Denis Geraads

Monte Mccrossin

Brenda Benefit

Bovidae (Mammalia) from the early Middle Miocene of Maboko, Kenya

Keywords: Bovidae, Miocene, Kenya, Biogeography

We describe here the Bovidae from the early Middle Miocene site of Maboko, Kenya, known for its rich and highly diverse assemblage of Primates. They are attributed to two taxa of slightly different sizes, Turcocerus africanus n.sp. and Kubanotragus pickfordi. In addition to Kenya, Turcocerus ranges through the Middle Miocene from Turkey to China; it might have originated in Africa but the lack of precise dating weakens any phylogeographic hypothesis.

Kubanotragus has similar chronological and geographic ranges. Both Maboko species retain primitively low levels of hypsodonty compared to their northern relatives and may have been less specialized for grazing than Eurasian taxa. The Maboko Bovidae illustrate an early diversification of the family in Africa that appears to pre-date the major diversification of Bovidae and appearance of the diverse clades that are characteristic of modern African pecoran faunas.

Introduction

Middle Miocene bovids from Maboko Island, Kenya are the oldest radiometrically dated Bovidae currently known, although they postdate molecular estimates for the origin of the family by at least 7 million years [START_REF] Chen | Large-scale ruminant genome sequencing provides insights into their evolution and distinct traits[END_REF]. Together with biostratigraphically dated fossils from Jebel Zelten, Libya [START_REF] Hamilton | The Lower Miocene ruminants of Gebel Zelten, Libya[END_REF], and Al-Sarrar and Ad Dabtiyah, Saudi Arabia [START_REF] Thomas | The lower Miocene fauna of Al-Sarrar (Eastern Province, Saudi Arabia)[END_REF][START_REF] Gentry | Ruminants from the Miocene of Saudi Arabia[END_REF], they document the early diversification of Afro-Arabian bovids, soon after their appearance at Arrisdrift, Namibia [START_REF] Morales | A new genus and species of Bovidae (Artiodactyla, Mammalia) from the early Middle Miocene of Arrisdrift, Namibia, and the origins of the family Bovidae[END_REF].

Historically, Maboko bovids were first discovered by Owen during the early 1930s and sold to the British Museum of Natural History. Precisely where on the island Owen collected his fossils is unknown. The excavation and mapping in place of 287 bovid fossils from intact Miocene sediments in three stratigraphic layers at Maboko Main by MM and BB from 1987 to 1997 allows us to make suggestions about the provenience of Owen's fossils (Benefit & McCrossin 1989;[START_REF] Geraads | A new rhinoceros, Victoriaceros kenyensis gen. et sp. nov., and other Perissodactyla from the Middle Miocene of Maboko, Kenya[END_REF]Arney et al. 2022: fig. 2). The general completeness, limited distortion and light color of the NHM specimens are consistent with fossils excavated from undisturbed Miocene strata at both the oldest fossiliferous stratum in the Maboko sequence Bed 3 (a yellow sandy clay) and Bed 5 white clay (5w) which occurs a meter higher in the stratigraphic column at the locality of Maboko Main. Horncore NHMUK M15543 in Owen's collection at the Natural History Museum in London has the same morphology as two of the most complete horncores excavated from intact Miocene yellow/green sand Bed 3 (KNM-MB 21849 and 35021), and horncore NHMUK M15544 closely resembles fairly complete horncores from both Bed 3 (KNM-MB 24892) and Bed 5w (KNM-MB 21504).

Provenience information for bovid fossils collected by Pickford between 1982 and1984 and by BB and MM follow Maboko locality and stratum designations established in the Kenya Palaeontology Gazetteer (Pickford 1984), with the additional distinction between upper (white clay) and lower (brown clay) levels of Bed 5 [START_REF] Benefit | Victoriapithecus, the key to Old World monkey and catarrhine origins[END_REF][START_REF] Retallack | Paleosols and paleoenvironments of the middle Miocene Maboko Formation, Kenya[END_REF]. Since 1982, 40 bovid specimens were collected from the surface of Maboko Main including from topsoil overlying intact Miocene sediments, and smaller collections come from Bed 6 surface exposures at the Maboko Cliffs locality immediately east of Maboko Main (n=1); Bed 12 at Maboko Southeast (n=7); and Beds 15-16 at Maboko Cliffs East (n=1). Incomplete locality information in the accession records of the National Museums of Kenya is available for 37 additional bovid specimens collected by Pickford, and for the 6 Maboko bovid specimens collected between 1932[START_REF] Thomas | The lower Miocene fauna of Al-Sarrar (Eastern Province, Saudi Arabia)[END_REF]by D.G. MacInnes (1933-34), L.S.B. Leakey (1949Leakey ( -1951) ) and Andrews and Pilbeam (1973). These expeditions are known to have focused their fossil collecting efforts at Maboko Main [START_REF] Andrews | The Miocene fossil beds of Maboko Island, Kenya: geology, age, taphonomy and palaeontology[END_REF], but it is not clear whether surface finds from other strata are included in the sample. Based on its morphology which differs from specimens excavated from Beds 3-5 at Maboko Main (see below), we suspect that KNM-MB 548 (field number 1934) collected by MacInnes may derive from a higher stratum .

Full provenience information is given in Supplementary Table 1 for the 350 Maboko bovid fossils that belong to the collections of the National Museums of Kenya (KNM) (Supplementary Table 1). This is especially important for the excavated material because the information is only recorded in field catalogs and on accession labels but not in the accession catalog of the KNM.

Radiometric dating of the Bed 8 tuff at Maboko Cliffs yields a 14.71±0. [START_REF] Artenay | [END_REF] Ma upper limit for the age of Bed 3-6 fossils [START_REF] Feibel | Age of the primate-bearing deposits on Maboko Island, Kenya[END_REF], but no absolute dates are yet obtainable for the base of the Maboko sequence. The presence of kenyapithecine apes, Victoriapithecus monkeys, choerolophodont gomphotheres, Climacoceras giraffoids, and listriodont suids in the lowest fossiliferous stratum at Maboko (Bed 3) suggests that the fauna is biostratigraphically more recent than c.17 Ma early Miocene deposits at Rusinga where these taxa are absent [START_REF] Pickford | Preliminary Miocene mammalian biostratigraphy for western Kenya[END_REF][START_REF] Andrews | The Miocene fossil beds of Maboko Island, Kenya: geology, age, taphonomy and palaeontology[END_REF]Peppe et al. 2009). Maboko Main is usually considered to be closer to 15 Ma and placed in Faunal Set III of the East African Faunal Zonation [START_REF] Pickford | Preliminary Miocene mammalian biostratigraphy for western Kenya[END_REF](Pickford , 1983)), but could theoretically be as old as 16 Ma. Bovids from Bed 12-16 are sandwiched between the Bed 8 Tuff and a phonolite which caps the Maboko Formation. Laser fusion 40 Ar/ 39 Ar dating of alkali feldspars of the Ombo phonolite at Maboko yields a youngest age of 13.8±0.04 Ma for fossils from Beds 9-20 [START_REF] Feibel | Age of the primate-bearing deposits on Maboko Island, Kenya[END_REF], the latter being the highest stratigraphic layer on the island (Pickford 1984). Although Maboko Bed 12 bovids are closer in age to the 13.7±0.3 Ma bovids from Fort Ternan [START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF][START_REF] Pickford | Refinement of the age of the Middle Miocene Fort Ternan Beds, Western Kenya, and its implications for Old World biochronology[END_REF], the presence of Climacoceras africanus rather than Climacoceras gentryi in Beds 12-16 indicates their fauna is less derived and somewhat older than Faunal Set IV at Fort Ternan.

