A new species of Mioeuoticus (Lorisiformes, Primates) from the early Middle Miocene of Kenya

Abstract

We here describe a prosimian specimen discovered from the early Middle Miocene (~15 Ma) of Nachola, northern Kenya. It is a right maxilla that preserves P⁴–M³, and is assigned to a new species of the Miocene lorisid genus Mioeuoticus. Previously, Mioeuoticus was known from the Early Miocene of East Africa. The Nachola specimen is therefore the first discovery of this genus from the Middle Miocene. The presence of a new lorisid species in the Nachola fauna indicates a forested paleoenvironment for this locality, consistent with previously known evidence including the abundance of large-bodied hominoid fossils (Nacholapithecus kerioi), the dominance of browsers among the herbivore fauna, and the presence of plenty of petrified wood.

Introduction

Although there are relatively abundant fossils of adapoid and omomyoid prosimians known from the Eocene of North America and Europe, prosimian fossils more closely related to living taxa are relatively rare in the fossil record. The fossil record of tarsiers is very poor (Gunnell and Rose, 2002; Rossie et al., 2006; Ni et al., 2013). Living lemurs are restricted to Madagascar, and no fossil lemurs are known prior to the Late Pleistocene (Godfrey and Jungers, 2002). Fossil lorisiforms are known from Africa and South Asia. In Africa, there are three genera (one lorisid: Mioeuoticus, and two galagids: Progalago, Komba) from the Early to early Middle Miocene (20–15 Ma), and one genus (Galago) from the Plio-Pleistocene (Phillips and Walker, 2000; Harrison, 2010). In Egypt, Pickford et al. (2006) reported the presence of Galago in the Late Miocene (~10 Ma). Two upper molars of galagid indet. were known from the Late Miocene (10–9 Ma) of Namibia (Conroy et al., 1993; Rasmussen and Nekaris, 1998). A galagid maxillary fragment with two upper molars was recently discovered from the Late Miocene of Kenya (~9.9 Ma) (Kunimatsu et al., 2017). Prior to the Miocene, stem galagids such as Saharagalago (~37 Ma) and Wadilemur (~34 Ma), and a possible lorisid or stem lorisiform (Karanisia; ~37 Ma) are known from the Fayum area in Egypt, supporting the hypothesis of an ancient Afro-Arabian origin for crown strepsirrhines and an Eocene divergence of extant lorisiform families (Seiffert et al., 2003, 2005; Seiffert, 2012). In addition, Pickford (2015) described a new lorisid genus, Namaloris rupestris, based on an upper molar disocvered from the Eocliff Limestone site EC 9 (Bartonian: 41.3–38.0 Ma) in Namibia. Another enigmatic primate, Notnamaia bogenfelsi, is known from the Lutetian (47.8–41.3 Ma) of Namibia (Pickford et al., 2008; Pickford and Uhen, 2013), and was originally considered to be an anthropoid (Pickford et al., 2008). Later, Godinot (2015) considered that N. bogenfelsi had overall similarities with European anchomomyin adapoids, although he also recognized that the former showed some significant differences from the latter.

In this article, we describe a new species of Mioeuoticus, based on material discovered from the early Middle Miocene deposits in Nachola, northern Kenya. Previously, this genus was known only from the Early Miocene, including two species, one from Napak I in Uganda (M. bishopi, ~19–18 Ma) and the other from Rusinga Island (M. shipmani, ~18 Ma) in Kenya (Phillips and Walker, 2000). In addition, Harrison (2010) proposed that the specimens previously assigned to Progalago songhorensis from Songhor and Rusinga in Kenya (~19–18 Ma) should be included in the hypodigm of M. bishopi. A talus from Koru (~20–19 Ma) and a calcaneus from Songhor (~19 Ma) in Kenya have been tentatively assigned to Mioeuoticus (Gebo, 1986, 1989). A loris-like distal humerus (MUZM 30) compatible in size with Mioeuoticus was reported from Napak (Gebo et al., 1997), but Pickford (2012) refuted its primate affinity, suggesting that it belongs to Paranomalurus bishopi, an arboreal rodent. The new species from Nachola is therefore the first discovery of this genus in the Middle Miocene.

