Arsinoitherium giganteum, Sanders & Kappelman & Rasmussen, 2004

Sanders, William J., Kappelman, John & Rasmussen, D. Tab, 2004, New large-bodied mammals from the late Oligocene site of Chilga, Ethiopia, Acta Palaeontologica Polonica 49 (3), pp. 365-392 : 369-372

Genus Arsinoitherium Beadnell, 1902 Arsinoitherium giganteum sp. nov.

Fig. 3 , Table 1.

Holotype: CH 69−1, maxilla with M2−3 ( Fig. 3A ).

Referred specimens: CH 3−1, distal half of upper molar, probably M3; CH 3−2, maxilla with P3−4; CH 3−8, right deciduous premolar (d2 or d3); CH 3−16, lower molar fragment; CH 3−52, left m1; CH 3−60, upper molar; CH 3−63, upper molar fragment; CH 3−94, partial mandible with right p2−m1, left p2−m2; CH 3−95, left M3 ( Fig. 3E ); CH 5−12, left P4; CHS6−V−1, proximal femur; CH 7−1, right deciduous premolar (d4?); CH 9−9, left p4 ( Fig. 3C ); CH 9−12, right p1; CH 9−14, right upper premolar; CH 9−15, molar fragment; CH 9−16, upper molar fragment; CH 10−3, left? p1?; CH 10−5, juvenile mandible with two deciduous? premolars ( Fig. 3B ); CH 10−6, M1; CH 15−V−6, right I1; CH 16−133, mid−cervical vertebra; CH 17−5, axis; CH 18−133, cervical vertebra; CH 18−134, cervical vertebra (axis); CH 19−6, thoracic vertebra; CH 25−17, left m2? ( Fig. 3F ); CH 25−18, right p2 or p3; CH 26−10, partial left nasal horn core ( Fig. 3D ); CH 26−12, acetabulum; CH 33−9, left femur; CH 33−22, cranial fragment; CH 35−7, premolar inside nodule; CH 35−13, molar inside nodule; CH 35−14, right m2; CH 35−16, molar inside nodule; CH 35−18, tooth fragment inside nodule; CH 35−19, molar inside nodule, CH 35−21, molar inside nodule; CH 35−35, thoracic vertebra; CH 35−36, femoral head; CH 35−37, distal femur; CH 35−39, left tibia; CH 51−1, right m3; CH 51−2, left m2; CH 51−3, distal femur; CH 56−1, proximal humerus; CH 76−1, parts of both lower jaws and one maxilla, each bearing two or three cheek teeth, embedded in a solid stone block, associated with multiple other blocks containing postcranial remains ( CH 76−3).

Etymology: From the Greek gigantos, gigantic.

Type locality: Chilga 69, Gahar Valley, Chilga region, northwest Ethiopia. Referred specimens are from other localities of the Gahar Valley, Upper Guang Section, and Middle Guang Area.

Age and distribution: Late Oligocene, 28–27 Ma. Only known from the Chilga region.

Diagnosis.—Differs from other species of Arsinoitherium in its larger tooth size ( Table 1).

Description.—The M2 of the holotype is fully erupted and in occlusion, while the M3 is incompletely erupted and has endured no occlusal wear ( Fig. 3A ). These are massive teeth, exceeding in size those of large “ Arsinoitherium andrewsi ” of the Fayum. Because of damage to the back of the maxilla, the entire, unworn crown of the distal loph in M3 can be measured—it is an impressive 120 mm high. Other lightly worn or intact distal lophs of upper molars measure 115 mm ( CH 3−60), 131 mm ( CH 3−95; Fig. 3E ), and 132 mm high ( CH 3−1). Crown height is also remarkably tall in other maxillary teeth ( Table 1). In two moderately to heavily worn specimens of P4, CH 3−2 and CH 5−12, buccal crown height measures 73 and 76 mm, respectively. The lingual crown heights of the same premolars are only 10–14 mm high. As Andrews (1906) pointed out, this suggests that the teeth pivot as they erupt, wearing unequally along the edges of the crown.

