2025 Volume 29 Pages 87-107
We review the Anthracotheriidae (Mammalia, Artiodactyla) from the lower part (ca. 15.0–14.4 Ma) of the middle Miocene Aka Aiteputh Formation in Nachola (northern Kenya), with a description of the specimens. The anthracotheriids of the Aka Aiteputh Formation in Nachola consist of two bothriodontine species, Brachyodus aequatorialis nacholaensis subsp. nov. and Nacholameryx baragoiensis gen. et sp. nov. The materials of B. aequatorialis nacholaensis consist of gnatho-dental, phalanx, metapodial, and scaphoid specimens that likely comprise a single individual. B. aequatorialis nacholaensis is distinguished from Brachyodus aequatorialis aequatorialis in having a lingual postprotocrista on P4 and a better developed cuspid on the p4 talonid and in lacking a buccal postprotocristid on p4. The materials of N. baragoiensis consist mainly of isolated teeth. N. baragoiensis is characterized by relatively small body size, tetracuspidate upper molars with a looplike mesostyle and without a lingual postprotocrista, lower molars with a preprotocristid and prehypocristid (cristid obliqua) reaching close to the lingual margin of the crown, a buccally positioned m3 hypoconulid, and a better-developed and isolated paraconid and metaconid (or entostylid) on p4 with an extra cuspid between the protoconid and the paraconid. Our phylogenetic analyses recover Nacholameryx as the sister taxon of the [Merycopotamus + Libycosaurus] clade within the Merycopotamini, raising the possibility that the [Merycopotamus + Libycosaurus] clade originated in Africa during the middle Miocene. This revision of the Nachola anthracotheriids demonstrates that the results of previous faunal analyses of the Nachola mammals should be reappraised.
ZooBank registration: urn:lsid:zoobank.org:pub:46DFEEF0-E0E8-4B80-BD7A-C9BE13B8D70D
Nachola is a fossil locality in northern Kenya that yields diverse and abundant mammalian fossils from the Miocene Aka Aiteputh Formation (e.g. Ishida, 1984; Ishida et al., 2001; Tsujikawa and Nakaya, 2005; Nakatsukasa and Kunimatsu, 2009). The Nachola locality is well known in paleoanthropology and paleoprimatology because it yields several species of primate (e.g. Nakatsukasa and Kunimatsu, 2009; Kunimatsu et al., 2017) and because it has yielded a great number of gnatho-dental and postcranial specimens of the primitive hominoid Nacholapithecus kerioi (e.g. Ishida et al., 1999, 2004; Kunimatsu et al., 2004; Nakatsukasa and Kunimatsu, 2009). Although the mammalian fauna of the Aka Aiteputh Formation in Nachola was briefly reviewed by Pickford et al. (1999), Tsujikawa et al. (2004), and Tsujikawa and Nakaya (2005), the fossil specimens collected from Nachola have not yet been formally described and studied except for the primates and some suoids (Ishida et al., 1984, 1991, 1999, 2004; Kunimatsu, 1992, 1997; Nakatsukasa et al., 1998, 2003a, 2003b, 2007, 2012; Kunimatsu et al., 2004, 2017; Pickford and Tsujikawa, 2005, 2019; Kikuchi et al., 2012, 2015, 2016, 2018; Pina et al., 2021).
Here, we review the Anthracotheriidae (Mammalia, Artiodactyla) of the Aka Aiteputh Formation in Nachola, with the description of the specimens. The family is likely paraphyletic and may be ancestral to hippos (Boisserie et al., 2005; Orliac et al., 2010; Tsubamoto et al., 2011; Alloing-Séguier et al., 2014; Lihoreau et al., 2015a; Rodrigues et al., 2020; Orliac et al., 2023; but see Pickford, 2008, 2015 for a different view). It is also considered to be a useful group for the study of mammalian biochronology and paleobiogeography during the Cenozoic (Rasmussen et al., 1992; Ducrocq and Lihoreau, 2006). According to Tsujikawa and Nakaya (2005), the Aka Aiteputh Formation in Nachola has yielded two species of anthracotheres, Afromeryx zelteni Pickford, 1991b and Brachyodus cf. aequatorialis MacInnes, 1951. Here, we reappraise these taxonomic identifications.
The fossils described here are mainly gnatho-dental specimens with some associated postcranial fragments. They were collected by the Kenya-Japan Joint Expedition through a long-term field project conducted since the early 1980s (e.g. Ishida, 1984; Pickford et al., 1984, 1999; Tsujikawa and Nakaya, 2005; Nakatsukasa and Kunimatsu, 2009; Kunimatsu et al., 2017; and references therein).
Repository.—The specimens described here are stored in the Palaeontology Section of the National Museums of Kenya (KNM) in Nairobi, Kenya. Specimen numbers that start with the acronym KNM-BG are the KNM accession numbers that are given to the specimens collected from fossil localities near Baragoi area (including Nachola) in northern Kenya. Specimen numbers that start with the acronyms BG‘J’, BG‘N’, BG‘O’, BG‘T’, and BG‘X’ are the field numbers used by Pickford et al. (1999) for the specimens collected from fossil sites near Baragoi area (including Nachola). The letters following BG indicate individual site numbers (see Ishida et al., 2001).
Dental terminology.—The dental terminology used herein is indicated in Figure 1 and was generally compiled from Bown and Kraus (1979), Lihoreau and Ducrocq (2007), and Tsubamoto et al. (2011).
Dental abbreviations.—DI/di, upper/lower deciduous incisors. I/i, upper/lower incisors. C/c, upper/lower canines. P/p, upper/lower premolars. M/m, upper/lower molars.
All the fossil specimens described here were recovered from several sites within the Miocene Aka Aiteputh Formation at the Nachola locality (approximately 1°50′ N, 36°40′ E [geodetic datum: WGS84]), west of the village of Nachola, which is located about 15 km west of the township of Baragoi, northern Kenya (Figure 2; Ishida, 1984; Sawada et al., 1999, 2001, 2006; Ishida et al., 2001; Kunimatsu et al., 2004). In the Nachola and Samburu Hills areas, the Neogene System overlies the Precambrian basement rocks and is divided into the following five formations: the Nachola, Aka Aiteputh, Namurungule, Kongia, and Tirr Tirr formations, in ascending order (Figure 2; Sawada et al., 1998, 1999, 2001, 2006). In Nachola, the Nachola Formation and the lower part of the Aka Aiteputh Formation are exposed (Sawada et al., 1999).
The Nachola Formation unconformably overlies Precambrian basement rocks (Figure 2; Sawada et al., 1999). It was dated within ca.19–15 Ma using K–Ar dating, and its upper part was dated as 15.4 Ma and 15.0 Ma (Sawada et al., 1998, 2006; Itaya and Sawada, 1999).