Nearly two-thirds of the excavated fossils derive from Bed 3 sediment (n=208; 5.9% of the 3,510 identifiable mammal fossils from this stratum). The small number of Bed 3 bovid individuals (MNI=8 based on the number of left scaphoids, or 10 based on half of horncore sample) is represented by at least 29 different postcranial elements (Table 1) indicating that a small number of disarticulated skeletons is preserved in the stratum. This is consistent with other taxonomic groups from Bed 3 including kenyapithecine apes [START_REF] Mccrossin | Recently recovered Kenyapithecus mandible and its implications for great ape and human origins[END_REF], 1994, 1997;[START_REF] Mccrossin | 19 -Fossil evidence for the origins of terrestriality among Old World monkeys and apes[END_REF] and rhinocerotids [START_REF] Geraads | A new rhinoceros, Victoriaceros kenyensis gen. et sp. nov., and other Perissodactyla from the Middle Miocene of Maboko, Kenya[END_REF]. The presence of ooids and oncoids within Bed 3 indicate that the fossils accumulated within a moderately low energy beach ramp or littoral environment associated with a freshwater lake of unknown depth and size [START_REF] Watkins | The middle Miocene Maboko Island primate locality: new data and the integration and reinterpretation of existing data suggest the paleoenvironment was lacustrine[END_REF]. Bed 3 fossils tend to be well preserved and undistorted. The 49 bovid fossils (1.4% of the 3,494 Bed 5b fossils; MNI=3 based on left m3s) excavated from the 90 cm thick Bed 5 brown clay (Bed 5b) overlying the Bed 4 calcrete at Maboko Main are less well preserved with tooth enamel that is often corroded. Bed 5b has been interpreted as representing a riparian woodland or localized wetland forest because of its association with a Dhero paleosol [START_REF] Retallack | Paleosols and paleoenvironments of the middle Miocene Maboko Formation, Kenya[END_REF]. Bed 5b mudstone is replaced a meter higher in the sequence by Bed 5 white clay which is associated with a vertisol and Yom paleosol (Arney et al. 2022: fig. 2), the latter of which is interpreted as a moist wooded grassland laid down during a time when rainfall was seasonal [START_REF] Retallack | Paleosols and paleoenvironments of the middle Miocene Maboko Formation, Kenya[END_REF]. This interpretation is supported by the discovery of a fossil grass seed (Figure 1; B. Jacobs personal communication) within Bed 5 white clay matrix attached to excavated Victoriapithecus partial mandible KNM-MB 18736. At present the seed has not been identified to a specific taxon and we do not know if it is a C 3 or C 4 grass. Although herbivore enamel isotopic values do not indicate the presence of C 4 grass [START_REF] Arney | Herbivore isotopic dietary ecology of the middle Miocene Maboko Formation, Kenya[END_REF], its abundance in the diets of Maboko herbivores may be too low to detect. Of the fifteen bird taxa identified in Bed 5w, ten are aquatic (cormorants, bitterns, pelicans, storks, flamingos, herons, lily-trotters, ducks, gulls and river kingfishers) indicating the presence of lakes, rivers, marshes or swamps [START_REF] Retallack | Paleosols and paleoenvironments of the middle Miocene Maboko Formation, Kenya[END_REF]Mayr 2014a); one is a hoatzin typically associated with flooded forest (Mayr 2014b); and three others (bustards, hornbills, and houbaras) suggest proximity to open woodland or savanna [START_REF] Retallack | Paleosols and paleoenvironments of the middle Miocene Maboko Formation, Kenya[END_REF]Mayr 2014a,b;[START_REF] Mccrossin | Avifaunal contributions to the paleoenvironment of Kenyapithecus, Victoriapithecus and other middle Miocene primates from Maboko Island, Kenya[END_REF]. The presence of the gastropod Limicolaria in the stratum reflects a seasonal pattern of rainfall during the time it was deposited [START_REF] Pickford | Fossil land snails of East Africa and their paleoecological significance[END_REF][START_REF] Retallack | Paleosols and paleoenvironments of the middle Miocene Maboko Formation, Kenya[END_REF]. The 30 bovid fossils excavated from Bed 5w (1.8% of the 1,645 mammals identified; MNI =3 based on right P4s) include well preserved horncores, jaws, and postcranial remains.

Material and Methods

Most of the material is housed in the National Museums of Kenya, Nairobi (KNM), but a few other specimens were also examined in the Natural History Museum, London (NHMUK). Comparative material is housed in the Muséum national d'Histoire Naturelle, Paris (MNHN). Dental nomenclature follows [START_REF] Geraads | Pliocene Giraffidae (Mammalia) from the Hadar Formation of Hadar and Ledi-Geraru, Lower Awash, Ethiopia[END_REF].

Systematic palaeontology

Family Bovidae Gray, 1821 Subfamily indet.

Genus Turcocerus [START_REF] Köhler | Boviden des türkischen Miozäns (Känozoikum und Braunkholen der Türkei. 28)[END_REF] Type-species. Oioceros (?) grangeri [START_REF] Pilgrim | Two new species of sheep-like Antelope from the Miocene of Mongolia[END_REF] Diagnosis. Bovidae of small size; horncores rather short, inserted directly above the orbits, moderately inclined backwards, slightly convergent in their basal parts but diverging higher up, with a weak to moderate homonymous torsion, slightly compressed transversely, their base somewhat swollen and thicker than the pedicle, which is long. Skull with weak craniofacial flexion, broad across the orbits, which are large and deep.

Turcocerus africanus n. sp. Fig. 2 Holotype. KNM-MB 24892, left horncore.

Type-locality. Bed 3, Maboko Main, Kenya, but other specimens were excavated from Bed 5w sediments three meters higher in the sequence at the same locality.

Diagnosis.

A species of Turcocerus of small size, with horncores distinctly compressed transversely, long premolars with transverse cristids, and brachyodont mandibular molars compared to extant bovids with weak goat-folds and convex lingual walls.

Description

Horncores. The holotype horncore KNM-MB 24892 (Fig. 2a) is rather short (~8 cm long) and almost straight and conical rather than cylindrical, as its antero-posterior diameter quickly decreases upwards; the basal part looks somewhat inflated in medial or lateral view. It is uprightly inserted, directly above the orbit. In medial view, its posterior border makes an angle of about 80° with the frontoparietal plane, its anterior border an angle of about 65°. In front view, the horncore first approaches its counterpart for about 5 cm, then diverges from it, so that the upper half of the horncores were slightly divergent. The horncore also has a slight forward curve, and a slight anticlockwise torsion (so-called homonymous, because the right horncore describes a dextral spiral). The basal cross-section is oval, with a main axis slightly oblique in respect to the sagittal plane and distinctly compressed transversely (Table 1), with stronger curvatures of the cross-section outline anteriorly and posteriorly; these almost become keels higher up. The pedicle is rather long relative to the length of the horncore proper. This is the only specimen that unambiguously includes the mid-frontal suture; it shows that the frontal is rather flat between the horncores, and that the orbit extends far medially, its medial wall being at most 10 mm from the sagittal plane, if not closer still. There is no postcornual fossa.

Most other specimens resemble KNM-MB 24892. KNM-MB 21504 (Fig. 2b) is ~9 cm long, has no hint of keels, is more inclined backwards than the holotype, and its pedicle is longer. A posterior longitudinal groove occurs between two well demarcated ridges, similar to that of T. halamagaiensis (see Li et al. 2021: fig. 2), but appears to be more medially positioned on the African specimen. There is an inconspicuous, shallow postcornual fossa.

The supra-orbital foramen is quite small, located medially to the ridge that extends in front of the horncore. The orbit is as large as that of KNM-MB 24892. KNM-MB 27599 (Fig. 3a) cannot be precisely oriented. It has a distinct torsion and conical shape; the section is compressed but without keels. In addition, the horncore is deeply grooved with the same medially positioned posterior groove as seen in KNM-MB 24892. Its base is much thicker than the pedicle, which is very long. KNM-MB 27603 is very stout, distinctly inclined backwards, and has a quickly decreasing anteroposterior diameter; the pedicle is short, and not much less thick than the horncore. KNM-MB 20347 (Fig. 3b) has a similar backward inclination toward the tip which is broken off, and preserves a strong posterior groove as well as incipient anterior keel. KNM-MB 29562 is a small horncore, rather strongly inclined backwards; it might be from a juvenile individual, but the moderate compression and lack of keels differ from the tip of adult horncores, in contrast to what would be expected in this case. KNM-MB 33211 much resembles the holotype; it shows additionally that the frontal bone is hollowed by a small sinus anteriorly. A few other, incomplete specimens are of the same type.

On average horncores excavated from Bed 3 are larger than those from Bed 5w, although specimens from both strata overlap in size (Fig. 4).

We also assign to this species NHMUK M15544 (Gentry 2010, fig. 38.8) that includes only the dorsal part of the braincase and the incomplete right horncore. Behind the frontals, on each side of the parietal, a pair of temporal lines first converge towards the midline and then run more and more parallel to each other backwards; however, the distance between the two lines of each pair slightly increases posteriorly, so that the more medial lines meet before reaching the (missing) occiput, while the lateral ones do not. The horncore is obliquely inserted above the orbit, and slightly divergent from its counterpart. It is compressed transversely, was certainly quite short, and is not curved backwards, as the outline of its anterior and posterior borders suggest instead that its tip might have been slightly curved forwards. Thus, it resembles the previous specimens except for its absence of torsion or increase of divergence upwards. However, these differences might well result from a slight distortion of the fossil (the frontal and parietal bones are heavily cracked) and we regard them as insufficient for species distinction. [START_REF] Thomas | Les Bovidés miocènes des rifts est-africains: implications paléobiogéographiques[END_REF] assigned this specimen to an Eotragus close to E. sansaniensis (recte: E. clavatus), but Gentry (2010) tentatively identified it as Homoiodorcas, a genus based upon H. tugenium [START_REF] Thomas | Les Bovidés miocènes de la formation de Ngorora du Bassin de Baringo (Rift Valley, Kenya)[END_REF], whose holotype horncore, from the pre-Hipparion levels of Ngorora, is strongly curved and inclined backwards, and does not show the quick decrease of antero-posterior diameter of NHMUK M15544. We therefore reject Gentry's identification.

