Chilgatherium harrisi, 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 : 372-375

Chilgatherium harrisi sp. nov.

Figs. 4 and 5 , Table 2.

Holotype: CH 35−3a−e, associated partial right m1 (d), right m2 (a) ( Fig. 5A ), partial right m3 (e), partial left m2 (b), left m 3 in crypt (c) ( Fig. 5B ).

Referred specimens: CH 4−2a,b, left p4 (a), left m1 (b); CH 9−7, left P3; CH 9−22, right P3 ( Fig. 4A ); CH 12−3, upper right molar fragment; CH 12−4, upper left molar fragment; CH 15−3, upper right molar fragment; CH 35−1, right M3 ( Fig. 4B ).

Etymology: In recognition of the many important contributions of Dr. John M. Harris to the study of deinothere evolution.

Type locality: Chilga 35, Gahar Valley, Chilga region, northwest Ethiopia.Referred specimens from Upper Guang and Gahar Valley Sections.

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

Diagnosis.—Diminutive deinotheres; teeth smaller than homologs in Prodeinotherium and Deinotherium ( Fig. 6 ). Differ from deinotheriines ( Deinotherium and Prodeinotherium ) in the following features: P3 with bunodont cusps that are more independent in occlusal distribution and that crowd the trigon basin, and with a weakly formed ectoloph; m2 with poor expression of cristids; m2, m3, and M3 with incipient development of a tritoloph(id). Distinguished from barytherioids ( Phosphatherium, Daouitherium , Numidotherium, Barytherium , all with bilophodont molars) by development of the m2 distocristid into an incipient third lophid; by development of a postentoconulid in m3; by greater expression of lingual cusps in P3 (shared with other deinotheres); and by the bunodont (P3) and bunolophodont (molars) condition of cheek teeth.

Description.—There are only a small number of deinothere cheek teeth in the Chilga sample. Typical deinothere features in these specimens include transversely continuous, sharpcrested loph(id)s, tapiroid “chisel−like” wear on loph(id) apices, and cristae(ids) extending posteriorly from the lateral edges of upper molar lophs and anteriorly from the lateral edges of lower molar lophids ( Figs. 4 and 5 ; see Bergounioux and Crouzel 1962; Harris 1975, 1978).

In deinotheres, D2 and P3 are very comparable morphologically and probably served similar masticatory functions ( Harris 1975). Specimens CH 9−7 and CH 9−22 are identified as P3 by their size relative to M1/m1 and to P 3 in other deinotheres; they are too large to be D 2 in this species ( Fig. 6A, B ). The P 3 specimens from Chilga differ from those of other deinothere species in having their buccal cusps connected only weakly by a postparacrista and premetacrista to form an incipient ectoloph ( Fig. 4A ; see Roger 1886; Gräf 1957; Bergounioux and Crouzel 1962; Harris 1975). These premolars have four inflated main cusps which largely fill the crown, rather than crested lophs. A narrow median sulcus divides the lingual and buccal halves of the crown, and a constricted transverse valley separates the lingual cusps from one another. Each buccal cusp is superficially subdivided into three apical digitations.

M3 is also distinguished from those of other deinothere species, by the nascent development of its tritoloph. The M3 tritoloph is composed of a transverse row of mammillons and is not fully independent of the metaloph ( Fig. 4B ). It is also narrower transversely and mesiodistally than the proto− and metalophs. The protoloph and metaloph are both well formed, anteriorly convex, and have transverse ridges with chisel−like wear along their anterior margins. The protoloph has a strong crescentoid on its lingual side that extends well into the transverse valley, and a narrow crest that runs along the posterior face of the buccal cusp toward the metaloph. Similarly, the metaloph has short cristae on its lingual and buccal sides that extend posteriorly toward the tritoloph. These features are characteristic of upper molars in deinotheres.

Lower cheek teeth of deinotheres from Chilga contrast with those of other confamilials, as well, particularly in the expression of their third lophids. Specimen CH 35−3a is the antimere of CH 35−3b (based on similar width; Table 2), which is identified as an m2 because it is serially associated anterior to an m 3 in its crypt. In occlusal view, the tooth in crypt is identified as an m3 by its distally tapered profile ( Fig. 5B ). The proto− and hypolophids of the m2 are dominated by large, rounded cusps located at the buccal and lingual edges of the crown that are transversely connected by sharp, anteroposteriorly narrow crests. The crests are anteriorly concave and are worn in a chisel−like manner on their posterior faces, as in other deinothere species. The buccal cusp (hypoconid) of the hypolophid has a long, low crescentoid that projects into the first transverse valley. However, there are no other cristids apparent on the crown. Differing from the condition in Prodeinotherium and Deinotherium m2 (which lacks a tritolophid) and d4 and m1 (which have complete tritolophids), the m2 tritolophid in Chilgatherium is composed of two rounded cusps (hypoconulid and postentoconulid) that are not connected by a transverse ridge and that are lower than the cusps in the first two lophids ( Fig. 5A ). The tritolophid has no accompanying distal cingulid. There is no ectoflexus laterally demarcating the boundary between the hypolophid and tritolophid.

Like m2, m3 has a weakly developed tritolophid; the cusps of the third lophid are low, narrow, and only tenuously connected by several mammillons, rather than a crest ( Fig. 5B ).

Unlike m2, in m3 the crown is widest at the protolophid rather than at the hypolophid, and it is also larger overall ( Table 2).