The Aka Aiteputh Formation conformably overlies the Nachola Formation. The Aka Aiteputh Formation in Nachola is composed of Units 1–4 in ascending order: Unit 1 and Unit 3 yield mammalian fossils, and Unit 2 and Unit 4 are basaltic layers (Figure 2; Sawada et al., 1999). Unit 3 was dated as 14.77 ± 0.10 Ma using 40Ar–39Ar dating (Nakatsukasa and Kunimatsu, 2009), and Unit 4 was dated as 14.4 Ma using K–Ar dating (Sawada et al., 2006) (Figure 2). Therefore, the geological age of the mammalian fossil-bearing horizons (Unit 1 and Unit 3) within the formation is estimated to be within ca. 15.0–14.4 Ma (Langhian, early middle Miocene). The mammalian fauna of the Aka Aiteputh Formation in Nachola now consists of seven orders and 28 species (Table 1). This newly updated faunal list (Table 1) was compiled from previous studies and this study, with recent observations of the specimens from Nachola by two of us (Y. Kunimatsu and H. Tsujikawa).
| Order | Family level | Genus and species | Unit 1 | Unit 3 |
|---|---|---|---|---|
| Primates | Galagidae | ?Komba sp. | X | |
| Lorisidae (= Loridae) | Mioeuoticus kichotoi | X | ||
| Victoriapithecidae | Victoriapithecidae gen. et sp. indet. | X | ||
| Nyanzapithecinae | Nyanzapithecus harrisoni | X | ||
| Hominoidea | Nacholapithecus kerioi | X | ||
| Rodentia | Thryonomyidae | Paraphiomys cf. pigotti | X | |
| cf. Apodector stromeri | X | |||
| Muroidea | Muroidea gen. et sp. indet. | X | ||
| Carnivora | Viverridae | Viverridae gen. et sp. indet. | X | |
| Mustelidae | Mustelidae gen. et sp. indet. | X | ||
| Hyracoidea | Titanohyracidae | Afrohyrax championi | X | X |
| Proboscidea | Gomphotheriidae | Archaeobelodon cf. filholi | ? | X |
| Deinotheriidae | Prodeinotherium hobleyi | X | X | |
| Perissodactyla | Rhinocerotidae (Elasmotheriini) | Elasmotheriini gen. et sp. indet. | X | X |
| Chalicotheriidae (Chalicotheriinae) | cf. Butleria rusingensis | ? | ? | |
| Artiodactyla | Anthracotheriidae (Bothriodontinae) | Brachyodus aequatorialis nacholaensis subsp. nov. | X | |
| Nacholameryx baragoiensis gen. et sp. nov. | X | X | ||
| Suidae | Kenyasus namaquensis | X | ||
| Suidae (Listriodontinae) | Kubwachoerus marymuunguae | X | ||
| Libycochoerus jeanneli (= Listriodon jeanneli) | X | X | ||
| Sanitheriidae | Diamantohyus nadirus | X | ||
| Diamantohyus sp. | X | |||
| Tragulidae | Dorcatherium pigotti | X | ||
| Dorcatherium chappuisi | X | |||
| Moschidae? | Walangania africanus | X | ||
| Climacoceratidae | Climacoceras sp. | X | ? | |
| Giraffidae | cf. Canthumeryx sirtensis | ? | X | |
| Bovidae | Bovidae gen. et sp. indet. | X |
Family Anthracotheriidae Leidy, 1869
Subfamily Bothriodontinae Scott, 1940
Genus Brachyodus Depéret, 1895
Type species.—Brachyodus onoideus (Gervais, 1859).
Revised diagnosis.—Modified after Pickford (1991b), Lihoreau and Ducrocq (2007), and Holroyd et al. (2010).
A large bothriodontine with a long and wide snout, a shallow mandible, a weakly developed angular process of the mandible, a fused mandibular symphysis, tusklike and sexually dimorphic upper central and lower lateral incisors, premolariform lower and upper canines, a short i3–c1 diastema, a long C1/c1–P1/p1 diastemata, P3 with a quadratic occlusal outline, pentacuspidate upper molars with a paraconule, molar styles pinched rather than looplike, no premetacristid on the lower molars, an anterior position (between C1 and P1) of the main palatine foramen, and a pentadactyl manus.
Differs from Bothriogenys Schmidt, 1913 by a reduction of its anterior teeth and a more selenodont morphology. Differs from Telmatodon Pilgrim, 1907, Afromeryx Pickford, 1991b, Nacholameryx gen. nov., Libycosaurus Bonarelli, 1947, and Merycopotamus Falconer and Cautley, 1947 in retaining a molar paraconule. Differs from Sivameryx Lydekker, 1883, Bothriodon Aymard, 1846, and Aepinacodon Troxwell, 1921 and further differs from Afromeryx, Nacholameryx gen. nov., Libycosaurus, and Merycopotamus in having a molar mesostyle that is pinched and “cusp-shaped” rather than looplike. Further differs from Afromeryx, Sivameryx, and Nacholameryx gen. nov. in being larger. Differs from Parabrachyodus Forster-Cooper, 1915 in having much less bunodont dentition. Further differs from Parabrachyodus and Telmatodon in having a steeper lingual slope of the molars in mesiodistal view.
Brachyodus aequatorialis MacInnes, 1951
Included subspecies.—Brachyodus aequatorialis aequatorialis MacInnes, 1951 and Brachyodus aequatorialis nacholaensis subsp. nov.
Revised diagnosis.—Modified after Pickford (1991b), Holroyd et al. (2010), and Miller et al. (2014).
Differs from the European species of the genus, Brachyodus onoideus and Brachyodus intermedius Mayet, 1908, in having small lower incisors that are spatulate with wrinkled enamel rather than being caniniform and in lacking the distal accessory cusp (“metaconule” in Pickford, 2020a) near the distal margin of the crown on P4. Differs from Brachyodus depereti (Fourtau, 1920) in being smaller (length of the lower molar row = ca. 115 mm in B. aequatorialis and ca. 143 mm in B. depereti). Differs from Brachyodus mogharensis Pickford, 1991b in being somewhat smaller (length of the upper molar row = ca. 90 mm in B. aequatorialis and ca. 114 mm in B. mogharensis) and in having larger upper premolars, mesiodistally more shortened P2–P3, a weaker buccal rib of the paracone on the upper premolars and molars, a weaker buccal rib of the metacone on the upper molars, and a weaker buccal cingulum on the upper premolars and molars.
Remarks.—This species was assigned to the genus Masritherium Fourtau, 1920 by Black (1978) and Pickford (2020a, 2020b). Later, Pickford (2022a) erected a new genus Rusingameryx Pickford, 2022a for this species, and Pickford (2022b) followed this classification. According to Pickford (2022a), Rusingameryx only differs from Brachyodus in its “platycephalic cranium.” However, the cranial characteristics of the species of Brachyodus except for the type species B. onoideus are unclear (Pickford, 2022a). In addition, those of the Nachola material are unknown. The dental characteristics of B. aequatorialis are at least comparable to B. onoideus at the genus level, in a general sense (Lihoreau and Ducrocq, 2007; Holroyd et al., 2010; Miller et al., 2014). Therefore, the erection of the genus Rusingameryx is not robustly supported, and here we conservatively use the genus Brachyodus for this species (MacInnes, 1951; Lihoreau and Ducrocq, 2007; Holroyd et al., 2010).
Brachyodus aequatorialis nacholaensis subsp. nov.
