Gomphotherium, 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 : 384-386

cf. Gomphotherium sp. nov.

Figs. 15 and 16 , Table 2.

Referred specimens: CH 14−V−14, partial r. m3 ( Fig. 16B ); CH 25−3, r. p4 ( Fig. 15A ; in same dentary as CH 25−V−12a); CH 25−V−12a, r. m2 ( Fig. 16A ; in same dentary as CH 25−3; antimere of CH 25−V−12b); CH 25−V−12b, l. m2 (antimere of CH 25−V−12a); CH 25−V−12c, r. P4 ( Fig. 15B ).

Age and distribution: Late Oligocene, ca. 28–27 Ma. Only known from the Upper Guang and Gahar Valley Sections, Chilga region, northwest Ethiopia.

Description.—Broken anteriorly, enough of the crown of CH 14−V−14 remains to show that it had at least three lophids ( Fig. 16B ). It also is damaged by compression fractures, but they have little affected crown dimensions and morphology. Enamel is thick ( Table 2) and marked by fine horizontal striations. There is no cementum. The posterior cingulid is expanded into a stout “heel” comprised of two large conelets, an anterior accessory central conule, and a low enamel ribbon on the pretrite side. The occlusal shape of the crown posteriorly, lack of a distal interproximal facet, narrowness relative to length, and slight longitudinal occlusal concavity of the crown indicate that CH 14−V−14 is an m3.

In lateral view, the lophids are torpedo−shaped, rather than pyramidal as in palaeomastodonts ( Fig. 16B ). The transverse valleys are occupied on the pretrite side by large posterior central accessory conules and smaller anterior central accessory conules. Pre− and posttrite half−lophids are each formed of a large, outer main conelet accompanied by a smaller, lower mesoconelet. The accessory central conules are located on the postero− and anteromedial sides of the main conelets, and at least in the last pretrite half−lophid would have formed a trefoil enamel figure with wear. Vestiges of buccal and lingual cingulids are present only as low enamel ribbons at the entrances of the transverse valleys.

The presence of an elephantoid at Chilga is confirmed by CH 25−V−12a ( Fig. 16A ) and b, right and left m2 antimeres. These specimens are complete, with three full lophids, and virtually unworn. They are small compared to m 2 in other elephantoids ( Fig. 12B ). In each, the pretrite half−lophid is accompanied by anterior and posterior accessory central conules, which would have formed trefoil enamel figures with wear. Each half lophid is formed of a large main, outer conelet and a smaller, lower mesoconelet. Anterior and posterior cingulids are narrow, low, and closely appressed to their nearest lophids. As in m3, the buccal and lingual cingulids are present only as low enamel ribbons restricted to the entrances of the transverse valleys. In occlusal view, the crown widens abruptly at the metaloph. There is a broad interproximal facet on the mesial face of the crown, and a much smaller, less distinct facet at the distal end. The alveolus anterior to the right m2 has a length of 58.6 mm and a width of 33.9 mm. The corpus below the m2 measures 94 x 94 mm.

A bilophodont right p4, CH 25−3 ( Fig. 15A ), is in place in the same dentary as CH 25−V−12a. It differs from p 4 in Phiomia major sp. nov. in its larger size ( Table 2) and greater expression of accessory conules, which with wear would have formed trefoil enamel figures on the pretrite side of the crown. The space between p4 and m 2 in this individual shows that m1 would have measured 66.5 mm in length.

A right P4 ( CH 25−V−12c; Fig. 15B ) is also likely from the same individual. It is morphologically similar to P 4 in Phiomia major sp. nov., but is larger ( Table 2) and has a better developed central cusp in its posterior loph. Also, the prominent enamel swelling on the anterior face of the second pretrite half−loph resembles the condition of P 4 in Gomphotherium ( Fig. 15B ; see Tassy 1985).