Due to the absence of association at Maboko between horncores and dentognathic and postcranial material, we cannot confidently assign any of this material to taxon. Although two subtly different dental morphs occur at Maboko (described below) they overlap in size and share a number of mandibular and dental traits that can be added to the T. africanus description. These include: premolar row long relative to molar row length with dimensions (Fig. 5) similar to N. gariepensis, Fort Ternan "Gazella", T. gracilis, and T. halamagaiensis; mandibular molars that are brachyodont with hypsodonty indices ranging from 0.75-1.04 based on m1-m2 metaconid height/crown length (Hall & Cote 2021 and our own measurements) and from 1.06-1.26 based on m3 metaconid height/crown width, with goat folds that are at most incipient and slightly convex lingual walls.

Comparisons

In their basal dimensions the horncores of T. africanus are similar to "Gazella" sp.

(KNM-FT 926 and 980) from Fort Ternan [START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF], and to larger specimens of both T. halamagaiensis (Li et al. 2021a, table 1) and E. clavatus (Fig. 4).

Horncores similar to those of Maboko were first described by [START_REF] Pilgrim | Two new species of sheep-like Antelope from the Miocene of Mongolia[END_REF] from the Middle Miocene (c. 14 Ma) of Tunggur (China), as "Oioceros (?) grangeri" and "O.(?) noverca". These species differ mostly in size and have short, stout horncores that resemble those of Turcocerus africanus, but are more circular in cross section. Associated teeth are distinctly high-crowned (especially so for Middle Miocene forms), the premolars are short (Fig. 5), upper teeth have flat labial walls and prominent styles, lower molars have a goat fold and flat lingual walls. [START_REF] Köhler | Boviden des türkischen Miozäns (Känozoikum und Braunkholen der Türkei. 28)[END_REF] erected the genus name Turcocerus for O.(?) grangeri.

From the Middle Miocene site of Çandır in Turkey, [START_REF] Köhler | Boviden des türkischen Miozäns (Känozoikum und Braunkholen der Türkei. 28)[END_REF], followed by [START_REF] Geraads | Ruminants, other than Giraffidae, from the middle Miocene hominoid locality of Çandır, Turkey[END_REF], described a few horncores similar to those of Turcocerus africanus. She called them T. gracilis. These horncores are more compressed than those from Maboko (Fig. 4), and we regard this as a valid difference at species level. In addition, at Çandır the premolar row is slightly shorter than molar row length and the molars are distinctly hypsodont, as at Tunggur.

The horncores of Turcocerus halamagaiensis (Ye, 1989) from the Middle Miocene of Botamoyin, China, whose teeth are also hypsodont, do not much differ from the Çandır ones (Li et al. 2021a). The species is also present in the Tabenbuluk area at Xishuigou, a site said to be of lower Miocene age (but see below). The horncores of Turcocerus kekemaidengensis [START_REF] Ye | A new species of Turcocerus from the Middle Miocene of the Northern Junggar Basin[END_REF]) from the late Middle Miocene are also similar [START_REF] Dmitrieva | Caprinae (Bovidae, Artiodactyla, Mammalia) from the Neogene of Mongolia[END_REF]Li et al. 2021a). Li et al. (2021a table 3) stated that these species differ from the Middle Miocene Eurasian genus Eotragus Pilgrim, 1939, in the presence of homonymous torsion, but the difference is far from clear, as torsion is absent in T. halamagaiensis (Ye 1989, pl. 1;Li et al. 2021a, fig. 2) while E. clavatus (Gervais, 1850) from Sansan, France, the type-locality of MN 6 biostratigraphic zone [START_REF] Made | Eotragus clavatus (Artiodactyla, Bovidae, Boselaphini) de Sansan[END_REF], may show a hint of incipient torsion (MNHN-Sa10799).

Other similar horncores have been described from Israel [START_REF] Tchernov | Miocene Mammals of the Negev (Israel)[END_REF] and Thailand [START_REF] Suraprasit | Middle Miocene bovids from Mae Moh Basin, Northern Thailand: first record of the genus Eotragus from Southeast Asia[END_REF]. Pseudoeotragus seegrabensis [START_REF] Made | The bovid Pseudoeotragus seegrabensis nov. gen., nov. sp. from the Aragonian (Miocene) of Segraben near Leoben (Austria)[END_REF] is similar to Eotragus clavatus, but the single horncore is more slender and the dentition more hypsodont.

Eotragus clavatus from Sansan (MNHN collections) further shares with Turcocerus africanus a skull that is broad across the orbits, salient orbital rims, a weak frontal flexure, temporal lines with an identical pattern, horncores inserted directly above the orbits, moderately inclined backwards, without backward curvature or torsion but with incipient outward and forward curvature. Most horncores from Sansan have no keel, but they may be present anteriorly (MNHN-Sa2494) or posteriorly (MNHN-Sa10804), so that the difference with Maboko is not great. Other differences are that E. clavatus is slightly smaller on the average, and that a sinus is probably always present in the frontal, as best shown by MNHN-Sa1183 and MNHN-Sa10799. Thus, although they are at similar evolutionary grades, E. clavatus shows a mixture of primitive and derived features compared to T. africanus; perhaps the clearest derived feature of the latter is the incipient torsion. Eotragus clavatus horncores are smaller on average than those of T. africanus, but its mandibular premolar and molar row lengths are larger than T. africanus, its dimensions overlapping in size with a single Bed 12 specimen that we currently cannot assign to genus (Fig. 5; see below).

Earlier forms of Eotragus, E. noyei [START_REF] Solounias | The oldest bovid from the Siwaliks, Pakistan[END_REF]E. minus Ginsburg et al., 2001, both from Pakistan, as well as Namacerus gariepensis Morales et al., 2003, from the early Middle Miocene of Namibia, have smaller and shorter horns (Figs. 4 and6) and the latter smaller teeth than T. africanus. The braincase of N. gariepensis resembles that of NHMUK M15544 in the course of the temporal lines.

Genus Kubanotragus Gabunia, 1973 Type-species. Kubanotragus sokolovi Gabunia, 1973 Diagnosis. A genus of small Bovidae with protruding large orbits, paired temporal lines, horncores inserted vertically above the middle of the orbits, long and slender, remarkably straight, almost parallel, with slight to moderate transverse compression, and with at most an incipient homonymous torsion. (Thomas, 1984) Fig. 7 1984 Nyanzameryx pickfordi Thomas, 1984a Holotype. NHMUK M15543, partial cranium with partial horncores.

Kubanotragus pickfordi

Type-locality. Maboko; the specimen was purchased from Archdeacon Owen, and likely came from Bed 3, the only stratum in which similar horncores are found.

Diagnosis. Braincase rounded, basioccipital lacking paired tuberosities, but with a central ridge flanked by depressions on either side. Cheek-teeth with low hypsodonty, long premolar to molar row length, lower premolars with oblique talonid cristids, and metastylid restricted to the top of the crown in the molars.

Description

Horncores. The holotype was described by Thomas (1984a) as a member of the Giraffoidea, but as previously noted by McCrossin et al. (1998: p. 379), [START_REF] Morales | A new genus and species of Bovidae (Artiodactyla, Mammalia) from the early Middle Miocene of Arrisdrift, Namibia, and the origins of the family Bovidae[END_REF] and Gentry (2010), it is in fact bovid as a clear distinction can be observed between the pedicle and the horncore proper. Besides this, not much can be added to Thomas' description. The horncores are slightly divergent in front view, and the partly preserved left one is virtually straight and apparently lacks torsion but close examination shows that it is partly reconstructed. The temporal lines are identical to those of NHMUK M15544, assigned above to Turcocerus africanus.

The almost complete right horncore KNM-MB 21849 (Fig. 7a) resembles the holotype in being uprightly inserted above the orbit, slightly compressed transversely, and long and slender; it fully lacks torsion. The length of the broken horncore is 12 cm, but is estimated to have been close to 22 cm long by projecting its anterior and posterior profile vertically. It differs in being slightly larger, and in that the frontal is narrower between the orbits and between the horncores, so that the orbits certainly approached each other more closely near the midline. KNM-MB 35021 (Fig 7B) is still more complete, and shares the same characters including tall height (~16.2 cm; Fig. 6).

