Sileem et al.—New Egyptian Oligocene Anthracothere
mesiolingually to merge with the distal junction between the postmetacristid and postprotocristid. There is no postentocristid and the posthypocristid run distolingually from the hypoconid to merge with the prehypocristulid that slopes down from the hypoconulid, leaving the longitudinal valley open. The hypoconulid is a distinct cusp, occupies the central part of the distal margin of the crown, and forms a very distinct third lobe. The posthypocristulid runs from the hypoconulid summit to end at the base of the entoconid. In labial view, a hypocristulid slopes mesiolabially toward the base of the hypoconid to merge with the labial cingulid. Occasionally there is a small knob developed on the posthypocristulid at the base of the hypoconid.
Materials.—From Quarry A: 13424, mandible with symphysis, left M1–2 and right broken P3–M1 and M2–3. From Quarry R: DPC 13442 (Fig. 4.1), left maxilla with P4–M2. From Quarry V: CGM 67200, a laterally crushed skull that preserves the upper dentition (right and left I2–3,C, P1–4,M1–3), except right and left I1 and left C (Table 1, Figs. 5–6); CGM 67201 (Fig. 7), right dentary with C, P1,P4 (broken), M1,M3, and left dentary with P2–P3; CGM 67202 (Fig. 8.2), left dentary fragment preserving M3 and part of the distal root of M2; DPC numbers: 6473 (Fig. 4.3), left maxilla with P3–M3; 8213, left maxilla P4–M3; 8410, left upper C; 9048 (Fig. 8.1), mandible with left and right C, P2–M3; 10197, left dentary with M3; 10527, left P4; 10668, right dentary with M2 (broken), M3; 10736, mandible with left dI1–3, dC, dP1–4,M1 and right dP3–4,M1; 10825, palate with left P4–M3, right P4–M2.
Comparison.—Members of Nabotherium exhibit a suite of distinctive features not seen among the other Fayum anthra- cotheres. Nabotherium differs from Bothriogenys in ways that suggest utilization of differing diets. Nabotherium has more bunodont and generally broader cheek teeth. The front of the skull is shortened and it lacks a canine-premolar diastema. Nabotherium possesses large, projecting, and relatively laterally compressed canines that are oval in cross section. In the upper molar row, Nabotherium differs from other Fayum anthra- cotheres in a large number of occlusal details, including the presence of cuspate mesostyles, well-developed labial and lingual cingula, distinctive labial surfaces of the para- and metacones (compare Fig. 4.1, 4.3 with 4.4) that have very strong barrels, and a well-developed lingual metacristule that joins with the lingual cingulum. This is combined with only weakly developed parastyles, metastyles, preparacristae, and postmetacristae. One specimen assigned to Nabotherium aegyptiacum from
Fayum Quarry A (DPC 13424) differs from other known specimens of this taxon in having more robust and larger premolars, and somewhat larger molars (Table 1). It is not clear whether these features might be taxonomically meaningful or whether they are better interpreted as representing idiosyncratic variation, but the specimen is assigned to N. aegyptiacum because, other than relatively larger tooth size, it is morpho- logically comparable to other members of the species. Relatively little can be said about the differences between
Nabotherium and Qatraniodon, due to the fact that Qatraniodon is not very well known, and the type preserves only M1–2.
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However, Nabotherium is clearly distinct from Qatraniodon in being larger and in having much more bunodont and relatively wider molars. Qatraniodon has a small and low, but distinct, cingular spur on M1 that is lacking in Nabotherium, and the posterior shelves ofM1–2 in Nabotherium are relatively broader than in Qatraniodon. Nabotherium further differs from the Asian Paleogene
anthracotheres including: Siamotherium Suteethorn, Buffetaut, Helmcke-Ingavat, Jaeger, and Jongkanjanasoontorn, 1988, Anthracokeryx Pilgrim and Cotter, 1916, and Anthracohyus Pilgrim and Cotter, 1916. Specifically, Nabotherium differs from Siamotherium in having a double rooted P1, and in having a well-developed lingual metacristule continuous with the lingual cingulum, a distolabially oriented paraconule, and more distinct mesostyles on the upper molars. The Egyptian taxon differs from Anthracokeryx (Colbert,
1938) in lacking diastemata between the lower canine–P1, P1–P2, and P2–P3, in having relatively shorter and broader cheek teeth, and in having a (albeit small) P2 protocone shelf. Nabotherium is distinct from Anthracohyus (known only
from a single upper molar; Colbert, 1938) in having a bulbous, cuspate mesostyle (Anthracohyus has only a tiny crest in the position of the mesostyle), a better developed parastylar area, distinct labial barrels on the labial surfaces of the paracone and metacone, and a distinct lingual cingulum.
Discussion
Anthracotheres have long been recognized as a family of artio- dactyls that likely originated in North America or Eurasia, at least by the late middle Eocene, and subsequently spread and diversified throughout Laurasia and Africa (Lihoreau and Ducrocq, 2007). The oldest known definitive anthracotheres have been identified from deposits in North America (ca. 42 Ma) and Myanmar (Burma) (ca. 40 Ma) (Khin Zaw et al., 2014), and although the origin of African anthracotheres is uncertain, their ancestry can probably be traced to a Eurasian form that reached Africa during or before the late Eocene. Three anthracothere lineages are known from the early
Oligocene deposits of the Fayum: a bunodont form (Nabother- ium), and two bunoselenodont ones (Bothriogenys, Qatranio- don). This same pattern, of coexisting bunodont and bunoselenodont lineages, seems to be fairly common; it has been noted previously among Eurasian and North American faunas (Macdonald, 1956; Lihoreau and Ducrocq, 2007) and is now documented in Africa as well. Of the bunoselenodont forms, Qatraniodon is known from
only a single specimen, which is a lower jaw with two molars. However, Bothriogenys is well represented in the Fayum faunal assemblage and exhibits features that include relatively longer and narrower cheek teeth, more complex premolars often with accessory crest development, occasional supernumerary teeth (GFG, ERM, personal observations), more complex molar teeth, with high crowns that sometimes show development of neo- morphic crests and cuspules, and small, more incisiform canines, combined with a long anterior canine-premolar dia- stema, and a mandibular symphysis that is canted anteriorly, resulting in an elongated, scoop-like anterior dental arcade. Dental adaptations such as these are typically seen in browsing
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