Nachola is located west of the township of Baragoi in northern Kenya. Through a long-term field project conducted since the early 1980s, the Kenya–Japan Joint Expedition has recovered hundreds of primate fossils, most of which are attributed to a large-bodied Miocene hominoid Nacholapithecus kerioi (Ishida et al., 1984, 1999, 2004; Nakatsukasa et al., 1998, 2003; Kunimatsu et al., 2004; Nakatsukasa and Kunimatsu, 2009). The Nachola primate fauna also includes a nyanzapithecine small catarrhine (Nyanzapithecus harrisoni) (Kunimatsu, 1992, 1997), a victoriapithecid monkey, and Komba sp. (Tsujikawa and Nakaya, 2005). In Nachola, fossils have been discovered from the lower part of the Aka Aithputh Formation. The chronological age of the fossiliferous deposits dated by the K–Ar method is 16.4–15.3 Ma (Sawada et al., 2006). A more recent analysis of anorthoclase grain samples from the fossil-bearing horizon, from which primates and the majority of other vertebrate fossils were obtained, yielded a ⁴⁰Ar–³⁹Ar age of 14.77 ± 0.10 Ma, in which the authors have confidence (Nakatsukasa and Kunimatsu, 2009). The fauna from this horizon (Tsujikawa and Nakaya, 2005) corresponds to the faunal set IIIb by Pickford (Pickford, 1981; Pickford and Morales, 1994), which is in accord with the results of K–Ar and ⁴⁰Ar–³⁹Ar dating.

Systematics

Order Primates Linnaeus, 1758

Suborder Strepsirrhini Geoffroy, 1812

Infraorder Lorisifomes Gregory, 1925

Superfamily Lorisoidea Gray, 1821

Family Lorisidae Gray, 1821

Subfamily Mioeuoticinae Harrison, 2010

Genus Mioeuoticus Leakey, 1962

Mioeuoticus kichotoi sp. nov.

Diagnosis

Upper P⁴ is subtriangular in occlusal outline, having two main cusps. Upper M¹ and M² have a square occlusal outline with the hypocone being well developed. Hypocone is not developed in upper M³, whose occlusal outline is consequently more triangular than in the anterior molars. Buccal cingulum is well developed on all upper molars.

Differential diagnosis

The new species is different from Komba and Progalago in the occlusal outline of the upper molars, which is subsquare in M¹ and M², and triangular in M³, lacking the concavity on the distal margin in the latter taxa. Hypocone on M¹ and M² located distal to the protocone, hence differing from the more distolingual position of the hypocone in Komba and Progalago. Differs from both M. bishopi and M. shipmani in having upper molars proportionally more elongate in mesiodistal direction and in relative molar size: In M. kichotoi, upper M² is slightly larger than M¹ and similar in size to M³, while the upper molars become smaller from M¹ to M³ in M. bishopi. In M. shipmani, M² is the largest molar with M¹ being similar in size to M³. Further differs from M. bishopi in having upper P⁴ slightly smaller and more triangular in occlusal outline, larger upper M²–M³, hypocone on M¹–M² being separated from the protocone by a groove, and better developed crista obliqua. Further differs from M. shipmani in smaller dental size, proportionally shorter upper P⁴, and narrower buccul cingulum on upper molars.

Holotype

KNM-BG 48081

Type locality

Site BG-K, Nachola village near Baragoi town, northern Kenya

Horizon

Lower part of the Aka Aiteputh Formation

Age

Early Middle Miocene (~15 Ma)

Etymology

After Mzee Kichoto, who has for years contributed much to maintaining the field camp in Nachola as the head of the local camp workers.

Hypodigm

Holotype only

Description

The holotype (KNM-BG 48081) is a right maxillary fragment with P⁴–M³ (Figure 1, Figure 2, Table 1). It preserves a part of the palatal process and orbital floor. There is a small foramen piercing the orbital floor reaching to the palatal ceiling near the mesiolingual corner of the M² crown. P⁴ is subtriangular in occlusal outline and is two-rooted (Figure 3). The buccal portion of the crown is longer mesiodistally than the lingual portion, with oblique and concave mesial and transverse distal margins. There are two main cusps (paracone and protocone). A chip of enamel is missing from the protocone. The paracone is larger in area than the protocone. The paracone is buccolingually narrower than the mesiodistal length of the cusp. The preparacrista is longer than the postparacrista, and a style is well developed at the base of the preparacrista.