These specimens resemble Turcocerus africanus in being of similar size, and in having a long pedicle, a faint postcornual fossa, a flat frontal, a large, deep orbit, paired temporal lines of similar course, and horncores that are slightly divergent upwards. They differ in having a less transversely compressed basal cross-section fully lacking keels, in being only slightly inclined backwards (raising almost vertically), in the absence of torsion, and in being long, slender, and almost cylindrical rather than conical (the diameters only slowly decrease upwards).

Comparisons

The shape of the basioccipital of Kubanotragus pickfordi is unlike that of most bovids, in which there are well-marked anterior tuberosities, replaced here by a pair of depressions on either side of a sagittal crest, but such a primitive morphology is also close to the one found in Namacerus [START_REF] Morales | A new genus and species of Bovidae (Artiodactyla, Mammalia) from the early Middle Miocene of Arrisdrift, Namibia, and the origins of the family Bovidae[END_REF] and probably Eotragus (Made 2012, and our observations), and it can be expected in early bovids. From the slightly younger site of Nyakach, Thomas (1984a) correctly assigned to the same species a horncore KNM-NC 7973. However, the mandible KNM-NC 7816 that he also assigned to N. pickfordi is much too large to go with this horncore; the hypoconid and entoconid remain isolated until late wear, as in the similarsized Bed 12 bovid KNM-MB 25148.

From the Middle Miocene of Belometchetskaya in Georgia, [START_REF] Sokolov | On the remains of Cavicornia (Bovidae, Mammalia) from the middle Miocene of Northern Caucasus[END_REF] described as Eotragus (?) martinianus (Lartet, 1851) three horncores, of which he figured one (Sokolov 1949, fig. 2), obviously of the same species as the holotype horncore of Kubanotragus sokolovi Gabunia, 1973, later described from the same locality. These horncores are so similar, in their remarkable straight, cylindrical shape to KNM-MB 21849 and KNM-MB 35021 from Maboko that genus identity is hardly questionable. Thomas (1984b) also reported K. sokolovi from Cheskewala in the Chinji of Pakistan. [START_REF] Hamilton | The Lower Miocene ruminants of Gebel Zelten, Libya[END_REF] assigned to Eotragus the horncores NHMUK-M26688 and NHMUK-M26689 from Jebel Zelten; they resemble Kubanotragus but are more inclined backwards.

We assign them to Kubanotragus ? sp.

The horncores of Kubanotragus gaopoensis [START_REF] Chen | Discovery of the genus Kubanotragus (Bovidae, Artiodactyla) from the Middle Miocene in Lantian District, Shaanxi Province, China[END_REF], from Gaopo, Lantian, are almost identical with those of K. sokolovi (the apparent difference in size might be due to the fact that Gabunia's horncore measurements were taken on the pedicle: Thomas 1984b), but associated dentitions much differ from KNM-MB 9204 and KNM-MB 20350 in their short premolars and hypsodont molars. From Thymiana on the Greek island of Chios, Bonis et al. No horncore from Çandır closely resembles Kubanotragus. By contrast, from the nearby site of Inönü that they regarded as somewhat younger than Çandır, [START_REF] Geraads | Middle Miocene Ruminants from Inönü, Central Turkey[END_REF] reported two types of horncores: larger and spiraled (similar to Turcocerus), smaller and straighter (similar to Kubanotragus), showing the co-existence of the two genera.

To sum up, a clear distinction can be made between Kubanotragus, present at Jebel Zelten, Nyakach, Thymania, Inönü, Cheskewala, and Gaopo, and Turcocerus present at Çandır, Inönü, Botamoyin (Ye 1989), Kekemaideng Fm [START_REF] Ye | A new species of Turcocerus from the Middle Miocene of the Northern Junggar Basin[END_REF]Li et al. 2021a), Ulan-Tologoi [START_REF] Dmitrieva | Caprinae (Bovidae, Artiodactyla, Mammalia) from the Neogene of Mongolia[END_REF], and Tunggur [START_REF] Pilgrim | Two new species of sheep-like Antelope from the Miocene of Mongolia[END_REF]).

Genus Indet.

The dimensions of one Maboko bovid horncore, KNM-MB 13827, are larger than expected for either T. africanus or K. pickfordi based on variation observed in other Middle Miocene bovid species (Fig. 4). This specimen is close in size to H. tanyceras from Fort Ternan, though it is more elongated relative to thickness than in the Fort Ternan taxon. It is a very badly eroded and broken specimen, preserving no significant morphological details. As a surface specimen, its provenience and age are less secure than the excavated material. Its identity awaits the discovery of more complete horncore specimens of the same size.

Dentognathic material

One partial maxilla with mandible, and five additional partial mandibles with teeth are preserved at Maboko. The only maxillary specimen KNM-MB 9204 from Bed 6 at Maboko Cliffs East preserves P4-M3 and its associated partial mandible p3-m3 but only the upper half of the corpus below these teeth. Upper and lower first molars are heavily worn but third molars show moderate wear. The KNM-MB 9204 P4 is about as bucco-lingually broad as M1 (Fig. 8a), and has a flat lingual surface between robust styles. Premolars are rather large relative to the molars, with an estimated lower Pm/M index of c. 66 (Table 1). The upper molars also have rather strong styles, a flat metacone, but a distinctly inflated paracone buccal Of the isolated upper molars, upper M3 KNM-MB 21536 (Fig. 8b) from Bed 3 (Table 2) likely represents a different species from KNM-MB 9204. The M3 is larger than molars of KNM-MB 9204, It is wider across the mesial than distal cusps due to greater lingual expansion of the metaconule, has more accentuated metastyle, mesostyle and parastyle, and a slight hint of a basal pillar which are not apparent on molars of KNM-MB 9204. This specimen might belong to the same taxon as KNM-MB 25148.

The two most complete mandibles were excavated from Bed 5w which thus far has produced only T. africanus horncores. They are of similar size, and differ only in subtle details of dental morphology. KNM-MB 21459 (Fig. 9B and10B) with left p4-m2, which preserves the symphysis to corpus below m2. Anteriorly the part of the corpus between p2 and estimated canine root is mesio-distally long (33 mm; 143% as long as the premolar series) and supero-inferiorly shallow (9.4 mm; 43% as deep as the ramus below m2). The mental foramen is long (7.5 mm). The corpus deepens posteriorly below p4 (6.4 mm), m1 (18.3 mm) and m2 (21.9 mm), making it different from Fort Ternan specimens assigned initially to "Gazella" sp. [START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF]: plate 15 KNM-FT 64.361), but similar to the larger "H". tanyceras in shape (Gentry 1970: plate 9). KNM-MB 21459 is also similar to the larger T. grangeri [START_REF] Pilgrim | Two new species of sheep-like Antelope from the Miocene of Mongolia[END_REF]: fig. 7) which has even longer diastema relative to premolar row (207% as long) that is supero-inferiorly shallow and a corpus that deepens posteriorly [START_REF] Pilgrim | Two new species of sheep-like Antelope from the Miocene of Mongolia[END_REF]: fig. 7). The premolar row of KNM-MB 21459 (23 mm) is also long (66% of estimated molar row length [35 mm]), plotting close to N. gariepensis, T. gracilis, T. halamagaeinsis and Fort Ternan "Gazella" (Fig. 5). Premolar row length is relatively shorter in other species attributed to Turcocerus and in other bovids from Fort Ternan. The p4 of KNM-MB 21459 is minimally worn but its mesiobuccal corner appears to be damaged by interstitial wear, obscuring details of the anteparacristid which is visible from the lingual perspective where it is separated from the paraconid. The p4's metaconid is tall with a steep anterior wall that extends close to the cervix, and is equal in height and crown area to the protoconid. The lingual side of the protoconid does not expand into the large gap between paraconid and metaconid, connected to the protoconid by the transverse cristid. The metasinusid gap occurs between the slightly oblique transverse cristid and entocristid. The telosinusid is enclosed by the intersection of lingual ends of the entocristid and hypocristid.

The entoconid is shorter than the metaconid and of equal height to the hypoconid which has a more expanded crown than the former. The slight to moderately worn m1 and very slightly worn m2 are brachyodont with a hypsodonty index (H/L) of 0.7 for m1 and 0.75 for m2.

Buccally, the m1 and m2 of this specimen have well developed basal pillars (ectostylids) in the median buccal cleft, somewhat wrinkled buccal enamel, and a hint of a goat fold.

