa in the latest Miocene (MN12-equivalent; Bernor et al., 2020; Cirilli et al., 2020). It persisted in Africa up to at least 1 Ma and eventually reached the very high MCH of about 90 mm (Bernor et al., 2010). Eurygnathohippus was initially a mixed feeder but became increasingly committed to grazing; by 4.4 Ma, E. woldegabrieli appeared as a dedicated grazer. Body size within the genus Eurygnathohippus ranged from medium to very large for hipparionines (mean body mass ca. 200–350 kg). Plesiohipparion (8 in Fig. 13) first appeared in Asia in the latest Miocene, evolving progressively higher crowned teeth, with MCH up to 75–80 mm. During the Pliocene it extended its geographic range both westward into western Asia and Europe and southward into South Asia (Cirilli et al., 2021). Plesiohipparion appears to have been a mixed feeder throughout its known history. The genus Plesiohipparion is characterized by increasing body size through time, with mean body mass estimates 17 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 Fig. 13. (A) Temporal distribution of the Old World hipparion genera measured crown height through time (upper and lower boundaries of the MN and MQ units are based on Hilgen et al., 2012). Leaf and grass symbols on the crown height distribution bars are represented with the estimated diet preferences of the genera coded with numbers. (B) Body mass estimates (in kg) of Old World hipparion genera (coded with colours) through MN and MQ units. (C) Mean ordinated hypsodonty curves of the Old World localities with and without Equidae, and hipparion bearing localities only through time. a greater degree of flexibility in feeding ecology than has usually been acknowledged (Rivals et al., 2015; Saarinen et al., 2021). The patterns of diet and body size of Old World hipparions can be summarized as follows (Figs. 11, 12 and 13): 1) Large body size evolved early during the evolution of the Old World hipparions (MN9–MN11) in the genera Hippotherium and Sivalhippus. Hippotherium had diets ranging from browsing to mixed-feeding. 2) During MN12–MN13 the diversity of Old World hipparions was at its highest, reflected in high body size range. Small species (e.g. Cremohipparion matthewi) were grazers while medium and large -sized taxa were mixed-feeders. Maximum, minimum and mean body size of Old World hipparions during the MN12–MN13 were lower than before or after. 3) During MN14, Old World hipparion diversity dropped. All the surviving lineages (Plesiohipparion, Proboscidipparion and Eurygnathohippus) had relatively large body sizes and they evolved into larger sizes during MN14–MQ1. Plesiohipparion and Proboscidipparion in Eurasia were mixed-feeders and occurred in a variety of habitats from shrublands or steppes to forests, suggesting ecological versatility enabled by flexible feeding adaptations, while the diets of Eurygnathohippus in Africa became increasingly grazing during the PlioPleistocene in response to the spread of C4 grassland environments during this time. All the Plio-Pleistocene Old World hipparion taxa (Plesiohipparion, Proboscidipparion and Eurygnathohippus) were relatively large-sized and increased in body size through time, but also many of the earliest species in Eurasia, including several species of Hippotherium and Sivalhippus, were of comparatively large sized. The smallest-sized species of the genus Cremohipparion, occurred in the circum-Mediterranean realm and Western Asia during the latter part of the Miocene (MN10–MN13). The highest body size variation occurred during MN11–MN12 in Eastern Mediterranean and East Asian hipparions, ranging from around 100 kg of small species of Cremohipparion to 200–300 kg of large species of Hippotherium and Shanxihippus. During MN 13 there was a decrease in body size in most of the Late Miocene hipparion taxa, after which only large-size species survived into the Pliocene (with the exception of Pliocene aged Cremohipparion licenti in East Asia). Previous studies have indicated that small size in Old World Late Miocene hipparionine equids is associated with populations that occupied relatively open environments. Scott et al. (2005) indicated, based on postcranial morphometrics, that forest populations of Hippotherium, such as the one from Höwenegg, Germany (MN9), had relatively large average body size, whereas populations in more open habitats, such as the one from Baltavar, Hungary (Hippotherium microdon, MN12), had smaller average body sizes. Moreover, Saarinen (2009) reported a negative correlation between mean ordinated hypsodonty of ungulate 18 R.L. Bernor et al. Earth-Science Reviews 221 (2021) 103784 communities and mean body size of Late Miocene Eurasian hipparions, indicating that hipparions were on average smaller in more open and arid environments. However, this pattern is driven by the abundance of small-sized hipparionine species in high-hypsodonty ungulate communities, rather than lack of large-sized species (Fig. 12). It is also evident that the smallest-sized hipparion species occurred in assemblages that had a high diversity of hipparion species. Based on mesowear analyses, the large-sized species of Old World hipparions (with body mass estimates exceeding 210 kg) had the most diverse diets, ranging from browse-dominated (some populations of Hippotherium primigenium and Cormohipparion sp.) to grazing (Eurygnathohippus hasumense), while medium-sized species (110–210 kg) were mostly mixed-feeders and the small-sized (with body mass estimates less than 110 kg) species of Cremohipparion (C. nikosi and C. periafricanum) were exclusively grazers (Fig. 12). Middle Miocene Cormohipparion from North America and Mexico, which are ancestors of the Old World hipparions, were of comparatively small size and had diets ranging from mixed-feeding to grass-dominated feeding (Fig. 12). Both the grazing African Eurygnathohippus and the mixed-feeding Eurasian Plesiohipparion and Proboscidipparion evolved into larger sizes during the Pliocene and Early Pleistocene. This pattern could reflect the effect of climatic deterioration on body size through several mechanisms that favored larger body size, such as benefits of large size under seasonally harsh conditions and lack of resources (e.g. Lindstedt and Boyce, 1985), or reduced plant defense mechanisms and thus increased diet quality and seasonally high productivity under decreased temperature and increased seasonality (Saarinen et al., 2014). Overall, the species richness of Old World hipparions increased steadily within the earliest late Miocene (MN9 equivalent), reaching a plateau during the MN10–12 interval, before peaking into the latest Miocene (MN13 equivalent; Fig. 13). Hipparion diversity became greatly reduced during MN13 and ultimately became further reduced at the Mio-Pliocene boundary (MN13/14 equivalent). The Early Pliocene, MN14, did not see a recovery in species diversity, but was in fact lower in the earliest Pliocene than at any time since MN9. Thereafter, there is a modest rise in hipparion diversity until the base of the Pleistocene (2.6 Ma), when it crashes in the wake of Equus entering Eurasia and undergoing its own aggressive radiation there (Cirilli et al., 2020). shown that the late Miocene was dominated by the evolution of Pikermian large mammal Old World Savanna Biome (OWSB). This evolution was provincial with Western Europe, Central Europe, Southeastern Europe-Southwest Asia, East Asia, South Asia, North Africa, East and South Africa evolving provincial faunas. During the 9–6 Ma interval, the Pikermian OWSB biome extended its geographic limits eastward into Asia, westward into Western Europe and Southward into Africa. While “mimicking” extant East African savanna mammal faunas, Pikermian mammal faunas evolved lineages that in many instances did not have African survivors past the late Miocene. Hence, African savannah large mammal faunas are convergent on Pikermian large mammal faunas. The Eurasian and African hipparion record exhibits a dynamic provincialism wherein lineages have restricted geographic areas of origin with some lineages remaining endemic, but others having undergone interprovincial to broad intercontinental-scale geographic extensions. Old World hipparions first appeared at the base of the late Miocene, 11.4–11.0 Ma with their ancestry securely rooted in a North American species of Cormohipparion of the C. occidentale group (Woodburne, 2007, 2009). Cormohipparion dispersed rapidly throughout Eurasia and Africa initially differentiating into higher latitude Eurasian Hippotherium with well identified species H. weihoense in China, H. primigenium in Europe and H. catalaunicum and H. koenigswaldi in Spain within the MN9 interval. Later MN9 records the first occurrence of Sivalhippus (S. nagriensis) in the Siwaliks, Pakistan, followed by an endemic radiation of the clade as well as extensions eastward into China (S. platyodus and S. ptychodus) and southward into Africa (S. turkanensis and S. macrodon). The later Vallesian (MN10) records the first occurrence of the Eurasian-North African lineage Cremohipparion (C. macedonicum) with several species evolving in Eurasia and North Africa during the Turolian and early Ruscinian. The Turolian (MN11–13), and especially the 9–6.8 (MN11–12) Ma interval for hipparions, witnessed an explosive radiation of additional hipparion lineages including Indian Subcontinent and African successors of Sivalhippus nagriensis Eurasian Cremohipparion, Hipparion s.s., Eurasian Plesiohipparion, African Eurygnathohippus and the endemic Chinese lineages of Shanxihippus and Baryhipparion. The end Turolian, MN13, records an initial phase of hipparion extinctions beginning 6.8 Ma reducing species diversity followed by a further extinction of the Hippotherium, Hipparion s.s., Sivalhippus and Shanxihippus lineages by the earliest Pliocene. Eurasian Plesiohipparion and African Eurygnathohippus survived into the Early Pliocene (MN14), the Chinese derived lineage Proboscidipparion originated and Cremohipparion survived only in China with the highly derived species Cremohipparion licenti (MN15 last occurrence). By MN16, only four hipparion lineages survived: Plesiohipparion, Proboscidipparion, Eurygnathohippus and the enigmatic Baryhipparion. Plesiohipparion and Proboscidipparion originated in Asia but succeeded in extending their range into West Asia and Europe. Eurygnathohippus originated in Africa by 7 Ma but made a brief appearance in the Indian Subcontinent at the end of the Pliocene, MN16 (Jukar et al., 2019). These