Figure 1

The holotype (KNM-BG 48081) of Mioeuoticus kichotoi, a right maxilla with P⁴–M³: (a) occlusal view (stereo); (b) linguocclusal view; (c) buccal view. Scale bar = 5 mm.

Figure 2

3D digital models of the holotype (KNM-BG 48081) of Mioeuoticus kichotoi, based on the CT scanning data: (a) occlusal view; (b) superior view; (c) buccal view; (d) lingual view; (e) posterior view; (f) anterior view. Scale bar = 5 mm.

Table 1 Dental measurements of Mioeuoticus species

Taxon	Acc. No.	Locality	P4		M1		M2		M3
			MD	BL	MD	BL	MD	BL	MD	BL
M. kichotoi	KNM-BG 48081	Nachola	2.1	2.8	3.5	3.7	3.5	4.1	3.5	4.0
M. shipmani	KNM-RU 2052	Rusinga	2.8	3.2	3.9	4.5	3.8	5.1	3.7	4.6
M. bishopi	NAP.I.3 6/58*	Napak	2.3	3.1	3.4	3.9	3.1	3.9	2.9	3.4
	KNM-SO 1312	Songhor	—	—	—	—	3.1	4.0	—	—
	KNM-RU 3415	Rusinga	—	—	—	—	3.2	4.2	—	—
	M. bishopi mean		2.3	3.1	3.4	3.9	3.1	4.0	2.9	3.4

*  Data from Phillips and Walker (2000).

Figure 3

CT image of the holotype maxilla in a horizontal section slightly above the cervical region of the cheek teeth, showing the roots of P⁴–M³. White arrows indicate the two roots of P⁴.

M¹ is tetracuspid, and is almost square in occlusal outline, but the buccal margin is slightly longer mesiodistally than the lingual margin with a slightly oblique mesial margin. The cusps are low. The protocone is the largest cusp, but is slightly lower than the buccal cusps. The paracone is slightly larger than the metacone, and both of them are buccolingually narrower relative to their mesiodistal length. The hypocone is well developed, occupying the distolingual corner of the crown, but is lower and more rounded than the other three cusps. The hypocone is separated from the protocone by a groove. The preprotocrista runs mesiobuccally and merges with the mesial marginal ridge. The postprotocrista runs distally and slightly buccally at first, and then turns to a more distobuccal direction. It meets with the lingual ridge from the metacone to make a low crista obliqua. Although the buccal surface of the crown is eroded, the preserved morphology indicates that the buccal cingulum is well developed, being almost continuous along the buccal aspect except that it might be briefly interrupted on the buccal face of the paracone. There is neither paraconule nor metaconule.

M² is basically similar to M¹, but the crown is relatively broader buccolingually (Table 2). Consequently, the trigon basin is also relatively broader. The hypocone is more lingually placed relative to the protocone, and the degree of lingual displacement of the hypocone is more marked than in M¹. A chip of enamel is missing from the metacone apex. The buccal surface is somewhat eroded, but it is apparent that the buccal cingulum is prominently developed all over the buccal aspect, though there may be a slight interruption on the buccal face of the paracone.

Table 2 Crown area (MD × BL) and proportion (MD/BL) of Mioeuoticus species

Taxon	Acc. No.	Locality	MD × BL (mm2)				MD/BL (%)
			P4	M1	M2	M3	P4	M1	M2	M3
M. kichotoi	KNM-BG 48081	Nachola	5.9	13.0	14.4	14.0	75.0	94.6	85.4	87.5
M. shipmani	KNM-RU 2052	Rusinga	9.0	17.6	19.4	17.0	87.5	86.7	74.5	80.4
M. bishopi	NAP.I.3 6/58*	Napak	7.1	13.3	12.1	9.9	74.2	87.2	79.5	85.3
	KNM-SO 1312	Songhor	—	—	12.4	—	—	—	77.5	—
	KNM-RU 3415	Rusinga	—	—	13.4	—	—	—	76.2	—
	M. bishopi mean		7.1	13.3	12.6	9.9	74.2	87.2	77.7	85.3

*  Calculated using data from Phillips and Walker (2000).