Lingually, the crowns are slightly convex. An unworn right m1 in mandible fragment, KNM-MB 7882 (Fig. 9E,10E) is similar in having a basal pillar, buccalward expansion of the hypoconid, and similar buccal and lingual contours as KNM-MB 21459. Its lingual cusps rise high (~3 mm) above crown height at intersection of the postmetacristid and preentocristid (5 mm). KNM-MB 20350 from Bed 5w (Figs. 9A,10A) is a left mandible preserving m1-m3, premolar alveoli, the corpus distal to the mental foramen and anterior to the superior aspect of the anterior-most ramus behind m3. Only the upper half of the corpus below m2-3 is preserved. The specimen is minimally larger than KNM-MB 21459 with a similarly long premolar relative to molar row length (Fig 5). The corpus is shallow anteriorly (9 mm) but deepens below m1 (21 mm). Molars of KNM-MB 20350 differ from KNM-MB 21459 in being somewhat narrower bucco-lingually relative to length (Fig. 11; L/W=1.52 for m1 and 2) compared to KNM-MB 21459 (L/W=1.41 for m1 and 1.42 for m2); having buccal cusps that are more columnar and vertically oriented with slightly less basal expansion in width compared to KNM-MB 21459; absence of m1 and m2 basal pillars though present on m3; smoother enamel buccally; and in being slightly more hypsodont (m2 H/L=0.84. m3 H/W=1.26). Yet these differences can occur within a single species. The relatively small m3 hypoconulid is positioned on the buccal side and separated from the entoconid by a strong entostylid distal to which the hypoconulid crown is concave with the curved prehypoconulid cristid and from the occlusal perspective creating a C-shaped ridge toward the entostylid from a furrow created by the concave lingual side of the crown due to cusp compression, a morphology also found on isolated m3 KNM-MB 20349. A similar m3 hypoconulid configuration is seen in several Miocene taxa including T. grangeri [START_REF] Pilgrim | Two new species of sheep-like Antelope from the Miocene of Mongolia[END_REF]: Fig. 7) and Fort Ternan "Gazella" [START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF]:plate 16-3) but not H. tanyceras [START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF]: plate 9-3).

The precise provenience on Maboko is not known for a third partial mandible that appears to have been collected in 1934, KNM-MB 548 (Figs. 9F, 10F;MB 1934in Whitworth 1958[START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF]. It preserves corpus depth below a highly worn and weathered dp4 and less worn m1-2 (Whitworth 1958; Gentry 1970; MM). Dimensions of the m1 and m2 do not fit perfectly within the Maboko Main sample, Length and Width dimensions for the m1 plotting with smaller m2s from Maboko Main and for the m2 plotting between Maboko Main m2s and m3s (Fig. 11). Both m1 and m2 of KNM-MB 548 lack basal pillars. Corpus depth below dp4 (15) and m1 (16 mm) In comparison, hypsodonty indices for Maboko Main 2s range from 0.75-0.92 (H/L, Hall & Cote, 2021, this study). KNM-MB 21666 (Fig. 9G), an unworn m3 from Bed 3 that is more elongated than most Maboko Main, has an intermediate hysodont index ( H/W=2.52) which is more similar to extant Tragelaphus buxtoni and T. euryceros, Given that the more advanced level of hypsodonty on KNM-MB 548 is not observed in the excavated sample from Maboko Main, we suggest that the specimen may have been collected from a higher stratum that younger than 14.7 Ma and possibly closer to 13.8 Ma in age. For now this remains an unproven hypothesis. KNM-MB 25148 from Bed 12 (Fig. 9D)is a left partial mandible with p3-m2 and part of m3. We refer to it as bovid indet. because its teeth are larger than all but one excavated specimen from Maboko Main (inferred m2 KNM-MB 29589 from Bed 3), and exceeds the range of variation observed in other middle Miocene species . Its molar row length is larger by 8 mm than any other Maboko specimen, as is its estimated premolar length (Fig. 6). With the exception of inferred m2 KNM-MB 29589 (Bed 3), m1 and m2 of this specimen are wider than other Maboko molars, and their length is greater than comparable elements . Although worn and weathered, the p4 is more elongated and complex anteriorly than either KNM-MB 21459 or 9420. The paraconid is expanded, partly filling the mesosinusid. The premolar talonid cristids are oblique and close off the telosinusid as in KNM-MB 21459, with a worn metaconid that appears to be smaller than the protoconid. On molars the buccal sinusid is deep and the entoconid remains isolated from the hypoconid until late wear. The metaconid of m1 is higher (4 mm) above intersection of postmetacristid and preentocristid than is crown height below this point (4 mm) than any other Maboko specimen including KNM-MB 7882.

Sorting the dentognathic assemblage into species is confounded by the lack of direct association between horncores and teeth, and the small number of maxillae and mandibles.

We do not feel confident assigning all the teeth and mandibles from Bed 5w to T. africanus, although all of the bovid horn cores from the stratum belong to this taxon and it is a possibility. T. africanus is more common (n=13) than K. pickfordi (n=3), making it more likely that most of the dental specimens belong to this taxon. The small sample of lower molars from Bed 3 is especially confusing because it includes one of the smallest m1s (KNM-MB 24927) and largest m3s (KNM-MB 21912 and 21666) from Maboko, along with both large and average sized m2s (Table 2). We are even less confident about assigning the larger teeth to K. pickfordi because its horn cores are smaller.

Postcranial material

Bovid postcrania from Maboko are not part of articulated specimens and therefore are more appropriate for ecomorphological than the taxonomic analysis that is the focus of this paper. At least 105 postcranial remains were excavated from Bed 3, consisting of 41 bones from 9 different elements of the upper limb, 43 bones from 11 different elements of the lower limb, 3 types of phalanges and 3 vertebras. In comparison, 13 bovid postcrania were excavated from Bed 5b and 15 from Bed 5w, and an additional 22 specimens were collected from the surface of Maboko Main. Of these, vertebral axis KNM-MB 27496 (Bed 3) is definitely more like the Fort Ternan second type of Gentry (1970 pl. 16, Fig. 7, bottom), but he did not identify it to taxon. A femur KNM-MB 24906 (Bed 3) lacks the cursorial characters of a notch between trochanter and articular head, very unequal lips of the distal trochlea, and deep supra-condylar fossa. Twenty-three astragali from Maboko are attributed to Bovidae with 13 from Bed 3, 2 from Bed 5b, 3 from Bed 5w, and 5 from the surface of Maboko Main. KNM-MB 21587 (Bed 3; Fig. 12A) is similar in size to those of the tragulid Dorcatherium chappuisi,. The majority are small and homogeneous in size, but their size range (medial height = 18.8 -23.8, distal width = 11 -14.7) would also be compatible with two species of slightly different sizes. However, KNM-MB 25378 from the surface Maboko Main is the size of a large gazelle astragalus. The foot exhibits primitive cursorial adaptations, although none occur in the femur. KNM-MB 22077 (Bed 5w), a left MC III/IV, has length (180 mm) and proximal medio-lateral breadth (21 mm) dimensions similar to those of the Eritrean gazelle Gazella tilonura, and relative breadth to length proportions (11.7) most similar to Gazella gazella, Antidorcas marsupialis, and Pelea capreolus (Scott 1985).

Although the MC III/IVs of Aepyceros melampus is slightly broader relative to length, it shares with KNM-MB 22077 a proximal end with the greatest antero-posterior thickness positioned asymmetrically and just medial to the sagittal midline; a magnum-trapezoid facet that is antero-posteriorly intermediate in length. Other partial metapodials, such as KNM-MB 21661 (Bed 3; Fig. 12E) are somewhat smaller.