ZooBank lsid: urn:lsid:zoobank.org:act:E6A94AA5-3DC2-471D-ADC8-287FEA836D8B
Brachyodus cf. aequatorialis, Tsujikawa and Nakaya, 2005, p. 605, table 1; Nakatsukasa and Kunimatsu, 2009, p, 105, table 1; Tsubamoto et al., 2020, p. 59, appendix 1.
Holotype.—KNM-BG 35340, left and right mandibular fragments with broken left p4–m3 and right p3–m3.
Paratypes.—KNM-BG 39918, a left maxillary fragment with P2–M2; KNM-BG 45220, right P3–M2; KNM-BG 39919 + 45210, broken left M3; KNM-BG 37553, a second phalanx; KNM-BG 45218a, a left scaphoid (radial carpal bone); KNM-BG 45218b, a second phalanx; KNM-BG 45218c, a second phalanx (worse preserved than KNM-BG 45218b); KNM-BG 45225, a distal part of second phalanx; KNM-BG 45219a; a distal part of abaxial metapodial; KNM-BG 45219b, a second phalanx. All these paratypes likely belong to the same individual as the holotype.
Type and only locality.—Kyoto University site number BG-11 (1°53′09.0″ N and 36°37′24.9″ E), Nachola locality, Baragoi area, northern Kenya (File S3; Ishida et al., 2001).
Horizon.—Unit 1 (Sawada et al., 1999) of the lower part of the Aka Aiteputh Formation (Figure 2).
Age.—Langhian (early middle Miocene, within ca. 15.0–14.8 Ma) (Figure 2; Sawada et al., 1998, 2006; Nakatsukasa and Kunimatsu, 2009).
Dental measurements.—Shown in Table 2.
| Upper dentition | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| KNM-NA | P2 | P2 | P3 | P3 | P4 | P4 | M1 | M1 | M1 | M2 | M2 | M2 | M3 | M3 | M3 |
| L | W | L | W | L | W | L | W-m | W-d | L | W-m | W-d | L | W-m | W-d | |
| 39918 | 19.3 | — | 23.5 | 22.5 | 20.5 | 25.1 | 31.2 | — | 32.1 | 32.8 | 37.8 | — | — | — | — |
| 39919 + 45210 | — | — | — | — | — | — | — | — | — | — | — | — | 33.8* | 39.6 | 35.8 |
| 45220 | — | — | 24.5 | 23.1 | 20.5 | 25.4 | 30.2 | 33.2 | 32.2* | 31.7 | 37.2 | 35.2 | — | — | — |
| Lower dentition | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| KNM-NA 35340 (holotype) | p3 | p3 | p4 | p4 | m1 | m1 | m1 | m2 | m2 | m2 | m3 | m3 | m3 |
| L | W | L | W | L | W-trd | W-tad | L | W-trd | W-tad | L | W-trd | W-tad | |
| right side | 26.3* | 17.2 | 26.0* | 19.2 | 31.2 | 20.8 | 21.3 | 31.8* | — | 24.1 | 48.1* | — | — |
| left side | — | — | 27.3* | 19.4 | — | 20.5 | — | 32.5* | 24.6 | 24.2 | 47.0* | 26.3 | 23.8 |
Other measurements.—KNM-BG 35340 (holotype; mandibles): symphysial length = 94.0 mm; mandibular depth below left p4 (buccal side) = 50.0 mm; mandibular depth below left m1 talonid (buccal side) = 52.3 mm; mandibular depth below left m3 hypoconulid (buccal side) = 58.9 mm. KNM-BG 37553 (second phalanx): length = 24.9 mm; proximal width = 25.1 mm; distal width = 23.4 mm. KNM-BG 45218b (second phalanx): length = 42.8 mm; proximal width = 36.5 mm; distal width = 31.0 mm. KNM-BG 45218c (second phalanx): length = 41.9 mm; proximal width = 32.3 mm; distal width = 30.1 mm. KNM-BG 45225 (second phalanx): distal width = 19.9 mm. KNM-BG 45219a (abaxial metapodial): distal width = 38.2 mm (estimate). KNM-BG 45219b (second phalanx): length = 39.9 mm; proximal width = 32.7 mm; distal width = 31.2 mm. KNM-BG 45218a (scaphoid): anteroposterior (dorsoventral/dorsovolar) height = 44.1 mm; proximodistal length = 31.5 mm; mediolateral width = 26.9 mm.
Etymology.—The subspecific name refers to Nachola, where the holotype was found.
Diagnosis.—Differs from the type subspecies of the species B. aequatorialis aequatorialis in having a lingual postprotocrista on P4 and a better developed cuspid on the p4 talonid and in lacking a buccal postprotocristid on p4 (Figure 5).
Comparative description.—The postcanine teeth are low crowned with a bunoselenodont/selenodont dentition.
P2 (Figure 4A) has a single cusp (paracone) lacking a protocone. The buccal rib of the paracone is very weak. The cingulum appears to surround the crown, but its buccal part is weak. The distal part of P2 is broken away. A weak crista originates at the middle of the preparacrista and extends mesially, connecting to the mesial cingulum.
P3 (Figure 4A, C) appears to have a similar shape to that of P2 and is roughly as wide as long. The buccal rib of the paracone is very weak as on P2. There are two mesial cristae from the paracone: the stronger crista extends mesiobuccally, connecting to the cingulum at the mesiobuccal corner of the crown, and the weaker one extends mesially, connecting to the mesial cingulum. The cingulum appears to surround the crown, but its buccal part is very weak. There is a shallow basin-like structure that is surrounded by the distal base of the paracone and the distolingual part of the cingulum.
P4 (Figure 4A, C) has two main cusps, a paracone and protocone. The buccal rib of the paracone is stronger than that on P2–P3 and is as weak as that of the metacone on the upper molars. The protocone has two distal cristae, postprotocrista and lingual postprotocrista. There is no style, although there is a small swelling at the mesiobuccal corner of the crown. The cingulum surrounds the crown, but its buccal part is very weak.
The upper molars (Figure 4A–C) are pentacuspidate with a paraconule. The buccal ribs of the paracone and metacone are relatively weak: the metacone rib is weaker than that of the paracone. The ectoloph (preparacrista + postparacrista + premetacrista + postmetacrista) is W-shaped, bearing a parastyle and a mesostyle. The metastyle is absent. The parastyle and mesostyle are not looplike but pinched and “cusp-shaped.” Some molars have a lingual premetacristule and others do not. The cingulum surrounds the crown, but its buccal part is weak.
The mandibles (Figure 3) are comparable to those of B. aequatorialis aequatorialis from the type locality of the species in Rusinga, southwestern Kenya. The mandibular symphysis is fused and anteriorly elongated (Figure 3A–F). There is only one mental foramen on each (left and right) side. Note that there are two mental foramina on each (left and right) side in B. onoideus from France (Pickford, 2020b). These left and right mental foramina are located not on the lateral side but on the ventral side of the symphysis (Figure 3E). In anterior view (Figure 3B), four alveoli for right and left i1–i2 are observed. The alveolus for i1 is smaller than that for i2. In dorsal view of the symphysial region (Figure 3D), the alveolus for i3 or c1 is located distal to the alveoli for i2. The anterior part of the symphysial region is concave ventrally (Figure 3B, E). There appears to be no diastema between p2–m3.