M³ is different from the anterior two molars in lacking the hypocone and having a more triangular occlusal outline. It is therefore a tricuspid tooth. The protocone is the largest cusp as in the anterior molars. The metacone is much smaller than the paracone. The distal cingulum and distal marginal ridge are developed between the distal bases of the metacone and protocone. The lingual end of the distal cingulum is slightly swollen. The postprotocrista runs distobuccally to meet the lingual ridge of the metacone, forming a crista obliqua. The buccal cingulum is well developed, forming a wide ledge that is continuous along the buccal aspect of the crown, although the cingulum fades out shortly on the buccal face of the paracone. A chip of enamel is missing from the paracone apex.

Comparisons

M. kichotoi is clearly distinguished from the two previously known species of the genus in having more elongate upper molars, and relative upper molar size from M¹ to M³, as described in the differential diagnosis. In dental size, M. kichotoi is smaller than M. shipmani, and is similar in M¹ and P⁴ and moderately larger in M² and M³ compared to the type species M. bishopi (Table 1, Table 2, Figure 4). The P⁴ morphology of M. kichotoi shows some resemblance to that of M. shipmani, such as less square occlusal outline, but the crown is relatively shorter than in M. shipmani and in this respect, the Nachola species is similar to M. bishopi. The MD/BL proportion of the upper molars (Table 2, Figure 5) appears to show different tendencies between M. kichotoi and M. shipmani. The former acquired more elongate and narrower molar crowns, and the latter shorter and broader crowns, if we assume that they were derived from the intermediate condition in the oldest species M. bishopi. However, given the paucity of available lorisid fossils, it may be premature to deduce much about their phylogenetic relationships.

Figure 4

Crown areas (MD × BL in mm²) of P⁴–M³ in Mioeuoticus species. As for M. bishopi, only the type specimen (NAP.I.3 6/58) is used in this graph.

Figure 5

Crown proportion (MD/BL in %) of P⁴–M³ in Mioeuoticus species. As for M. bishopi, only the type specimen (NAP.I.3 6/58) is used in this graph.

Body mass estimates

Body mass estimates for M. kichotoi (KNM-BG 48081), M. shipmani (KNM-RU 2502) and M. bishopi (UMP-NAP. I.3 6/58) were calculated from crown areas (MD × BL) using the formulae for extant prosimians in Egi et al. (2004). The body mass estimates vary depending on which tooth is used (Table 3). The ranges of body mass estimates are 0.46–1.39 kg for M. kichotoi, 0.75–1.72 kg for M. shipmani, and 0.58–0.84 kg for M. bishopi. The multiregression formula gives body mass estimates as 0.70 kg for M. kichotoi, 1.05 kg for M. shipmani, and 0.61 kg for M. bishopi. M. shipmani is apparently much heavier than the other two species. M. kichotoi is roughly similar to or slightly heavier than M. bishopi.

Table 3 Body mass estimates (kg)

	M. kichotoi	M. shipmani	M. bishopi
P4	0.46	0.75	0.59
M1	0.64	0.93	0.66
M2	0.75	1.13	0.58
M3	1.39	1.72	0.84
Multiregression	0.70	1.05	0.61

Discussion

The new prosimian material discovered from the Aka Aiteputh Formation in Nachola is attributed to a new species of Mioeuoticus. At present, the fossil record of lorisoid primates is fairly poor, and Mioeuoticus is the only genus attributed to the Lorisidae in the Miocene African fossil record, although there are a few specimens of indeterminate lorisids known from Chamtwara (~19 Ma), Fort Ternan (~14 Ma), and Lukeino (~6 Ma) in Kenya (Harrison, 2010; Pickford, 2012). The previously known species of Mioeuoticus were confined to a relatively short period of the Early Miocene (19–18 Ma) (Harrison, 2010). The new species M. kichotoi from Nachola (~15 Ma) is the first discovery of Mioeuoticus from the Middle Miocene, and is the latest known occurrence of this genus in the fossil record, extending the chronological distribution of the genus by ~3 million years younger.

A left maxilla with P⁴–M³ of a lorisid is known from another Middle Miocene locality, Fort Ternan, in Kenya, but its dental morphology is so different from that of Mioeuoticus that the Fort Ternan lorisid is considered to belong to a distinct genus and species which is more derived than Mioeuoticus and is most similar to extant Perodicticus (Harrison, 2010). The discovery of M. kichotoi from Nachola increases the diversity of prosimians in the African Middle Miocene.