Paleoecology

A browsing diet for the Maboko bovids is suggestedby their small body size [START_REF] Gagnon | Dietary preferences in extant African Bovidae[END_REF] which is estimated to have been 22-26 kg [START_REF] Mihlbachler | Body size distribution and habitat structure of Maboko Island, middle Miocene, Kenya[END_REF], long diastema with shallow mandibular corpus below [START_REF] Wang | Geometric morphometrics of mandibles for dietary differentiation of Bovidae (Mammalia:Artiodactyla)[END_REF], and generally low levels of molar hypsodonty that are nonetheless higher than observed in Maboko tragulids and giraffoids [START_REF] Hall | Ruminant mesowear reveals consistently browse-dominated diets throughout the early and middle Miocene of eastern Africa[END_REF]this study). Enamel carbon isotope values are consistent with a browsing diet for Maboko bovids [START_REF] Arney | Herbivore isotopic dietary ecology of the middle Miocene Maboko Formation, Kenya[END_REF]. Of the bovid dental sample, carbon values for the largest teeth from Maboko Main are the least negative of all the species sampled, indicating that they foraged in the most open canopy woodland habitats preserved at Maboko. Coincidently, the highest hypsodonty values observed for the Maboko bovids are from the largest m3s KNM-MB 548 and 21666. Enamel carbon isotope value for Maboko and Fort Ternan bovids overlap significantly (Arney et al. 2022: fig. 6), as do their levels of hypsodonty [START_REF] Hall | Ruminant mesowear reveals consistently browse-dominated diets throughout the early and middle Miocene of eastern Africa[END_REF]. Carbon values for the largest Maboko bovid overlap those of H. tanyceras which also has the least negative values of the Fort Ternan bovids, and values for other Maboko bovids overlap "Gazella" from Fort Ternan. Environmental inferences from isotope values are more difficult to make for middle Miocene sites because although C 4 grasses were present in Africa 15 million years ago [START_REF] Morgan | Carbon isotopic evidence for the emergence of C4 plants in the Neogene from Pakistan and Kenya[END_REF], they were not as abundant or widespread as they would later become and are more difficult to detect (Retallack et al. 1992;[START_REF] Cerling | Dietary and environmental reconstruction with stable isotope analysis of herbivore tooth enamel from the Miocene locality of Fort Tenan, Kenya[END_REF]. Carbon isotope values do not differentiate between the very different C 3 dominated environments represented at Maboko, Fort Ternan, and Langebaanweg (Arney et al. 2022: fig. 6), or between the different environments represented in the Maboko Main sequence (Arney et al. 2022: fig. 3, 5). The presence of grass at Maboko (this study) and Fort Ternan is undisputed [START_REF] Retallack | Fossil soils and grasses of a middle Miocene East African grassland[END_REF][START_REF] Dugas | Middle Miocene fossil grasses from Fort Ternan, Kenya[END_REF]), but whether Fort Ternan grasses used a s C 4 photosynthesis pathway as suggested by their taxonomy, or a C 3 pathway as indicated by soil and herbivore enamel isotope signatures, is hotly debated [START_REF] Cerling | Fossil soils, grasses and carbon isotopes from Fort Ternan: grassland or woodland[END_REF][START_REF] Cerling | Dietary and environmental reconstruction with stable isotope analysis of herbivore tooth enamel from the Miocene locality of Fort Tenan, Kenya[END_REF]Retallack 1992). Either way, at least one of the Maboko gomphotheres is documented to have been a grass-dominated mixed feeder based on macro and microwear analysis of molar teeth [START_REF] Saarinen | The ecology of browsing and grazing[END_REF][START_REF] Saarinen | Aridity, cooling, open vegetation, and the evolution of plants and animals during the Cenozoic[END_REF]. It is possible that Middle Miocene bovids at Maboko and Fort Ternan could have consumed C 3 or C 4 grasses as a fallback food during seasons when browse was limited. Today the C 3 grass Phalaris arundinacea is found in wetlands of North Africa (and much of the northern hemisphere) where it provides hay for mammals and seeds for birds.

Bovids are important markers distinguishing early from middle Miocene faunas in Africa. Based on present evidence, their numbers and diversity in terms of body size and horncore morphology increase from the earliest to latest middle Miocene in Africa. Bovids are far less abundant at Maboko (total NISP=352 and MNI=17) than at Fort Ternan (NISP=3,165 and MNI=250;[START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF]. Although bovids persist throughout the Maboko sequence from Bed 3 to Bed 15, they are rare compared to other mammalian groups (5.9% of Bed 3, 1.4% of Bed 5b, and 1.8% of Bed 5w fauna). In contrast, they are the most dominant mammal at Fort Ternan where they comprise 64.4% of the mammalian fauna [START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF]Shipman et al. 1981;Arney et al. 2022: table 4). This difference is in part due to depositional environment. Deposits at Maboko are associated with large expanses of open water including lakes (Bed 3), densely wooded wetlands (Bed 5b), and more open woodland associated with seasonal rainfall and the presence of grass (Bed 5w) [START_REF] Retallack | Paleosols and paleoenvironments of the middle Miocene Maboko Formation, Kenya[END_REF][START_REF] Arney | Herbivore isotopic dietary ecology of the middle Miocene Maboko Formation, Kenya[END_REF]).

These environments were preferred primate habitats that supported large populations of the semi-terrestrial monkey Victoriapithecus macinnesi (24% of the mammal fauna of Bed 3; 40% of Bed 5b, and 32.7% of Bed 5w) and large-bodied ape Kenyapithecus africanus (3.4% of Bed 3, 3.8% of Bed 5b and 1.2% of Bed 5w mammals) [START_REF] Mccrossin | Maboko Island and the evolutionary history of Old World monkeys and apes[END_REF], 1997;[START_REF] Mccrossin | 19 -Fossil evidence for the origins of terrestriality among Old World monkeys and apes[END_REF], as well as highly arboreal dendropithecids (3.5% of Bed 3, 2.2% of Bed 5b, and 0.9% of Bed 5w mammals) and nyanzapithecine apes (only found in Bed 5b where they comprise 5% of the mammalian fauna) [START_REF] Mccrossin | An oreopithecid proximal humerus from the Middle Miocene of Maboko Island, Kenya[END_REF][START_REF] Mccrossin | Proximal humeral evidence for partitioning of locomotor substrates by four catarrhine species from the middle Miocene of Maboko Island, Kenya[END_REF]. In contrast, Fort Ternan depositional environments are strongly tied to volcanic activity along the flanks of the Tinderet volcano and are thought to be associated with more open woodland and grassland environments that supported far fewer primates (3% of the total fauna based on MNI) than Maboko and no cercopithecoid monkeys (Shipman et al. 1981;Arney et al. 2022: table 4). At Fort Ternan, tragulids (1.56% of the fauna) and giraffoids (3.7% of the fauna) are as rare as the primates, but at Maboko the smaller Dorcatherium pigotti and larger Dorcatherium chappuisi (estimated body mass 7-9 kg. for the former and 26 kg for the latter; [START_REF] Mihlbachler | Body size distribution and habitat structure of Maboko Island, middle Miocene, Kenya[END_REF]) make up 9.9% of Bed 3 mammal specimens, 27% of Bed 5b, and 14% of Bed 5w. Tragulid teeth are adapted to consume softer, less fibrous items than those of bovids and giraffoids, and their limbs for movement in more densely wooded or forested environments (Geraads 2010: fig. 36.1). Together tragulids and primates make up a combined 78% of the mammalian fauna in Bed 5b based on NISP, but only 40% of the fauna where primates and tragulids are the least common and make up only 40% of the mammalian fauna (Bed 3), a pattern similar to rhinocerotids [START_REF] Geraads | A new rhinoceros, Victoriaceros kenyensis gen. et sp. nov., and other Perissodactyla from the Middle Miocene of Maboko, Kenya[END_REF].

Conclusions

The Bovidae material from Maboko consists mostly of isolated teeth and postcranial remains, but includes no taxonomically significant cranial part besides horncores, and only a few frontals, mandibles and tooth-rows. Horncores allow for their unambiguous identification to Turcocerus and Kubanotragus, genera originally named on the basis of Miocene material from Eurasia. This has important biogeographic implications because each genus is dispersed over three continents (Table 3, Figure 14). Whether the Maboko bovids evolved from precursors in Africa or Eurasia is an open question, as primitive bovids are known from both regions. Miocene evidence for incipient horncores in ruminants is slender. Li et al. (2021b) described as Amphimoschus a partial cranium from Xishuigou showing a short supraorbital process. The Xishuigou fauna is said to be in the range 17-19.7 Ma [START_REF] Wang | Neogene Mammalian Biostratigraphy and Geochronology of the Tibetan Plateau[END_REF], an age that would fit the evolutionary stage of this cranium, but it also has Turcocerus halamagaiensis, a species defined in the significantly younger Halamagai fauna (Ye 1989), which makes such an early age unlikely (we may note that its Platybelodon dangheensis would also be the earliest of its genus). Thus, it could in fact be similar in age to Jiulongkou from where [START_REF] Chen | Miocene mammalian fossils from Jiulongkou, Cixian, Hebei Province[END_REF] described as Oioceros (?) jiulongkouensis an adult cranium with very small supraorbital protuberances and a strong sagittal crest, as in Amphimoschus. It is conceivable that these forms gave rise to proto-Eotragus forms ("Eotragus" noyei, "Eotragus" minus, and Namacerus), whose horncores are larger but remain smaller than in European primitive Eotragus, reported from the Spanish sites of Buñol, Córcoles, and Tarazona [START_REF] Alférez | Primera cita en España del antílope más antiguo de Europa, Eotragus artenensis Ginsburg y Heintz, 1968 (Mammalia, Bovidae), procedente del Orleaniense de Córcoles[END_REF][START_REF] Moyá-Solá | Los Boselaphini (Bovidae, Mammalia) del Neógeno de la Península Ibérica[END_REF][START_REF] Astibia | Los macromíferos del Mioceno medio de Tarazona de Aragón (Depresión del Ebro, provincia de Zaragoza)[END_REF][START_REF] Mazo | Fauna y bioestratigrafia del yacimiento aragoniense de Montejo de La Vega de La Serrezuela (Segovia)[END_REF]. Further increase in size led to Eotragus clavatus, well-known from Sansan [START_REF] Made | Eotragus clavatus (Artiodactyla, Bovidae, Boselaphini) de Sansan[END_REF].