four derived hipparion lineages co-occurred with first occurring Equus which appeared at the base of the Pleistocene, 2.58 Ma and persisted longest in E. Asia and Africa, up to about 1.0 Ma. Although Old World hipparions are descendants of medium-sized grazing North-American species of Cormohipparion, they became predominantly large-sized browse-dominated feeders and mixed-feeders during the early Late Miocene in Eurasia and Africa, such as Hippotherium. The browsing forest populations of at least the Hippotherium lineage had on average relatively large body sizes in comparison to more mixed-feeding populations in more open environments. Towards the latest Miocene, the proportion of grass-dominant feeders increased within the Old World hipparions, and the grazing species (especially within the genus Cremohipparion) were small-sized compared to browsedominated and mixed-feeding species. During the Pliocene and Pleistocene, the diversity of hipparions diminished, and body size increased in all the remaining lineages of Plesiohipparion, Proboscidipparion in Eurasia and Eurygnathohippus in Africa, probably in response to 7. Conclusions Traditionally, the genus Hipparion was broadly defined as horses with hypsodont cheek teeth, isolated protocones and tridactyl feet. Skinner and MacFadden (1977) initiated a revolution in hipparion systematics with the recognition of a new North American genus, Cormohipparion. Woodburne (2007) provided a detailed systematic revision of North American Cormohipparion and identified a California Cormohipparion sp. as the most likely source of the “Hipparion Datum”. Woodburne and Bernor (1980) extended inquiry into the Old World record recognizing 4 superspecific groups of Old World hipparions. Through a series of studies extending over 40 years time, numerous authors cited herein have expanded our understanding of the diversity of hipparions that occurred in the later Neogene and Pleistocene of Eurasia and Africa. Our contribution here chronicles the history behind our recognition of 10 superspecific groups of Eurasian and African hipparions documenting, to whatever extent possible, their geographic and chronologic ranges, climatic and paleoecological contexts. We believe that there are likely several other lineages of Old World hipparions yet to be identified that participated in the extensive evolutionary radiation that occurred between 11.4 and < 1.0 Ma. Late Neogene Eurasian and African large mammal faunal evolution was regulated by geographic heterogeneity and long-term climatic change. Bernor (1983, 1984), Bernor et al., 1996a, Fortelius et al. (1996), Eronen et al. (2009) and Kaya et al. (2018) have reconstructed the biogeography, paleoclimates and paleoecology of Old World large mammal faunal evolution in the later Neogene. These studies have 19 Earth-Science Reviews 221 (2021) 103784 R.L. Bernor et al. increasingly harsh and seasonal environments. Especially Proboscidipparion had a very wide distribution in Eurasia, and it occurred in a wide range of habitats. While the Eurasian lineages had mixed-feeding diets, Eurygnathohippus in Africa became purely grazing during the Pliocene and Pleistocene in response to the spread of C4 grasslands. The 63 hipparion species belonging to 10 genus-rank clades that we recognize here no doubt underestimates the diversity of Old World hipparion species and lineages. Our presentation here is limited to those clades that we have been able to recognize from our own studies and well documented studies of the same by other authors. 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The Höwenegg hipparionine horses: systematics, stratigraphy, taphonomy and paleoenvironmental context. Andrias 10, 1–230. Bernor, R.L., Armour-Chelu, M., Kaiser, T., Scott, R.S., 2003a. An Evaluation of the late MN 9 (late Miocene, Vallesian Age), Hipparion Assemblage from Rudabánya (Hungary): Systematic Background, Functional Anatomy and Paleoecology. Col. Paleo. Vol. Extra. 1, 35–46. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Bernor wishes to acknowledge research funding by NSF EAR grants 8806645, 0125009, 1113175, 1558586, DBI grant 1759882 for the FuTRES database, and support from HERC, U.C. Berkley, the Smithsonian Human Origins Program. We thank the many museums in Europe, Asia, Africa and North America that supported this study, measurement and imaging of their extant and fossil equid specimens: American Museum of Natural History (New York, USA), Natural History Museum (London, UK), Geological and Paleontological Museum (Florence, Italy), Institute of Vertebrate Paleontology and Paleoanthropology (Beijing, China), State Museums of Natural History (Karlsruhe and Darmstact, Germany), Nairobi National Museum (Kenya), Geological Institute (Budapest, Hungary), National Natural History Museum (Tehran, Iran), National Museum of Natural History (Paris, France), National Museum of Ethiopia (Addis Ababa), Natural History Museum (Vienna, Austria), National Museum of Tanzania (Dar Es Salam), Museum of Evolution (Uppsala, Sweden), South African Museum (Capetown), Senckenberg Museum (Frankfurt, Germany), State Museum of Natural History Stuttgart (Germany), Tianjin Natural History Museum (China). 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