Tragelaphus rastafari, Bibi, 2011

Bibi, Faysal, 2011, Tragelaphus nakuae: evolutionary change, biochronology, and turnover in the African Plio-Pleistocene, Zoological Journal of the Linnean Society 162 (3), pp. 699-711 : 701-703

TRAGELAPHUS RASTAFARI SP. NOV. ( FIG. 2 )

Tragelaphus aff. nakuae Gentry, 1981

Tragelaphus nakuae (in part) Gentry, 1985

Tragelaphus nakuae (in part) Harris, Brown & Leakey, 1988

Tragelaphus nakuae (in part) Harris, 1991

Tragelaphus nakuae ‘early stage’ Bibi, 2009

Tragelaphus aff. nakuae Reed & Bibi, 2011

Holotype: KNM-TH 32833, calvarium with complete left horn core and most of the right horn core, and left upper second and third molar (M 2–3) ( Fig. 2A ). This specimen derives from the Chemeron Formation at Baringo ( Kenya), from site BPRP 150 , which is located about 10 m below a tuff dated to 2.898 Mya (J. Kingston, pers. comm.). Average sedimentation rates provide a rough date of 2.96 Mya for KNM-TH 32833 (J. Kingston, pers. comm.).

Etymology: Named after the late Emperor Haile- Selassie, whose pre-coronation title and name was Ras Tafari Mekonnen, and who encouraged the formation of the International Omo Research Expedition of 1967–76.

Diagnosis: A large-sized species of Tragelaphus diagnosed by horn cores that are long and slender, with three keels, moderate to strong anteroposterior compression (DT/ DAP range = 1.13–1.50), moderately inclined against the braincase (inclination angle ~45– 55°), and undergoing between three-quarters to half a whorl of torsion (270–180°); in lateral view, posterior curvature of horn core bases is weak; large supraoccipital bar absent (weak development variable); premolar row length reduced to between 48 and 63% of the molar row length. Females horned ( Reed & Bibi, 2011).

Differs from T. moroitu in: larger size and more robust cranium; less helical horn core coiling; horn cores with reduced torsion; greater anteroposterior compression and weaker anterior keel; stronger inclination; premolar row more reduced. Differs from T. kyaloae in: horn cores with reduced torsion; more triangular horn core cross-section and stronger anterior keel; stronger inclination. Differs from T. saraitu in: horn cores with reduced torsion and stronger inclination against the braincase. Differs from T. nakuae in: horn cores with greater torsion and absence of a pronounced supraoccipital bar. Compared to the youngest specimens of T. nakuae (corresponding to Shungura Member G or younger levels), the horn cores in T. rastafari are relatively longer, with less anteroposterior compression on average, absence of rapid diminution of distal horn cores, more pronounced posterior curvature of the horn core bases, and much less drastic changes in horn core crosssection from the tips to the bases. Tragelaphus rastafari differs from T. eurycerus in horn cores that are longer on average, more widely set apart, more upright, with greater total torsion, and looser coiling.

Main horizons present: Omo Shungura: Member B. Middle Awash: WEE-VP-5, MAT-VP-6. Koobi Fora: Lokochot Member, Lower Tulu Bor Member. Baringo: Chemeron Formation BPRP site 150. Hadar: Basal Member, Sidi Hakoma Member, Denen Dora Member, Kada Hadar Member ( Fig. 1 ).

First appearance datum: Between 3.58 and 3.40 Mya (Lokochot Member, WEE-VP-5).

Last appearance datum: 2.95 Mya (Shungura Unit B10).

Specimens of significance: Hadar: A large and wellpreserved collection of T. rastafari specimens derives from Hadar. These specimens, including several partial crania with horns, were described and referred to T. aff. nakuae by Gentry (1981) and Reed & Bibi (2011). The Hadar T. rastafari specimens derive from the Basal to the Kada Hadar members, with an age range between> 3.4 and 2.95 Mya. These all show diagnostically long, slender horns completing between 270 and 180° of torsion, and a braincase lacking a supraoccipital bar.

Shungura: Specimens from Member B described by Gentry (1985) as T. nakuae are here referred to T. rastafari . From the new collections of the Omo Group Research Expedition ( Boisserie et al., 2008) come two specimens from site OMO 112 (unit B10) attributable to T. rastafari . These specimens (along with the specimen from Baringo, below) constitute the latest certain appearance datum of T. rastafari .