Potentially, minor changes could easily have led to Turcocerus: some thickening of the horncore base, homonymous twisting, and broadening of the anterior part of the basioccipital, although this part of the skull is not preserved before the age of Tunggur. We may surmise that the sagittal crest, present at Xishuigou and Jiulongkou was lost in derived forms (Sansan, Seegraben), but reappeared at Tunggur in T. grangeri, perhaps because of its large size.

Alternatively, proto-Eotragus forms, rare but widespread from Namibia to China, could have given rise by vicariance in Eurasia to Eotragus (first definitely known from Spain), and in Africa to Turcocerus (first represented there by Turcocerus africanus) before its dispersal to Eurasia. This hypothesis looks more satisfactory because no definite Eotragus is known in Africa, but is weakened by the lack of evidence of the genus in Africa earlier than in Eurasia.

The affinities of Kubanotragus are unclear, because its remains are restricted to teeth and horncores; it is perhaps more closely related to Hypsodontus, best illustrated by H. pronaticornis from Çandır, Paşalar [START_REF] Gentry | Ruminant artiodactyls of Paşalar, Turkey[END_REF], and Prebreza [START_REF] Pavlovic | Miozän-Säugetiere des Toplica-Beckens -paläontologischstratigraphische Studie[END_REF], than to Turcocerus, even though Hypsodontus is larger. These three genera are often regarded as members of a Subfamily Hypsodontinae, but this view is questionable, and we prefer not to follow it, because the shared characters of these early bovids: upright horncore insertion, absence of backward curvature or heteronymous torsion, absence of cranial flexure, shape of the basioccipital, etc., are probably just primitive.

The dental remains of these forms tell an interesting story. All Eurasian dentitions associated with Turcocerus, Kubanotragus, and Hypsodontus are hypsodont for their geological age, and have flat lingual walls, goat folds, and shortened premolars, and are thus clearly adapted for grazing. The single mandible from Jebel Zelten (NHMUK M26685) displays similar features. By contrast, in dentitions from Maboko, the molars are lowercrowned, their lingual walls are more convex, the central valleys disappear later in wear, and the premolars are relatively longer. Thus, even though horncore characters show that they belong to the same group, the teeth from Maboko do not display the grazing features widespread in Eurasia, and could have evolved from the earlier African bovid N. gariepensis.

It may be that in the Middle Miocene of Kenya, Kubanotragus and Turcocerus filled the niches that were occupied in Eurasia by taxa absent from Maboko, especially Tethytragus, unknown in Africa, and Gentrytragus, which only appears at Fort Ternan [START_REF] Gentry | The Bovidae (Mammalia) of the Fort Ternan fossil fauna[END_REF](Gentry , 2010;;[START_REF] Azanza | Tethytragus nov. gen. et Gentrytragus nov. gen. Deux nouveaux Bovidés (Artiodactyla, Mammalia) du Miocène moyen. Relations phylogénétiques des Bovidés ante-vallésiens[END_REF]. Gentry, 1970, Moralas and[START_REF] Morales | A new genus and species of Bovidae (Artiodactyla, Mammalia) from the early Middle Miocene of Arrisdrift, Namibia, and the origins of the family Bovidae[END_REF] and from Li et al., (2021a). Comparative data from Gentry, 1970, Moralas and[START_REF] Morales | A new genus and species of Bovidae (Artiodactyla, Mammalia) from the early Middle Miocene of Arrisdrift, Namibia, and the origins of the family Bovidae[END_REF] and from Li et al., (2021a). Li et al. 2021a);42, Koujiacun Fm (Chen & Wu 1976;Deng et al. 2012); 43, Gaopo [START_REF] Chen | Discovery of the genus Kubanotragus (Bovidae, Artiodactyla) from the Middle Miocene in Lantian District, Shaanxi Province, China[END_REF]); 44, Mae Moh [START_REF] Suraprasit | Middle Miocene bovids from Mae Moh Basin, Northern Thailand: first record of the genus Eotragus from Southeast Asia[END_REF].

(

  1998) described as Hypsodontus cf. gaopense [sic] a similar frontlet; as at Gaopo, the molars are hypsodont and the premolars are short.

  wall.Lower third and fourth premolars of 10D) are not very different from each other, with distinct paracristid, anteparacristid and entocristid. They differ from the p4 of partial mandible KNM-MB 21459(Fig 9B; see below) in having an entocristid and hypocristulid that are unfused lingually leading to lingual opening of the telosinusid, as well as in having an unexpanded metaconid. From occlusal and lingual views it is clear that the lingual side of the p4 protoconid cusp is inflated and extends into the mesosinusid, a feature almost absent in KNM-MB 21459. From the buccal perspective the p4 hypoconid of KNM-MB 9204 is only weakly separated from the protoconid by a narrow cleft that extends a shorter distance from the occlusal surface unlike the p4 hypoconid of KNM-MB 21459 which is separated from the protoconid by a wider cleft that extends from the occlusal surface to just above the cervix. Lower molars of KNM-MB 9204 have convex lingual walls, almost no metastylid, a minute ectostylid, basal pillars, and only a hint of a goat fold; the metaconid connects the hypoconid in early wear, the m3 talonid is unicuspid, and the m3 hypoconulid lacks the strong entostylid and lingual concavity of KNM-MB 20350 and 20349 (see below).

  are shallower than observed in KNM-MB 21459 and 20350, but increases to 19 mm below m2. First and second molars of KNM-MB 548 differ from most excavated specimens in having goat folds below which strong parastylid pillars extend close to the cervix and less convex lingual crowns. Molar enamel is also more finely wrinkled than that of KNM-MB 21459. The m2 of KNM-MB 548 is the most hypsodont of the Maboko bovids (H/L=1.04, Hall & Cote 2021; H/W=3.58) its height to width index being comparable to hypsodonty indices calculated for m3s of extant Gazella dorcas, Nanger granti and Gazella thomsoni (Damouth & Janis 2011; C. Janis pers. Comm).

in

  Bed 3 and 48% in Bed 5w, strongly supporting interpretations of Bed 5b as more densely wooded or forested than other strata in the Maboko sequence. Compared to tragulids, Maboko bovid and giraffoid molars are better adapted to the consumption of fibrous browse, and their limbs (including fused and elongated metacarpals) to movement in more open canopy woodland. Both have their lowest frequency in Bed 5b where Climacoceras africanus makes up 2.5% and bovids 1.4% of the mammalian fauna. In comparison C. africanus makes up 16% of the fauna in Bed 3 and 10% in Bed 5w. Clearly, at Maboko bovids and giraffoids are least abundant where primates and tragulids are most common (Bed 5b), and most abundant

Fig. 2 .

 2 Fig. 2. Turcocerus africanus, Maboko. A, KNM-MB 24892 (Bed 3), holotype left horncore in A1, medial view, A2, front view, A3, posterior view, A4, lateral view, A5, crosssection of horncore at base. B, left horncore KNM-MB 21504 (Bed 5w) in B1, medial view, B2, front view, B3, posterior view, B4, lateral view, B5, cross-section of horncore at base. Abbreviations: a, anterior, l, lateral. Scale = cm for Figs. A-B, 5 cm for Fig. C.

Fig. 3 .

 3 Fig. 3. Turcocerus africanus, Maboko. A, right horncore KNM-MB 27599 (Bed 3), A1, anterior view, A2, posterior view. B, left horncore KNM-MB 20347 (Maboko Main surface), B1, anterior view, B2, posterior view, B3, medial view, B4, lateral view.

Fig. 4 .

 4 Fig. 4. Bivariate plot of middle Miocene horncore dimensions. Comparative data from

Fig. 5 .

 5 Fig. 5. Bivariate plot of middle Miocene bovid lower premolar to molar row lengths.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Bivariate plot of middle Miocene horncore height against a-p length. Comparative data from Gentry, 1970, Moralas and Pickford (2003) and from Li et al., (2021) Fig. 7. Kubanotragus pickfordi, Maboko. A, KNM-MB 21849 (Bed 3), right horncore in A1, medial view, A2, front view, A3, posterior view, A4, lateral view, A5, cross-section of horncore at base. B, KNM-35021 (Bed 3), left horncore in B1, anterior view, B2, lateral view, B3, cross-section of horncore at base.

Fig. 8 .

 8 Fig. 8. Maboko bovid upper dentition. Left: KNM-MB 9204 maxillary p3-M3 A1, occlusal view, A2, lingual view. Right: KNM-MB 20536, B1, lingual, B2, buccal, B3, occlusal views.

Fig. 9 .

 9 Fig. 9. Maboko bovid mandibles and lower dentition. Occlusal (A1-E1, G2) and lingual (A2-G2 and F) views of Morph 2 KNM-MB 20350 (A1, A2), Morph 1 KNM-MB 21459 (B1, B2), KNM-MB 9204 (C1, C2), KNM-MB 25148 (D1, D2), KNM-MB 7882 (E1, E2), KNM-MB 548 (F), KNM-MB 21666 (G1, G2).