The distal end of p3 (Figure 3I–J) is broken. The preprotocristid extends mesially, connecting to the mesial end of the cingulum, where the lingual and buccal cingula are linked. A short crest extends distally from slightly above the base of the preprotocristid. The postprotocristid extends distolingually, curves mesially at the distolingual base of the protoconid, and connects to the short crest from the preprotocristid, forming a shallow trigonid basin. At the middle of the postprotocristid, a cristid originates and extends distally. The buccal postprotocristid is absent. The buccal cingulum is distinct mesially and distally but is only weakly expressed at the middle of the buccal aspect. The lingual cingulum is divided into mesial and distal parts by the formation of the continuous crest between the preprotocristid and the postprotocristid mentioned above, and both portions are developed strongly.
The p4 (Figure 3G–J) has a similar shape to p3 but is somewhat wider buccolingually. The trigonid basin is deeper than that of p3. There is a relatively large extra cuspid on the talonid. This cuspid has a mesiolingually oriented cristid, which connects to the middle of the postprotocristid.
The lower molars (Figure 3G–J) lack a paraconid. The preprotocristid extends mesiolingually, disappearing at the mesial base of the metaconid. The postprotocristid extends lingually, connecting to the metaconid. There is no premetacristid. The prehypocristid (cristid obliqua) meets the distal trigonid wall below the notch between the protoconid and the metaconid. The notch is closer to the metaconid than to the protoconid. The preentocristid extends mesiobuccally, connecting to the prehypocristid. The posthypocristid and postentocristid connect to each other at the distal talonid notch. On m1–m2, a distally oriented cristid originates at the distal talonid notch, connecting to the distal cingulum or faint hypoconulid. There are mesial and distal cingula. The buccal and lingual cingula are absent. The m3 has a distally elongated hypoconulid.
The second phalanges (Figure 4D–H) are morphologically comparable to those of the Brachyodus-like material from Moroto (Uganda) described by Pickford (2020a). They are also roughly comparable in morphology to those of hippos and are distinguished from the mesiodistally shorter second phalanges of rhinos (Walker, 1985). They are somewhat compressed dorsoventrally (dorsovolarly/plantarly). The proximal/distal articulation is triangular to trapezoidal in proximal/distal view, with a central ridge on the proximal side and a concave distal side.
The distal part of the metapodial (KNM-BG 45219a; Figure 4I) is comparable to the distal part of the abaxial metapodial of the Brachyodus-like material from Moroto (Uganda) described by Pickford (2020a). It is mediolaterally asymmetrical in distal view. There is a central ridge on the volar/plantar side of the distal end. This central ridge fades out towards the dorsal side.
The scaphoid or radial carpal bone (KNM-BG 45218a; Figure 4J) is mediolaterally compressed and dorsoventrally (dorsovolarly) elongated. It is morphologically comparable to those of hippos, pigs, cervids (e.g. chital), giraffes, and bovids (e.g. nilgai and blackback) and more mediolaterally compressed than those of rhinos, elephants, and camels (Walker, 1985; Choudhary et al., 2013, 2015; Bharti and Singh, 2018; Georgitsis et al., 2022). There are large concave proximal (for radius) and distal (for magnum) articular facets. The distal articular facet is more concave than the proximal one. The distal articular facet is single and is not divided into two facets, suggesting that it does not articulate with the trapezoid. There are flat facets or surfaces on both the mesial and lateral surfaces.
Discussion.—All the Brachyodus specimens from Nachola described here likely belong to a single individual. The overall morphology and size of the Nachola specimens are comparable to those of Brachyodus aequatorialis from the type locality (MacInnes, 1951), which was dated to the late early Miocene (ca. 18–16 Ma: Werdelin, 2010). The Nachola specimens are distinguished from the other species of Brachyodus: they differ from B. onoideus and B. intermedius in lacking the distal accessory cusp (“metaconule” in Pickford, 2020a) near the distal margin of the crown on P4; they differ from B. depereti in being smaller; they differ from B. mogharensis in being somewhat smaller and in having larger upper premolars, mesiodistally shorter P2–P3, a weaker buccal rib of the paracone on P2–M3, a weaker buccal rib of the metacone on M1–M3, and a weaker buccal cingulum on P2–M3. Therefore, the Nachola Brachyodus specimens can be assigned to B. aequatorialis.
Although there is no significant morphological difference between the mandibles and upper and lower molars of the Nachola Brachyodus specimens and B. aequatorialis from the type locality, they are distinguished from each other in P4/p4 morphology. The Nachola specimens differ from B. aequatorialis from the type locality in that P4 has a lingual postprotocrista and in that p4 has a better developed cuspid on the talonid and lacks a buccal postprotocristid (Figure 5). However, some bothriodontine species have relatively large intraspecific variations on P4/p4 morphology (Dineur, 1982; Hellmund, 1991; Pickford, 2020a). Therefore, it seems that these morphological differences in P4/p4 morphology described above cannot be concluded with certainty as interspecific differences. Nevertheless, these morphological differences could be explained by the chronological gap between the Brachyodus specimens from Nachola and Rusinga (ca. 15–4.8 Ma vs. ca. 18–16 Ma, respectively). Furthermore, except for the Nachola specimens, all African occurrences of Brachyodus are from the early Miocene (before ca. 16 Ma: Holroyd et al., 2010). Therefore, we consider these relatively minor but distinct morphological differences to be differences at the chronological subspecies level and erect a new subspecies, B. aequatorialis nacholaensis, within the species B. aequatorialis for the Nachola specimens. Although subspecies are usually used for geographic variants in extant taxa and are rarely created in mammalian paleontology, some paleontologists (e.g. Haile-Selassie, 2001) use subspecies to recognize different segments of a lineage through time.
Tribe Merycopotamini Lydekker, 1883 (sensu Lihoreau et al., 2016)
Genus Nacholameryx gen. nov.
ZooBank lsid: urn:lsid:zoobank.org:act:B4DEFACD-113C-4ED8-8CD1-6E3C86D03720
Type species.—Nacholameryx baragoiensis sp. nov. by monotypy.
Etymology.—Nachola is the fossil locality where the holotype was found, and ‘-meryx’ is from the Greek for ruminant and is a common suffix for bunoselenodont/selenodont artiodactyls. The gender of the genus is masculine.
Distribution.—Middle Miocene of Nachola, Kenya.