The lower molars are also extremely high−crowned ( Table 1), with typical development of the distinctive form of arsinoithere bilophodonty ( Court 1992b). Teeth identified as m1 rise relatively straight from their bases, while m2 and m3 show greater degrees of curvature on the buccal side of the lophid pillars. All molars have a slight fossa and paracristid located in front of the mesial lophid representing a relictual trigonid (see CH 25−17, Fig. 3F ).

Comments.— Arsinoitherium giganteum sp. nov. is the largest and the geologically youngest species of arsinoithere. It is the second sub−Saharan record of the genus, the other being tooth fragments from Malembe, Angola, that are apparently of Oligocene age ( Pickford 1986a). The geographically disparate occurrences at Malembe, and in Arabia ( Thomas et al. 1989 a, 1999), Egypt ( Andrews 1906), Libya ( Wight 1980), and highland Ethiopia, indicate that Arsinoitherium was a widespread herbivore in Afro−Arabia during the early Tertiary. This distribution is unexpected given detailed functional morphological study of the postcranium suggesting that Arsinoitherium was adapted to semi−aquatic swampland environments ( Court, 1993). The lower adult and deciduous premolars from Chilga ( Fig. 3B, C ) resemble those known from Fayum arsinoitheres. The deciduous premolars are nearly perfect small replicas of the adult molars. The dental specimens of arsinoitheres from Chilga, however, are larger than all specimens from the Fayum, with one exception: the holotype of the Fayum’s controversial large species, “ A. andrewsi ” ( Lankester 1903; Andrews 1906; El−Khashab 1977), which overlaps the lower range of variation seen in the Chilga arsinoitheres. The largest specimen examined by us attributed to the common Fayum species, A. zitteli (CGM 8802), has an m1 length about eight millimeters shorter than the smallest Chilga m1 ( Table 1). The longest Chilga lower molar is over 90 mm long, while no specimen of A. zitelli exceeds 67 mm.

It seems likely that the large holotype of “ A. andrewsi ” simply represents the largest known individual of A. zitteli . Large herbivores are almost always sexually dimorphic in body size, especially those with head ornaments indicating mating competition ( Fig. 3D ). If “ A. andrewsi ” were a legitimate species, one might still expect variation in A. zitteli to exceed CV values of 8–10, an amount easily accommodated in monomorphic and moderately dimorphic species ( Plavcan and Cope 2001). However, excluding the holotype of “ A. andrewsi ,” the CV for remaining specimens of A. zitteli is quite low (ranging from 5.0 to 8.2 for those dental measures with n> 8). With the holotype of “ A. andrewsi ” added in to the calculations, CVs for the same tooth dimensions increase to a range of 7 to 15, not too high to encompass within a single species, and indeed, comfortably within the range expected for dimorphic species ( Plavcan and Cope 2001). The early justification to recognize “ A. andrewsi ” as a distinct α width of loph(id) of greatest breadth.

species was that it was “nearly half again as large” ( Andrews, 1906) as A. zitteli , but this ratio is not compelling. Similar size disparities are expected in large herbivores; e.g., among extant rhinocerotids the largest adult individuals are well over 50% larger than the smallest ( Nowak and Paradiso, 1983).

The fairly large sample of arsinoithere teeth from Chilga allow us to conclude that the population from Chilga was greater in body size than that from the Fayum, with slight overlap between only the largest specimen from the Fayum and the smallest from Chilga, suggesting that there is a specific distinction between the two samples.

Figures

Fig. 3. Cranio−dental remains of Arsinoitherium giganteum sp. nov. A. Maxilla fragment with M2−3, CH69−1, holotype, in lateral view; anterior is to the right. (A2, diagram of A1). B. Juvenile dentary with two deciduous premolars, CH10−5, in buccal (B1) and occlusal (B2, B3) views; anterior is to the right. C. Left p4, CH9−9, in buccal (C1) and occlusal (C2) views; anterior is to the left.D. Partial left nasal horn core, CH26−10. E. Left M3 in posterior view, CH3−95. F. Left m2, CH25−17, in buccal (F1) and occlusal (F2, F3) views; anterior is to the left. Scale bars 3 cm.