Comments.—These specimens are morphologically distinct among Chilga elephantiform teeth. They differ from cheek teeth of palaeomastodonts and resemble elephantoid teeth by the prominent posterior “heel” in m3, in having highly reduced bucco−lingual cingulids, lophids that are torpedo−shaped rather than pyramidal in lateral view, last lophids with distinct abaxial and adaxial conelets on both pre− and posttrite sides, and by the presence of anterior and posterior accessory central conules throughout the length of the crown. Overall, they most closely resemble cheek teeth of Gomphotherium , particularly by their rounded conelets, transverse continuity of half−lophs, and trefoil arrangement of outer main conelets and accessory conules on the pretrite side.

It is difficult to more precisely assess the affinities of these specimens because the m3 ( CH 14−V−14) is incomplete and many features diagnostic of Elephantoidea are cranial (Shoshani 1996; Tassy 1996b). Although the presence of a fourth lophid in m3 is considered a defining trait of elephantoids by Shoshani (1996), this feature is variable in Gomphotherium ( Tassy 1996c; Lambert and Shoshani 1998; Sanders and Miller 2002). Consequently, although the number of lophids originally present in CH 14−V−14 is uncertain, even with only three it could still be elephantoid.

These teeth constitute the earliest appearance of the superfamily Elephantoidea , and considerably extend its temporal range. The oldest prior record of elephantoids is that of Eozygodon from the earliest Miocene (see above); the most ancient certain occurrences of Gomphotherium are slightly younger and penecontemporaneous in Africa and Eurasia, from the interval ca. 20–18 Ma ( Tassy 1985, 1986, 1989, 1994b, 1996c; Pickford 1986b; Welcomme 1994; Mazo 1996; Sanders and Miller 2002). If a molar from the Aquitanian of Sicily is really that of Gomphotherium (Checchia− Rispoli 1914), the genus would be older yet by several million years, but still geologically younger than the Chilga specimens.

Specimen CH 14−V−14 is also the smallest m3 formally attributed to Elephantoidea , though m3 of a proboscidean of similar morphological grade from slightly younger deposits at Dogali, Eritrea is even more diminutive ( Shoshani et al. 2001). If CH 14−V−14 had three lophids, it could reasonably be reconstructed to a length of approximately 100 mm, slightly shorter than the smallest, early–mid Miocene “pygmy” gomphotheres from Ghaba, Oman ( Roger et al. 1994), Siwa, Egypt ( Hamilton 1973), Gebel Zelten, Libya (Arambourg 1961; Gaziry 1987), and Kabylie, Algeria ( Depéret 1897). CH 14−V−14 thus resembles other elephantiform molars in the Chilga assemblage in being intermediate between earlier Oligocene palaeomastodonts and Miocene elephantoids not only in age but in size.

Figures

Fig. 15. Dental specimens of Chilga elephantoids, cf. Gomphotherium sp. nov. A. Right p4, CH25−3, in lingual (A1) and occlusal (A2, A3) views. B. Right P4, CH25−V−12c, in occlusal view. Anterior is to the left. Scale bars 1 cm.

Fig. 16. Dental specimens of Chilga elephantoids, cf. Gomphotherium sp. nov. A. Right m2, CH25−V−12a, in lingual (A1) and occlusal (A2, A3) views. B. Partial right m3, CH14−V−14, in buccal (B1) and occlusal (B2, B3) views. Anterior is to the left. Broken areas indicated by dotted lines. Scale bars 1 cm.

Fig. 12. Bivariate plots of natural log−transformed M3, m3, and m2 crown length versus width in Chilga and other fossil elephantiforms. Comparative dimensions are from Fourtau (1920), Cooper (1922), Osborn (1936), Bergounioux and Crouzel (1959), Arambourg (1961), Hamilton (1973), Gaziry (1976, 1987), Tassy (1977b, 1983a, 1983b, 1985), Tassy and Pickford (1983), Gentry (1987), Roger et al. (1994), Göhlich (1998), and Sanders and Miller (2002). A. M3. B. m2. C. m3.