OMO 112/3-10006 ( Fig. 2D ): a partial cranium with complete right horn core, partial left horn core, and posterior portion of braincase. Horn cores are long and slender, with triangular cross-section throughout, torsion of 180°. There is no rapid diminution of the horn core at the tips. The posterior braincase bears plastic deformation but its dorsal surface is well preserved, being flat with no supraoccipital torus. The mastoid region is not prominent in its lateral projection, and the occipital surface is relatively tall and narrow.

OMO 112/3-10001: dorsal cranial fragments (frontals, parietals–supraoccipital) and both horn cores almost complete. Left horn core with 225° torsion preserved, estimated at 270° when complete. The supraoccipital dorsal area is flat, lacking torus.

Baringo: KNM TH-32833 is a calvarium with complete left horn core and most of the right horn core, and left M 2–3. This is here taken as the holotype specimen of T. rastafari . KNM TH-32833 includes a right horn core midshaft fragment that may be from a different individual. Horns are long and slender, and torsion is just over 180°. Cross-section is triangular with weak compression distally, to transverse oval with strong anteroposterior compression (DT/ DAP left horn core: 73.53/51.93 = 1.42) and weak to absent anterior keel proximally. Dorsal aspect of posterior braincase is flat (there is no supraoccipital torus or associated anterior depression at all). A strong intercornual torus is present. Mastoid region is enlarged .

Koobi Fora: Specimens from the Lokochot and Lower Tulu Bor members described by Harris (1991) as T. nakuae are here referred to T. rastafari . These include: KNM ER-2950 and KNM ER-5394 from the Lower Tulu Bor Member (3.36–2.90 Mya), and KNM ER-1589 and KNM ER-2825 from the Lokochot Member (3.5–3.36 Mya).

Middle Awash: From the site of Wee-ee 5, dated to 3.4 Mya ( Renne et al., 1999: fig. 8), WEE-VP-5/15 comprises right and left frontlets with half the right horn core and an almost complete left horn core, left temporal fragment, basioccipital fragment, and some other cranial fragments. The horn cores are slender, the left horn core preserves just over 180° torsion (estimated c. 210° complete), horn core cross-section is triangular, with a relatively prominent anterior keel with moderate anteroposterior compression (DT/DAP left horn core: 58.25/46.66 = 1.25). The intercornual torus is weak. Left mastoid region is very enlarged as a result of an enlarged exterior aspect of the petrous (rather than mastoid bone). The basioccipital fragment preserves anterior tuberosities that are relatively flattened round raised rugosities, located close together with thin crests extending posteriorly. Specimen does not include posterior braincase.

Notes: Geraads & Thomas (1994) attribute horn cores from the Warwire Formation in Uganda to T. aff. nakuae . These are described as being similar to the Hadar Formation ‘ T. aff. nakuae ’. In basal horn core dimensions the Warwire tragelaphin overlaps with both T. rastafari and T. saraitu . Without more complete material, it is not possible to confirm assignment to either species.

Female individuals of T. rastafari are most likely to have been horned. Despite the large collections of these fossil bovids, no hornless crania have ever been recovered. Reed & Bibi (2011) showed that the range of size variation in T. rastafari is similar to that in living tragelaphins with horned females ( T. oryx , T. eurycerus ).

Figures

Figure 1. Age of sites and stratigraphical units mentioned in the text. The temporal ranges of Tragelaphus rastafari sp. nov. and Tragelaphus nakuae are also shown. Known specimens from between 2.95 and 2.74 Mya are not sufficiently well preserved to be assigned to either T. rastafari or T. nakuae. After 2.3 Mya, T. nakuae begins to exhibit highly derived morphology, represented here by the change in colour.

Figure 2. Tragelaphus rastafari sp. nov. A, KNM TH-32833 (holotype) calvarium in posterior, anterior, and left lateral views, and M2–3 in occlusal view. B, MAT-VP-6/24 horn cores in anterior view and braincase in dorsal view. C, KNM ER-1460 horn cores in anterior and braincase in right lateral view. D, OMO 112/3-10006 right horn core in anterior view and braincase in left lateral view. Note that the dorsal braincase in T. rastafari is flat, bearing no supraoccipital torus. Scale bar = 10 cm for horn core views, 4 cm for braincase views, and 2 cm for teeth.