Tragelaphus nakuae, Reed & 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 : 703-704

TRAGELAPHUS NAKUAE ARAMBOURG, 1941

Revised diagnosis: A large-sized tragelaphin characterized by horn cores that are strongly inclined, completing between 90 and 225° of torsion; with triangular cross-section and strong anteroposterior compression basally. A large, transversely orientated torus is present on the dorsal surface of the cranium at the posterior limit of the braincase (supraoccipital torus). Associated with this is a deep depression just anteriorly, between the supraoccipital and the horn cores. Mastoid region strongly protruding laterally. In lateral view, the horn cores remain straight as they arise from their bases, with little or no trace of posterior curvature. Younger specimens of T. nakuae , particularly beginning in Member G and equivalents (2.33 Mya), tend to possess horns that are relatively shorter and stouter, completing no more than 180° torsion, with weaker anterior keel, greater anteroposterior compression, stronger intercornual torus, and exaggerated diminution of the distal horn core ( Fig. 3 ). With increasing anteroposterior compression, the horn core cross-section sees greater changes between distal and basal sections: distal sections of horn cores retain a more regular triangular shape, becoming more compressed as they grow proximally (see Arambourg, 1947: fig. 64). Reduced premolar row to molar row ratio (0.48–0.63). Females most likely horned.

Syntypes: A partial cranium with horn cores, listed alternatively as MNHN 1933-9 - 89 or 1939-9-89 (‘Type A’); a partial right mandible with p4-m3, listed alternatively as MNHN 1933-9 - 106 or 1939-9-106 (‘Type B’). Both specimens are recorded to come from Bourillé, on the left bank of the Omo River ( Arambourg, 1941: 343–345) .

FAD: 2.74 Mya (Shungura Unit C4).

LAD: ~1.8 Mya (Shungura H or lower part of KBS Member).

Main horizons present: Omo Shungura: Unit C4 to Member H. Koobi Fora: Upper Tulu Bor, Lower Burgi, Upper Burgi, KBS Member. Middle Awash: Matabaietu 5, Wilti Dora 2.

Apomorphies relative to T. rastafari and T. eurycerus: Prominently raised supraoccipital torus present, intercornual torus more consistently developed. Basal horn core compression (DT/DAP) is greater on average than in T. rastafari , with values of> 1.50 first recorded ( Fig. 3B ). Specimens of T. nakuae younger than about 2.3 Mya show stouter, shorter horns with stronger distal taper, and reduction of horn torsion to less than 180° and reaching a minimum of 90° in most derived specimens.

Specimens of significance: Omo Shungura: From the new collections of the Omo Group Research Expedition is OMO 3/2-10058 ( Fig. 4E ) from Unit C4, representing the oldest recognized appearance of T. nakuae . This specimen constitutes cranial fragments including posterior braincase and parts of the left and right horn cores. The posterior cranium preserves a supraoccipital torus and associated depressed area on the dorsal cranium just anterior to it.

Torsion in degrees

Figures

Figure 3. Morphological change amongst Tragelaphus saraitu, Tragelaphus rastafari sp. nov., and earlier and later representatives of Tragelaphus nakuae. Plots showing: decreasing horn core torsion (A); increasing horn core anteroposterior compression (B); and decreasing relative horn core length (C) over time. Specimens of T. nakuae older and younger than 2.3 Mya are distinguished by different symbols.

Figure 4. Tragelaphus nakuae. A, MAT-VP-5/29 in anterior and dorsal views. B, KNM ER-1968, showing most clearly the supraoccipital torus. C, KNM ER-17359. D, KNM ER-272 braincase in right lateral view. E, OMO 3/2-10058 braincase. F, WIL-VP-2/20 in anterior and left lateral views. Scale bar = 10 cm for top row, 5 cm for bottom row.