The extant lorisids are restricted to tropical/subtropical forests in Sub-Saharan Africa and South to Southeast Asia (Mittermeier et al., 2013). They are arboreal primates, and their locomotion is characterized by slow and cautious movements in trees. The extant African lorisids include angwantibos (Arctocebus spp.), pottos (Perodicticus potto), and false pottos (Pseudopotto martini), while slender lorises (Loris spp.) and slow lorises (Nycticebus spp.) are distributed in tropical rainforests to deciduous forests in Asia (Schwartz, 1996; Groves, 2001; Butynski et al., 2013; Mittermeier et al., 2013; Kingdon, 2015). The estimated body weight of ~0.7 kg for M. kichotoi is slightly smaller than P. potto (0.8–1.6 kg), but considerably larger than angwantibos (0.23–0.47 kg for A. calabarensis; 0.20–0.27 kg for A. aureus) (Kingdon, 2015).

M. bishopi was originally known from Napak, Uganda (Harrison, 2010). Recently, Harrison (2010) suggested that some specimens from Songhor and Rusinga, Kenya, should also be assigned to this taxon. M. shipmani is known from Rusinga (Philips and Walker, 2000). The fossil terrestrial gastropods collected from Napak indicate the presence of woodland to forest environments in this area around 20–18.5 Ma, although some taxa suggest that patches of more open environments may have existed occasionally (Pickford, 2004). The abundance of tragulid fossils at Napak (Pickford, 2002) may indicate the presence of forested environments, although caution is necessary as the microwear analysis of the early Miocene African tragulids from Rusinga and Songhor shows diverse diets from grazers to browers, suggesting that those fossil tragulids may have been adapted to more diverse environments than extant tragulids (Ungar et al., 2012).

The vertebrate fossil assemblages from Rusinga include both forest dwellers and those that more likely inhabited open environments, suggesting that the environments of Rusinga may have been variable lowland forests including both wetter/closed and drier/open habitats (Andrews and Couvering, 1975; Andrews et al., 1997; Nesbit Evans et al., 1981; Ungar et al., 2012). Paleobotanical and paleosol studies also support the paleoenvironmental reconstruction for Rusinga as having forested to woodland environments in a strongly seasonal and warm climate (Collinson et al., 2009; Maxbauer et al., 2013).

Plenty of petrified wood was discovered from the same stratigraphic level as the primate fossils in Nachola, suggesting the presence of forests in the paleoenvironment (Suzuki, 1987). A large-bodied hominoid N. kerioi is the most abundant taxon in the Nachola fauna (Tsujikawa and Nakaya, 2005). The locomotion pattern of N. kerioi is reconstructed as arboreal quadrupedalism with enhanced functions of the forelimbs for climbing and clambering (Rose et al., 1996; Ishida et al., 2004; Nakatsukasa et al., 1998, 2007, 2012; Nakatsukasa and Kunimatsu, 2009), suggesting that they would have needed relatively dense tree covers. The majority of the herbivorous mammals from Nachola are browsers. Although there are a few hypsodont taxa such as a rhinocerotid and a bovid, they are rare in the Nachola mammalian fauna (Tsujikawa and Nakaya, 2005).

The presence of a new lorisid species M. kichotoi described in this article further supports a forested paleoenvironment for Nachola, which is consistent with the abundance of a large-bodied, arboreal quadrupedal hominoid (N. kerioi) and the dominance of browsers among herbivore mammals in the mammalian fauna of this locality, as well as the presence of plenty of petrified wood (Suzuki, 1987; Tsujikawa and Nakaya, 2005).

Acknowledgments

We thank the government of Kenya for permitting us to carry out our research in Kenya, and the staff of the National Museums of Kenya for their assistance in laboratory and fieldwork. Our gratitude goes to the people of the village of Nachola for their long-term assistance during our fieldwork. The Nairobi Research Station of the Japan Society for the Promotion of Science has provided us with much assistance for the fieldwork in Kenya. We are also grateful to T. Harrison for the information on the specimens of M. bishopi from Songhor and Rusinga. We also thank the Primate Research Institute of Kyoto University for the support through the Cooperation Research Program. This study was financially supported by the Grants-in-Aid for Scientific Research (#13375005, #20247033, #22257408, #24570254).

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