Comments.—Deinotheres have heretofore been represented in the fossil record by Prodeinotherium and Deinotherium ( Harris 1978; Shoshani et al. 1996). The most ancient deinotheres previously known are placed in P. hobleyi and derive from early Miocene sites in Kenya and Uganda, dated to 20–18 Ma or possibly a little older, depending on the age of Moroto ( Harris 1978; Pickford 1981; Tassy 1986; Gebo et al. 1997; Pickford et al. 1999). Unfortunately, the geological age of a small deinothere m2 from Adi Ugri, Eritrea ( Vialli 1966) has not been established definitively. However, this specimen, assigned to “ Deinotherium cf. hobleyi ” ( Vialli 1966: 447) , is a close match in size and occlusal structure to P. hobleyi m2 from the early Miocene locality of Nyakongo Uyoma, Kenya ( Pickford 1986b), and from the middle Miocene site of Gebel Zelten, Libya ( Harris 1978), giving no morphological reason to suspect it had a greater antiquity. Prodeinotherium was succeeded in Africa at the beginning of the late Miocene by the larger, more cursorial Deinotherium ( Harris 1973, 1978; Hill et al. 1985; Nakaya 1993; Leakey et al. 1996).

Prodeinotherium also made its first appearance in Europe towards the end of the early Miocene, in mammal faunal zone MN 4 ( Antunes 1989; Ginsburg 1989; Tassy 1989), and is documented as well from the early Miocene of South Asia ( Cooper 1922; Raza and Meyer 1984). In both Europe and Asia, as in Africa, Deinotherium replaced Prodeinotherium by the start of the late Miocene ( Bergounioux and Crouzel 1962; Sarwar 1977). Although Deinotherium disappeared from the Asian fossil record around 7 Ma ( Barry and Flynn 1989), the genus persisted in Europe until the mid Pliocene ( Sarwar 1977) and in Africa until about 1 Ma ( Behrensmeyer et al. 1995). Thus, the late Oligocene age of the Chilga discoveries extends by more than a third again the prior temporal range of deinotheres. While a mammalian fauna is known from the similar−aged Eragaleit beds in the Lothidok Range of northern Kenya (ca. 27.5–24.0 Ma; Boschetto et al. 1992; Leakey et al. 1995), Prodeinotherium from the area (see Madden 1972; Van Couvering and Van Couvering 1976) is securely documented only from the younger Lothidok Formation (<17.8 Ma; Boschetto et al. 1992). For now, the Chilga deinotheres are the oldest yet recorded.

Given their great age and unusual morphology, the Chilga deinothere teeth are especially important for exploring the early evolution of the Deinotheriidae . Although the phylogenetic root of deinotheres remains a mystery ( Harris 1978; Shoshani et al. 1996), the expression of cusps and crown organization of the Chilga deinothere molars suggest derivation from a bunolophodont, rather than lophodont, form. For example, in P3 from Chilga the cusps and accessory conelets are inflated and largely independent, and the cusps of the buccal ectoloph are only weakly interconnected. In contrast, Prodeinotherium and Deinotherium typically have the paracone, metacone, and metaconule fully incorporated into an ectoloph ornamented by mammillons, creating a more continuous and sharper crest ( Roger 1886; Gräf 1957; Bergounioux and Crouzel 1962; Harris 1975). In P3 of these deinotheres, the other cusps are also more sharply crested apically. Additionally, in Chilga deinothere m2 and m3, the tritolophid is formed of two cusps which are not fully interconnected by a transverse ridge, and the proto− and hypolophids are dominated laterally by large, rounded cusps. Ab− e, estimated dimension; () numbers in parentheses indicate the loph(id) at which the crown has its greatest width; +, indicates original dimension was greater sence of a posterior cingulid in m2, and posterior association of a cingular ridge with the last loph in M3 support Harris’ (1969, 1975, 1978) hypothesis that in deinotheres, the tritolophid of m1 evolved via hypertrophy of the posterior cingulid while the tritoloph of M1 developed from postmetaloph ornamentation. It is apparent that the tritoloph(id)s of second and third molars in deinotheriines were secondarily reduced. The primitive expression of these features in the new Chilga specimens indicate that they are reasonably separated at the subfamilial level from other deinotheres.

Figures

Fig. 4. Cheek teeth of Chilgatherium harrisi sp. nov. A. Right P3, CH9−22, in buccal (A1, A2) and occlusal (A3, A4) views. B. Right M3, CH35−1, in buccal (B1) and occlusal (B2, B3) views. Anterior is to the left. Scale bars 1 cm.

Fig. 5. Cheek teeth of Chilgatherium harrisi sp. nov. A. Right m2, part of type specimen, CH35−3a, in lingual (A1) and occlusal (A2, A3) views; anterior is to the left. B. Left m3, part of type specimen, CH35−3c, in occlusal view. Scale bar 1 cm.

Fig. 6. Bivariate plots of natural log−transformed crown length versus width of selected cheek teeth in Chilgatherium harrisi sp. nov. and other deinothere species. Comparative dimensions are from Bachmann (1875), Weinsheimer (1883), Roger (1886), Andrews (1911), Cooper (1922), Palmer (1924), Éhik (1930), MacInnes (1942), Gräf (1957), Sahni and Tripathi (1957), Symeonidis (1970), Harris (1973, 1977, 1983, 1987), Gaziry (1976), Tobien (1988), Tsoukala and Melentis (1994), Huttunen (pers. com.; 2000), Sach and Heizmann (2001), and Sanders (2003). A. Specimens CH9−7 and CH9−22 compared with D2 in other deinotheres. Note that in this comparison, the Chilga specimens appear anomalously large, relative to size contrasts between other cheek teeth from the Chilga deinothere sample and those of Prodeinotherium and Deinotherium. B. P3. C. M3. D. m2. E. m3.