Diagnosis.—A small merycopotamin (sensu Lihoreau et al., 2016) with a low-crowned dentition. Differs from Sivameryx and Afromeryx in having a better-developed and isolated paraconid and metaconid (paraconid < metaconid) on p4 with an extra cuspid between the protoconid and the paraconid and in lacking a lingual postprotocrista on the upper molars. Further differs from Sivameryx in lacking a molar paraconule. Differs from Gonotelma Pilgrim, 1908 in lacking a lingual postprotocrista on the upper molars. Further differs from Afromeryx and Gonotelma in having lower molars with the preprotocristid and prehypocristid (cristid obliqua) reaching close to the lingual margin of the crown and a more buccally positioned m3 hypoconulid. Differs from the primitive merycopotamin Hemimeryx Lydekker, 1883 in being smaller and in having upper molars that are wider than long. Differs from derived merycopotamins such as Merycopotamus and Libycosaurus in being smaller and in having a less selenodont dentition, a molar premetacristid, and a less angular mesiobuccal corner of the p4 crown. Differs from Parabrachyodus and Telmatodon in being much smaller, in having a steeper lingual slope of the molars in mesiodistal view, and in lacking a lingual postprotocrista on the upper molars. Further differs from Parabrachyodus in having a much less bunodont dentition.
Additionally, differs from the contemporaneous African bothriodontine Brachyodus in being smaller, in having a looplike molar mesostyle and a molar preparacrista that connects to the parastyle buccally rather than lingually, and in lacking a molar paraconule. Differs from Elomeryx Marsh, 1894 and Bothriogenys in lacking a molar paraconule and in having a more mesiodistally compressed talonid on the lower molars. Further differs from Bothriogenys in having a looplike molar mesostyle and lower molars with the preprotocristid and prehypocristid (cristid obliqua) reaching close to the lingual margin of the crown. Differs from Bothriodon and Aepinacodon in being smaller, in having less selenodont dentition, and in lacking a molar paraconule.
Nacholameryx baragoiensis sp. nov.
ZooBank lsid: urn:lsid:zoobank.org:act:4703C805-D239-4B3F-8AA6-473483AEF0F1
Afromeryx zelteni Pickford, 1991b (in part). Pickford, 1999, p. 147, 153–154, figs. 10–11; Tsujikawa and Nakaya, 2005, p. 605, table 1; Tsubamoto et al., 2020, p. 59, appendix 1.
Holotype.—KNM-BG 39988, right p4.
Paratypes.—KNM-BG 17857, left P4; KNM-BG 14802 (= BG‘X’ 456’84), right M3?; KNM-BG 35482, right m1?; KNM-BG 14790 (= BG‘X’ 474’84), a right m3 hypoconulid; KNM-BG 14798 (= BG‘X’ 66’84), a talonid of left m1 or m2; KNM-BG 14799 (= BG‘X’ 345’84), an upper molar fragment; KNM-BG 14800 (= BG‘X’ 61’84), an upper molar fragment; KNM-BG 14801 (= BG‘X’ 472’84), a trigonid of a right lower molar; KNM-BG 14803 (= BG‘X’ 390’84), a fragment of upper postcanine tooth?; KNM-BG 14804 (= BG‘X’ 393’84), a right upper molar fragment; KNM-BG 14805 (= BG‘X’ 243’84), a talonid of left m1 or m2; KNM-BG 14806 (= BG‘J’ 534’84), an upper premolar fragment; KNM-BG 14807 (= BG‘X’ 260’84), an upper molar fragment; KNM-BG 14808 (= BG‘X’ 56’84), an upper molar fragment; KNM-BG 14809, a left m3 hypoconulid; KNM-BG 14810 (= BG‘X’ 287’84), a trigonid of a left lower molar; KNM-BG 14811 (= BG‘X’ 71’84), a molar fragment; KNM-BG 14812 (= BG‘X’ 473’84), a talonid fragment of a right lower molar; KNM-BG 35346, a right m3 talonid; KNM-BG 35453, a lingual part of P4; KNM-BG 37176, a tooth fragment; KNM-BG 41991, a right m3 hypoconulid; KNM-BG 45530, a right upper molar fragment.
Tentatively referred material.—KNM-BG 15097 (= BG‘X’ 475’84), a right upper incisor (I2?); KNM-BG 16917 (= BG‘X’ 304’84), a right upper incisor (I2?); KNM-BG 35443, an incisor; KNM-BG 17873, a phalanx; KNM-BG 42182, the distal part of a phalanx; KNM-BG 45586, the distal part of a phalanx. These specimens are excluded from the type series.
Type locality.—Kyoto University site number BG-C (1°50′05.5″ N and 36°40′47.2″ E), Nachola locality, Baragoi area, northern Kenya (Table 3; File S3; Ishida et al., 2001).
| Site no. | Unit | GPS data (WGS84) | Specimen no. (KNM-BG) |
|---|---|---|---|
| BG-A | Unit 1 | 1°48′20.5″ N, 36°40′54.0″ E | 35346, 35453 |
| BG-C | Unit 1 | 1°50′05.5″ N, 36°40′47.2″ E | 39988 (holotype), 35482, 41991 |
| BG-J | Unit 1 or 3 | 1°52′05.6″ N, 36°39′48.9″ E | 14806 |
| BG-K | Unit 3 | 1°52′08.1″ N, 36°39′35.4″ E | 42182, 45586 |
| BG-O | Unit 3 | 1°48′44.5″ N, 36°40′10.3″ E | 37176, 35443 |
| BG-X | Unit 3 | 1°49′13.3″ N, 36°39′12.0″ E | 14790, 14798–14805, 14807–14812, 15097, 16917, 17873 |
| BG-11 | Unit 1 | 1°53′09.0″ N, 36°37′24.9″ E | 45530 |
| BG-13 | Unit 3 | 1°52′18.5″ N, 36°36′58.0″ E | 17857 |
Horizon.—Unit 1 and 3 (Sawada et al., 1999) of the lower part of the Aka Aiteputh Formation (Figure 2; Table 3).
Age.—Langhian (early middle Miocene, within ca. 15.0–14.4 Ma) (Figure 2; Sawada et al., 1998, 2006; Nakatsukasa and Kunimatsu, 2009).
Measurements.—KNM-BG 39988 (holotype; right p4): length = 16.2 mm; width = 10.3 mm. KNM-BG 17857 (left P4): length = 11.6 mm; width = 14.2 mm. KNM-BG 14802 (paratype; right M3?): length = 18.8 mm; mesial width = 20.6 mm (estimate); distal width = 19.7 mm. KNM-BG 35482 (paratype; right m1?): length = 17.6 mm; trigonid width = 9.7 mm; talonid width = 11.3 mm. KNM-BG 14798 (paratype; left m1? talonid): width = 9.0 mm. KNM-BG 14801 (right lower molar trigonid): width = 12.7 mm. KNM-BG 14804 (paratype; right upper molar): length = 15.0 mm. KNM-BG 14805 (paratype; left m1? talonid): width = 9.3 mm. KNM-BG 14810 (paratype; left lower molar trigonid): width = 12.4 mm. KNM-BG 35346 (paratype; right m3 talonid): width = 14.4 mm. KNM-BG 15097 (right I2?): length = 9.4 mm; width = 5.0 mm. KNM-BG 16917 (right I2?): length = 8.7 mm; width = 5.1 mm. KNM-BG 35443 (incisor): length = 6.8 mm; width = 4.6 mm. KNM-BG 17873 (phalanx): length = 14.6 mm; proximal width = 11.4 mm; distal width = 10.2 mm. KNM-BG 42182 (distal part of phalanx): distal width = 11.5 mm. KNM-BG 45586 (distal part of phalanx): distal width = 11.8 mm.