Fig. 10 .

 10 Fig. 10. Buccal views of Maboko bovid mandibles and lower dentition. A. Morph 2 KNM-MB 23050, B. Morph 2 KNM-MB 21459, C. KNM-MB 7882, D. KNM-MB 9204, E. KNM-MB 21666, E. KNM-MB 548.

Fig. 11 .

 11 Fig. 11. Bivariate plot of lower molar mesio-distal Length (L) and maximum bucco-lingual width (W) for specimens in mandibles and isolated teeth that are tentatively assigned to element based on the range of dimensions for molars in mandibles.

Fig. 12 .

 12 Fig. 12. Bivariate plot of Maboko bovid molar Length (L) versus Width (W) compared to samples from other Miocene species. Comparative data from Moralas and Pickford(2003) and fromLi et al., (2021).

Fig. 13 .

 13 Fig. 13. Examples of Maboko bovid and other ruminant postcrania. Upper row: Anterior views of right astragali A. bovid KNM-MB 21587, B. Dorcatherium chappuisi KNM-MB 21953, C. Dorcatherium piggotti KNM-MB 21593, D. Climacoceras africanus KNM-MB 21588. Lower row: distal metapodial KNM-MB 21661 in anterior (E1), posterior (E2) and side view (E3).

Fig. 14 .

 14 Fig. 14. Distribution map of the bovid taxa discussed in the text. Present-day localities areprojected on a 15 Ma paleogeographic map (Scotese 2016) using GPlates[START_REF] Müller | GPlates: Building a virtual Earth through deep time[END_REF]. Numbers refer to the following localities: 1, Maboko (this paper); 2, Nyakach (this paper; Thomas 1984a); 3, Jebel Zelten[START_REF] Hamilton | The Lower Miocene ruminants of Gebel Zelten, Libya[END_REF]; 4, Arrisdrift[START_REF] Morales | A new genus and species of Bovidae (Artiodactyla, Mammalia) from the early Middle Miocene of Arrisdrift, Namibia, and the origins of the family Bovidae[END_REF] 5, Buñol (Moyá-Solá 1983;[START_REF] Made | Eotragus clavatus (Artiodactyla, Bovidae, Boselaphini) de Sansan[END_REF]; 6, Córcoles(Alférez et

(

Table 1 .

 1 Horncore measurements. APD = anteroposterior diameter, TD = transverse diameter

	Accession #	Bed	Taxon	APD	TD	APD/TD
	KNM-MB 13827	Main	Genus indet.	41	25	1.64
	KNM-MB 27602	3	Turcocerus africanus	30.7	20.8	1.48
	KNM-MB 24892	3	Turcocerus africanus	30	22.1	1.36
	KNM-MB 27603	3	Turcocerus africanus	29.5	22	1.34
	KNM-MB 27599	3	Turcocerus africanus	28.3	21.7	1.30
	KNM-MB 33211	3	Turcocerus africanus	28	20.3	1.38
	KNM-MB 10061		Turcocerus africanus	27.5	20.5	1.34
	KNM-MB 20351	5w	Turcocerus africanus	27.3	17.2	1.59
	KNM-MB 20347	Main	Turcocerus africanus	26	17	1.53
	KNM-MB 21504	5w	Turcocerus africanus	25.9	20.6	1.26
	KNM-MB 27601	3	Turcocerus africanus	25	17.5	1.42
	KNM-MB 36684	3	Turcocerus africanus	25	19	1.32
	KNM-MB 29562	5w	Turcocerus africanus	23.3	17.3	1.35

Table 2 .

 2 Measurements of the main dental elements. L=mesiodistal length, W=maximum bucco-lingual width, Hyp=metaconid H/L for m1 and 2, and Metaconid H/W for m3. When two Hyp values are given the former is for m2 and the latter for m3. Upper dentition in bold.

	accession #		m3	m2	m1	M1-3	m1-3	p2-p4	Hyp
		Bed	L/W	L/W	L/W	L	L	L
	KNM-MB 25148 12 c. 20	~15/8.5 ~11/8		c. 46	c. 30
	KNM-MB 20350 5w 14.3/7	11/6.7	9.6/6.3		35	c. 23	0.84/1.26
	KNM-MB 34204 5b 14.5/7	11/6.7	9.2/6.4		c. 34.7	mine=
	KNM-MB 9204 KNM-MB 21459 5w	14.7/6.5 10.8/6.5 9.7/6.9 10.5/7.4 9.9/7	29.8 33.3	c. 22.1 20.5 22.4	0.75
	KNM-MB 548			13.5/6.8 11.1/4.6
	KNM-MB 20349 5w 14.6/6.9
	KNM-MB 24927	3			10/5.9
	KNM-MB 21912	3 c. 19/7.5
	KNM-MB 21666	3 18.7/5.6						/2.52
	KNM-MB 13828		16
	KNM-MB 25490 12 15.3/6.8
	KNM-MB 35022 5w 14.8
	KNM-MB 20921		14.7
	KNM-MB 29589	3		12.8/8.5
	KNM-MB 35330	3		12.3/6.8					0.9
	KNM-MB 34487	3		12/7.6					0.92
	KNM-MB 24923	3		11.6/6.7
	KNM-MB 24928	3		11.9/7.2
	KNM-MB 24929	3		11.9
	KNM-MB 32360			11.1/6.9					0.88
	KNM-MB 34200			11.1/7.1					0.81
	KNM-MB 34202 5b			10.4/6.9
	KNM-MB 7882				10/6.5				0.8

Table 3 .

 3 Main localities dicussed in the text, roughly arranged by increasing age. N° refers to the numbers in Fig.3. C and D, grass seed taken at 25x. Seed measures ~0.9 mm in length and 0.3 mm in width.

	locality	country	N° taxon
	Koujiacun	China	42 Turcocerus lishanensis
	Kekemaideng	China	39 Turcocerus kekemaidengensis
	Tunggur	China	41 Turcocerus grangeri; Turcocerus noverca
	Gaopo	China	43 Kubanotragus gaopoensis
	Toril 3A	Spain	7 Eotragus sp.
	Mae Moh	Thailand	44 Eotragus lampangensis
	La Grive	France	14 Eotragus sp.
	Çandır	Turkey	30 T. gracilis; Hypsodontus pronaticornis
	Manchones	Spain	7 Eotragus sp.
	Mordoğan	Turkey	27 Hypsodontus pronaticornis
	Ulan Tologoi	Mongolia	37 Turcocerus kekemaidengensis
	Sansan	France	12 Eotragus clavatus
	Arroyo de Val	Spain	7 Eotragus sp.
	Prebreza	Serbia	23 Hypsodontus serbicus
	Inönü	Turkey	29 T.gracilis; Kubanotragus sp.; H. pronaticornis
	Cheskewala	Pakistan	35 Kubanotragus sokolovi
	Göriach	Austria	18 Eotragus cf. clavatus
	Veliko Selo	Serbia	22 Eotragus sp.
	Paşalar	Turkey	28 Hypsodontus pronaticornis
	Belometchetskaya Russia	33 Kubanotragus sokolovi
	Leoben	Austria	19 Pseudoeotragus seegrabensis; Eotragus sp.
	Veltheim	Switzerland 17 Eotragus sp.
	Labitschberg	Austria	20 Eotragus sp.
	Gračanica	Bosnia-Herz. 21 Eotragus sp.
	Popovac	Serbia	24 Eotragus sp.
	Rimbez	France	13 Eotragus sp.

Acknowledgments

We are grateful to George Koufos, Aristotle University of Thessaloniki, Greece, and to Jorge Morales, CSIC, Madrid, Spain, for sharing photos of the Thymania and Arrisdrift material, respectively, to Yikun Li for his help with Chinese sites, to Christine Janis for access to her m3 hypsodonty data for extant ungulates, to Bonnie Jacobs for identification of the seed as a grass, to Christine Argot, Frederick Kyalo Manthi and Spyridoula Pappa for giving access to the MNHN, KNM, and NHMUK collections, respectively, and to Matthew Porter (NHMUK), for providing 3D models of NHMUK-M15543 and M15544. In addition, we are grateful to the Kenyan excavation team at Maboko including Blasto Onyango, Stephen Gitau, Joseph Onyango Miumi, Cercil Odera, Paul Odera Abong, Godfrey Wadem Dondo, Charles Omondi Agak, Monica Awasi and more than a hundred other residents of Maboko and Majiwa who are responsible for the excavation and detailed mapping of each specimen. The excavations were funded by grants from the Wenner-Gren Foundation, National Geographic Society, L.S.B. Leakey Foundation, Fulbright Collaborative Awards, and the National Science Foundation.