Arsinoitherium BEADNELL , 1902

Pickford, Martin, 2017, Arsinoitherium (Embrithopoda) And Other Large Mammals And Plants From The Oligocene Of Tunisia, Fossil Imprint 73 (1 - 2), pp. 172-181 : 173-175

Genus Arsinoitherium BEADNELL, 1902

Arsinoitherium cf. zitteli BEADNELL, 1902

M a t e r i a l. ONM T-68, part of distal loph of a right upper molar of Arsinoitherium comprising the lingually expanded metacone which extends across to the lingual side of the crown ( Text-fig. 2 ).

L o c a l i t y. Site 68-1, ± 50 metres above base of SS unit, Grès de Fortuna, on divide between Oued Cherichera and Oued Grigema. In order to distinguish this locality from a site called Cherichera that yielded a mandible of a gomphothere ( Errington de la Croix 1877, Gaudry 1891a, b) I will refer to it as Grigema.

M e a s u r e m e n t s. Breadth of tooth – 34 mm; mesiodistal thickness of loph – 14 mm; preserved height of fragment – 55 mm; difference in height between the cervix on the distal and lingual sides of the crown – 46 mm.

D e s c r i p t i o n. Tooth T-68 from Grigema is partly coated in a ferruginous matrix, especially adherent on the dentine. The tooth is deeply worn, to the stage where the lingual part of the cervix is just beneath the occlusal wear facet. In occlusal view the enamel is thin (ca. 1.3 mm). In distal view, the tooth is slightly convex from cervix to apex and lightly concave from buccal to lingual ( Text-fig. 2 ). It possesses a shallow distal furrow that curves gently lingually as it goes from the apex towards the root. The enamel surface is marked by closely spaced horizontal striations (perikymata) and where unworn or unabraded the enamel is lightly rugose or pitted with shallow depressions. On the lingual side, the enamel-dentine junction is observed to run almost vertically from the distal surface towards the occlusal surface ( Text-fig. 2b ), revealing that the tooth had an extremely hypsodont distal surface accompanied by an extremely brachyodont lingual side. The broken anterior side of the specimen retains a small zone of enamel which corresponds to the fossette which separates the mesial and distal lophs from each other. The loph is 14 mm in mesiodistal thickness. In buccal view the enamel is observed to curve from the rather flat distal surface, first anteriorly and then lingually to form a loph-like structure.

D i s c u s s i o n. The tooth fragment from the interfluve between Oueds Cherichera and Grigema, despite its limited nature, retains enough morphology to indicate with little doubt that it belongs to an arsinoithere. The enamel-dentine junction is almost vertical on the disto-lingual corner of the tooth, revealing that the crown had an extremely hypsodont distal part and an extremely brachyodont lingual side. Such a combination is highly unusual among mammals, the best example known being Arsinoitherium zittelli from the Fayum, Egypt ( Andrews 1906, Court 1992a) ( Text-fig. 3c ). Other features of the tooth underline the arsinoithere-like morphology of the specimen, including the double curvature of the distal loph (concave bucco-lingually; convex cervicoapically), the layout of the perikymata and the thin enamel.

Eocene arsinoitheres such as Palaeoamasia from Turkey ( Sen and Heintz 1979), and Namatherium from Namibia ( Pickford et al. 2008), possess molars in which the cervix is horizontal all the way round the tooth. Arsinoitherium is the only known genus in which the distal and buccal surfaces of the molars are extremely hypsodont at the same time that the mesial and lingual sides are brachyodont ( Andrews 1906, Court 1992a). It is therefore reasonable to suggest that this is the genus represented by the Grigema tooth. In terms of dimensions, the specimen falls within the range of metric variation of Arsinoitherium zitteli, but considering its fragmentary nature, it is referred to Arsinoitherium cf. zitteli.

Arsinoitherium is best known from the Fayum in Egypt ( Andrews 1906, Court, 1992a, b), but has also been reported from Malembe ( Angola north of the Congo River) ( Pickford 1986), Dor el Talha ( Libya) ( Wight 1980), Chilga ( Ethiopia) ( Kappelman et al. 2003), Losodok ( Kenya) ( Rasmussen and Guttierez 2009) and Oman ( Pickford 2015a). Al-Sayigh et al. (2008) recorded the presence of Arsinoitherium at Aydim ( Oman), but the specimen (an isolated ulna) could belong to the barytherioid Omanitherium ( Seiffert et al. 2012) (see also Pickford 2015b). All these localities are of Oligocene age, and this is the most likely age of the tooth from Oued Grigema. Arsinoitheres of Eocene age (Lutetian) are known from Turkey ( Palaeoamasia and Hypsamasia ), Romania ( Crivadiatherium ) ( Radulesco et al. 1976) and Namibia ( Namatherium ) ( Pickford et al. 2008) but these genera do not have the combination of hypsodont buccal and distal crown surfaces and brachyodont mesial and lingual ones, making it unlikely that the Oued Grigema specimen is as old as Lutetian.

The Tunisian specimen, even though fragmentary, is important as it shows that arsinoitheres were widespread in the zone immediately south of the Tethys ( Text-fig. 4 ). Vialle et al. (2013) reported the possible presence of Arsinoitherium (?) at Bir Om Ali, not far from Chambi, Tunisia. The present paper removes the residual doubt that might remain concerning the presence of the genus in the country.

Figures

Text-fig. 2. ONM T-68, Arsinoitherium cf. zitteli right upper molar fragment from the divide between Oued Cherichera and Oued Grigema, Central Tunisia. a) distal view (apical part towards bottom of page), b) lingual view (apex down, distal surface to right of page) (note the almost vertical enamel-dentine junction), c) occlusal view (lingual part towards top of page).

Text-fig. 3. Right maxilla of a young adult individual of Arsinoitherium zitteli from the Fayum, Egypt (M 8802, NHM collection, London). The dotted lines indicate the homologous part preserved in the specimen from Grigema, Tunisia. a) stereo occlusal view, b) stereo buccal view, c) stereo lingual view.

Text-fig. 4. Distribution of arsinoitheres in Africa. Reconstruction of Arsinoitherium is adapted from Pomerol (1973) (the body in the image is probably too similar to that of an elephant, but the reconstruction gives an idea of the dimensions and possible body plan of Arsinoitherium).