Etymology.—The specific name refers to the town of Baragoi, which is located near Nachola where the holotype was found.
Diagnosis.—As for the genus by monotypy.
Remarks.—Several specimens described here were preliminarily reported by Pickford et al. (1999) as “Afromeryx zelteni.” The currently missing specimens from Nachola listed as “A. zelteni” by Pickford et al. (1999) are as follows: BG‘X’ 13’84, the distal end of a metapodial; BG‘X’ 57’84, an upper molar fragment; BG‘O’ 3’84, a partial astragalus; BG‘X’ 135’84, right DI2; BG‘X’ 64’84, left DI2; BG‘T’ 433’84, right I2; BG‘X’ 305’84, right I2; BG‘N’ 13’84, an astragalus; BG‘X’ (no numerical number), an m3 talonid.
Comparative description.—The specimens consist of isolated postcanine teeth, with three incisors and three phalanges that are tentatively referred to the species. The overall size and morphology of the postcanine teeth are roughly similar to those of Sivameryx and Afromeryx, which are small-sized merycopotamins. The postcanine teeth are low-crowned and bunoselenodont/selenodont.
P4 (Figure 6B) has three roots and two main cusps, a paracone and protocone. The preparacrista extends mesiobuccally and turns mesially, connecting to the mesiobuccal margin of the crown. The postparacrista extends distobuccally, connecting to the distobuccal margin of the crown. These two cristae make a V-shaped ectoloph. The preprotocrista extends mesiobuccally, merging with the mesial cingulum near the parastyle. The postprotocrista extends distobuccally, connecting to a swelling on the distal cingulum. There are continuous mesial, lingual, and distal cingula. The lingual cingulum is stronger than the mesial and distal cingula. There is a weak buccal cingulum at the buccal base of the paracone.
The upper molars (Figures 6C, 7G, T) are bunoselenodont/selenodont and tetracuspidate, lacking a paraconule. The paracone and metacone have buccal ribs. The ectoloph is W-shaped. The preparacrista extends mesiobuccally/buccally, connecting to the parastyle buccally rather than lingually. The preprotocrista extends mesiobuccally, connecting to the parastyle/mesial cingulum. The postprotocrista extends buccally, stopping at the lingual base of the paracone. The lingual postprotocrista is absent. The premetacristule extends mesiobuccally, disappearing at the median transverse valley. The postmetacristule extends distobuccally, merging with the distal cingulum. The parastyle is well developed. The mesostyle is looplike. The metastyle is weak. The median transverse valley is deep and well invaded into the mesostyle. There are continuous mesial, lingual, and distal cingula. The buccal cingulum is weak.
The p4 (Figure 6A; holotype) has three main cusps, a protoconid, paraconid, and metaconid (or entostylid). The protoconid is the largest and tallest cusp. The paraconid and metaconid are well developed and are isolated from the cristids of the protoconid. The metaconid is larger than the paraconid. The preprotocristid extends mesiolingually, bearing a small extra cuspid at the distobuccal base of the paraconid. The postprotocristid extends distolingually, connecting to the postmetacristid. The preparacristid extends mesiolingually, connecting to the mesiolingual margin of the crown. The buccal paracristid extends buccally, connecting to the mesiolingual base of the small extra cuspid. The premetacristid extends mesially, disappearing at the distolingual base of the paraconid. The postmetacristid extends distobuccally. At the distal trigonid wall, a cristid originates below the V-shaped notch between the protoconid and the metaconid and extends distally. This cristid connects to the distal cingulum, bearing a talonid swelling. The cingulum surrounds the crown, but it is weakened at the buccal and lingual parts of the trigonid. The trigonid basin is narrow and deep.
The lower molars are poorly preserved. The paraconid is absent. The preprotocristid and prehypocristid (cristid obliqua) reach close to the lingual margin of the crown. In KNM-BG 14801 (Figure 7E), both the premetacristid and lingual premetacristid are present, and in KNM-BG 14810 (Figure 7M), the lingual premetacristid is absent. The preentocristid extends mesiobuccally, connecting to the prehypocristid. There is a lingual postentocristid and no postentocristid. There are mesial, buccal, and distal cingula. The lingual cingulum is absent. The m3 hypoconulid (Figure 7P) is elongated, buccally situated, single cuspid, and looplike with pre- and posthypocristulids, and it lacks a cingulum. The m3 length is roughly estimated to be ca. 30 mm.
The three incisors (KNM-BG 15097, 16917, and 35443) and three phalanges (KNM-BG 17873, 42182, and 45586) are tentatively referred to the hypodigm of N. baragoiensis. The crowns of the incisors are somewhat flattened buccolingually and are convex buccolingually in occlusal view (Figure 7X–Z). In KNM-BG 15097 (a right upper incisor), the tooth neck is extended diagonal to the extension of the tooth root in buccal and lingual views, and the mesial part is more heavily worn than the distal part. KNM-BG 15097 and 16917 were identified as right I2 by Pickford et al. (1999). The phalanges are somewhat flattened dorsoventrally (dorsovolarly/plantarly) and have distal articular surfaces (Figure 7U–W), suggesting that none of them are distal (i.e., first) phalanges. The proximal parts of KNM-BG 42182 and 45586 are broken away (Figure 7V–W).
Discussion.—The overall size and morphology (relatively small-sized, low-crowned, and bunoselenodont/selenodont dentition) of the Nachola specimens are comparable to those of the primitive Merycopotamini (sensu Lihoreau et al., 2016) such as Sivameryx, Afromeryx, and Gonotelma, which are relatively small-sized genera. Several specimens described here were assigned to Afromeryx zelteni, which is a small-sized species of the genus, by Pickford et al. (1999) and Tsujikawa and Nakaya (2005), mainly based on its small size and on the lack of the upper molar paraconule. The Nachola specimens are distinguished from more derived merycopotamins (Merycopotamus and Libycosaurus) as well as Bothriodon and Aepinacodon mainly in having less selenodont dentition and in being smaller. They are distinguished from another merycopotamin Hemimeryx in having upper molars that are wider than long and in being smaller. They are further distinguished from Merycopotamus and Libycosaurus in having a molar premetacristid (Figure 8; Holroyd et al., 2010) and a less angular mesiobuccal corner of the P4 crown. Also, they are distinguished from the contemporaneous African bothriodontine Brachyodus mainly in having tetracuspidate upper molars and a looplike molar mesostyle. They are distinguished from Elomeryx and Bothriogenys mainly in having tetracuspidate upper molars and a more mesiodistally compressed talonid on the lower molars. Gernelle et al. (2023) included Parabrachyodus and Telmatodon from Asia in the Merycopotamini and concluded that these two genera constitute the most basal clade of the Merycopotamini (sensu Gernelle et al., 2023). The Nachola specimens are markedly distinguished from Parabrachyodus and Telmatodon in being much smaller, in having a steeper lingual slope of the molars in mesiodistal view, and in lacking a lingual postprotocrista on the upper molars. They are further distinguished from Parabrachyodus in having much less bunodont dentition.
Whereas the Nachola specimens show the greatest morphological similarity with Sivameryx, Afromeryx, and Gonotelma (primitive Merycopotamini) among the known bothriodontines, our observation of the present specimens led us to erect a new genus and species for them and to reconsider their phyletic relationships.
The molars from Nachola are distinguished from those of Afromeryx and Gonotelma in lacking a lingual postprotocrista, in that the preprotocristid and prehypocristid (cristid obliqua) reach close to the lingual margin of the crown, and in that the m3 hypoconulid is buccally situated (Figure 8). In Afromeryx and Gonotelma, the preprotocristid and prehypocristid of the lower molars do not reach the lingual border of the crown, and the m3 hypoconulid is situated centrally rather than buccally (Figure 8; Lihoreau and Ducrocq, 2007; Holroyd et al., 2010).
Although the molars from Nachola are more comparable to Sivameryx than to Afromeryx and Gonotelma in overall morphology, they are distinguished from those of Sivameryx (Figure 8; Lihoreau and Ducrocq, 2007; Holroyd et al., 2010). The lower molars from Nachola are comparable to Sivameryx in that the preprotocristid and prehypocristid (cristid obliqua) of the lower molars reach close to the lingual margin of the crown and that the m3 hypoconulid is buccally situated. These lower molar characteristics of the Nachola specimens and Sivameryx are more derived than those of Afromeryx and Gonotelma. However, the upper molars from Nachola are distinguished from Sivameryx in lacking a paraconule and a lingual postprotocrista.
The Nachola specimens are diagnosable particularly based on p4 morphology. The p4 of the Nachola specimens is distinguished from those of Afromeryx and Sivameryx in having a better developed and isolated paraconid and metaconid (the paraconid is smaller than the metaconid) with an extra cuspid between the protoconid and the paraconid (Figures 6A, 8). Sivameryx has a better developed paraconid and metaconid on p4 than Afromeryx does. However, the Nachola specimens still have a better developed paraconid and metaconid on p4 than Sivameryx does (Figures 6A, 8; Pickford, 1991b; Miller et al., 2014). The p4 morphology of Gonotelma, which is an Asian taxon, is unknown (Pilgrim, 1912; Forster-Cooper, 1924; Gernelle et al., 2023). It is noteworthy that this p4 morphology of the Nachola specimens with an isolated and large paraconid and metaconid and an extra small cuspid between the protoconid and the paraconid is reminiscent of that of Merycopotamus and Libycosaurus (Figure 8; Ducrocq et al., 2001; Lihoreau et al., 2004, 2007, 2019). The combination of the above-mentioned characteristics of the Nachola specimens indicates that they are not referrable to any known anthracotheriid genus, so we here erect a new genus and species for them.
Nacholameryx is likely more closely related to more derived Merycopotamini such as Libycosaurus and Merycopotamus than to Sivameryx, Afromeryx, and Gonotelma. Nacholameryx is more derived than Sivameryx in having tetracuspidate (no paraconule) upper molars and is also more derived than Afromeryx and Gonotelma in that the preprotocristid and prehypocristid (cristid obliqua) of the lower molars reach close to the lingual margin of the crown and that the m3 hypoconulid is buccally situated. Note that another primitive merycopotamin Hemimeryx, which is considered to be the sister taxon of Sivameryx (e.g. Lihoreau and Ducrocq, 2007; Lihoreau et al., 2016, 2019; Gernelle et al., 2023), has an autoapomorphic characteristic that the upper molars are longer than wide (Lihoreau and Ducrocq, 2007). In contrast, Nacholameryx shares the following derived characteristics with Libycosaurus and Merycopotamus: tetracuspidate upper molars without a lingual postprotocrista and an isolated and large paraconid and metaconid on p4 with an extra cuspid(s) between them.
Phylogenetic analysis.—We performed parsimony phylogenetic (cladistic) analyses using the data matrix of Gernelle et al. (2023) to test the phyletic position of Nacholameryx. We added Nacholameryx to the character-taxon matrix of Gernelle et al. (2023) using Mesquite 3.81 (build 955) (Maddison and Maddison, 2023). The revised data matrix used here consists of 77 taxa and 224 characters, which is available as a NEXUS file (File S4). The data matrix was analyzed using PAUP 4.0a (build 169) (Swofford, 2003) and TNT (version 1.6) (Goloboff and Morales, 2023).
The PAUP analysis (File S5) was performed with the heuristic search option (optimality criterion = parsimony; character-state optimization = accelerated transformation or ACCTRAN). All characters were coded as unordered and unweighted, and Gujaratia, Bunophorus, and Homacodon were chosen as outgroup taxa (Gernelle et al., 2023). It recovered 20 most parsimonious trees (MPTs) with a length of 1580 steps (File S5). Each most parsimonious tree has a consistency index (CI) of 0.1975, homoplasy index (HI) of 0.8025, retention index (RI) of 0.6346, and rescaled consistency index (RC) of 0.1253. The 50% majority-rule consensus tree of the 20 equally most-parsimonious trees are shown in File S6.
For the TNT analysis, the NEXUS data file (File S4) was transformed to TNT file using Mesquite (the ‘gaps’ were converted to ‘missing’). The TNT analysis (File S7A) was performed with the traditional and new technology search options. The traditional search recovered 20 MPTs with a length of 1571 steps, and the new technology search recovered four MPTs with a length of 1573 steps. The tree lengths of the MPTs returned by the TNT parsimony analyses (1571 and 1573) are slightly shorter than those returned by the PAUP parsimony analysis (1580) (Files S5, S7A). This difference is likely due to the difference of the algorithms used by the programs among them, and this is not fundamentally problematic in the discussion here. 50% majority-rule consensus trees of the MPTs returned from the traditional and the new technology searches are shown in File S7B and File S7C, respectively.
Within the [Bakalovia + Elomeryx + Merycopotamini] clade, the topologies of the consensus trees by both the PAUP and TNT parsimony analyses were the same (Figure 9; Files S6, S7B–C). Nacholameryx is located within the Merycopotamini (sensu Lihoreau et al., 2016) and is positioned as the sister taxon of the [Merycopotamus + Libycosaurus] clade (Figure 9). Therefore, the results of our phylogenetic analysis raise the possibility that the [Merycopotamus + Libycosaurus] clade (the oldest record is ca. 13.9 Ma in South Asia; Lihoreau et al., 2007, 2015b) originated in Africa during the middle Miocene.
The Anthracotheriidae (Mammalia, Artiodactyla) from the lower part of the middle Miocene (ca. 15.0–14.4 Ma) Aka Aiteputh Formation in Nachola (northern Kenya) consists of two bothriodontine genera represented by two species, Brachyodus aequatorialis nacholaensis subsp. nov. and Nacholameryx baragoiensis gen. et sp. nov. The fossils of B. aequatorialis nacholaensis were collected only from Kyoto University site number BG-11. The fossils of N. baragoiensis were collected from several sites, including BG-11 in Nachola (Table 3). Therefore, these two anthracotheriid taxa co-occur at BG-11 (Table 3; Ishida et al., 2001). B. aequatorialis nacholaensis differs from B. aequatorialis aequatorialis from Rusinga (the type locality of the species) in P4/p4 morphology. This morphological difference could be attributed to be a chronological gap (ca. 15.0–14.8 Ma vs. ca. 18–16 Ma) between the two subspecies. According to the results of our phylogenetic analyses, Nacholameryx is the sister taxon of the [Merycopotamus + Libycosaurus] clade (Figure 9). This raises the possibility that the [Merycopotamus + Libycosaurus] clade originated in Africa during the middle Miocene.
Most of the mammalian species from the Aka Aiteputh Formation in Nachola are recorded either only in Unit 3 or in both Unit 1 and Unit 3 (Table 1; Tsujikawa and Nakaya, 2005). Among the 28 mammalian species known from the Nachola locality (Table 1), just three are recorded only in Unit 1: B. aequatorialis nacholaensis, Diamantohyus sp., and Climacoceras sp. It is noteworthy that B. aequatorialis nacholaensis is likely represented only by a single individual and that another species of Diamantohyus, Diamantohyus nadirus, is recorded in Unit 3. Unit 1 has also yielded far fewer specimens than Unit 3: approximately ten times as many fossil specimens were obtained from Unit 3 as from Unit 1 (Nakatsukasa and Kunimatsu, 2009). Furthermore, Climacoceras sp. is represented only by a fragmentary ossicone and two astragali from Unit 1, and there is a possibility that Climacoceras sp. is also recorded in Unit 3 (Table 1). Tsujikawa et al. (2004) and Tsujikawa and Nakaya (2005) suggested that there is a chronological difference between Unit 1 and Unit 3, on the basis of the mammalian faunal difference: Unit 1 is correlated to the faunal set II or IIIa (ca. 18–16 Ma) of Pickford (1981, 1986, 1991a, 1991b) and Pickford and Morales (1994), and Unit 3 is correlated to faunal set III or IIIb (ca. 16–14 Ma). However, this must be carefully interpreted. Indeed, according to radioisotopic ages (Figure 2; Sawada et al., 1998, 2006; Nakatsukasa and Kunimatsu, 2009), both Unit 1 and Unit 3 are dated as within ca. 15.0–14.4 Ma, and there is likely no significant chronological difference between them. One of the reasons for the correlation of Unit 1 to the faunal set II or IIIa (ca. 18–16 Ma) by Tsujikawa et al. (2004) and Tsujikawa and Nakaya (2005) is the presence of “Afromeryx zelteni” and Brachyodus aequatorialis (both of which are mainly recorded in the lower Miocene). However, our reappraisal of the Nachola anthracotheriids revealed that Brachyodus aequatorialis from Nachola is morphologically distinguished from the specimens from the type locality of the species in Rusinga (lower Miocene) and that specimens from Nachola previously referred to “Afromeryx zelteni” actually represent a distinct new genus and species, Nacholameryx baragoiensis. Therefore, the results of the mammalian faunal analysis by Tsujikawa et al. (2004) and Tsujikawa and Nakaya (2005) should be reappraised.
We are grateful to the National Commission for Science, Technology and Innovation (NACOSTI) of the Government of Kenya for giving research permission to us. We thank Mary Gikungu, Fredrick Kyalo Manthi, Emmanuel K. Ndiema, Sarah Wanjiku Kimani, Margaret Omoto, Mary Muungu, and Francis Ndiritu Muchemi (National Museums of Kenya, Nairobi, Kenya) for their help in examining the specimens in the museum. We are also grateful to the personnel of Japan Society for the Promotion of Science, Nairobi Research Station for their help during our stay in Kenya and to the support of the Domestic Research Program from Ryukoku University. Thanks are also due to Killian Gernelle (Université Montpellier, Montpellier, France) for providing the NEXUS file from the phylogenetic analysis of Gernelle et al. (2023). This manuscript was improved by two reviewers, Pat Holroyd (University of California, Berkeley, USA) and Fabrice Lihoreau (Université Montpellier, Montpellier, France), who both provided useful comments. This research was supported by JSPS KAKENHI Grant Numbers 25257408, 16H02757, and 23H02562 (to M. Nakatsukasa) and by Bilateral Programs Joint Research Project (JSPS-NACOSTI).
File S1. Brachyodus aequatorialis nacholaensis subsp. nov. from the Miocene Aka Aiteputh Formation of Nachola (Kenya), occlusal view (stereo pair) of the lower postcanine dentition of KNM-BG 35340 (holotype). A, left p4, trigonid of m1, m2, and m3 without the hypoconulid; B, p3–m2; File S2. Brachyodus aequatorialis nacholaensis subsp. nov. from the Miocene Aka Aiteputh Formation of Nachola (Kenya), upper postcanine dentition. A, KNM-BG 45220, right P3–M2, occlusal view (stereo pair); B, KNM-BG 39918, left P2–M2. C, KNM-BG 39919 + 45210, left M3: C1, occlusal view (stereo pair); C2, distal view; File S3. Topographic map of Nachola area (Kenya), showing the fossil localities of the Anthracotheriidae within the Miocene Aka Aiteputh Formation (Table 3; after Ishida et al., 2001); File S4. NEXUS file of the phylogenetic analysis used in this article (after Gernelle et al., 2023); File S5. Some results of the PAUP analysis performed in this article; File S6. Phylogenetic trees returned by the PAUP analysis. A, 50% majority-rule consensus tree of the 20 equally most parsimonious trees analyzed in this article using PAUP 4.0a (build 169) (Swofford, 2003). The tree length is 1580 steps. Note that Nacholameryx gen. nov. is the sister taxon of the [Merycopotamus + Libycosaurus] clade. B, The Mesquite tree view of 50% majority-rule consensus tree of the 20 most parsimonious trees analyzed in this article using PAUP (File S6A). Note that the tree length is 1746 although the tree length of the PAUP analysis is 1580 steps. C, A manually edited Mesquite tree view of the tree of File S6B. The positions of Epirigenys and Myaingtherium were changed. Note that the tree length is 1743, which is smaller than that in File S6B; File S7. TNT analysis. A, Some results of the TNT analysis performed in this article. B, 50% majority-rule consensus tree of the 20 most-parsimonious trees analyzed in this article using TNT version 1.6 with the traditional search option (Goloboff and Morales, 2023). Note that Nacholameryx gen. nov. is the sister taxon of the [Merycopotamus + Libycosaurus] clade. C, 50% majority-rule consensus tree of the four most-parsimonious trees analyzed in this article using TNT version 1.6 with the new technology search option (Goloboff and Morales, 2023). Note that Nacholameryx gen. nov. is the sister taxon of the [Merycopotamus + Libycosaurus] clade; File S8. Some results of the maximum-likelihood analysis using PAUP. Note that Nacholameryx gen. nov. is positioned as the sister taxon of the [Merycopotamus + Libycosaurus] clade as in the parsimony analyses (Files S6, S7B–C).
T. T. organized this study, analyzed data, and was primarily responsible for the taxonomic aspects. T. T., Y. K., and H.T. examined the specimens. Y. K., H. T., and M. N. conducted geological and paleontological field research and collected fossil specimens. All authors contributed to the writing of the paper.
