Dissacus zanabazari, Geisler & Mckenna, 2007

Geisler, Jonathan H. & Mckenna, Malcolm C., 2007, A new species of mesonychian mammal from the lower Eocene of Mongolia and its phylogenetic relationships, Acta Palaeontologica Polonica 52 (1), pp. 189-212 : 190

Dissacus zanabazari sp. nov.

Figs. 1–11 , Tables 1–4.

Holotype: MAE−BU−97−13786 ( Figs. 1–11 ), partial skeleton including skull with mandibles; 5 vertebrae:atlas, axis, 2 posterior lumbar, and the first sacral; partial right humerus and ulna; right radius; complete distal left forelimb except for distal phalanges; pelvis, complete right hind limb except for distal ends of metatarsals and distal phalanges; proximal end of left femur. Collected on August 23, 1997 by the joint expedition of the Mongolian Academy of Sciences and the American Museum of Natural History.

Type locality: Tsagaan Khushuu (Tsagan Khushu), Nemegt Basin, Omnogov Aimag (Province), Mongolia.

Derivation of name: In honor of Undur Geghen Zanabazar (1635– 1723), a renowned leader of Mongolia who initiated a renaissance in Mongolian religious art, literature, and architecture.

Age and distribution.— The holotype and only known specimen of Dissacus zanabazari was found several meters above the base of the Bumban Member of the Naran−Bulak Formation. Dashzeveg (1988) correlated the mammalian fauna of the Bumban Member with early Eocene faunas of North America and Europe. An early Eocene age for the Bumban fauna is also supported by the faunal cluster analysis of Meng and McKenna (1998) .

Diagnosis.—Short face; trilobed lower incisors; no diastemata between lower premolars and molars except between p1 and p2; m3 metaconid subequal to protoconid; lingual face of M3 protocone nearly vertical; foramen for superior ramus of stapedial artery entirely within petrosal; anterior halves of 3 rd and 4th metacarpals abut each other; medial edge of lesser trochanter of femur not thickened. In addition to the above listed autapomorphies, Dissacus zanabazari differs from D. argenteus in having a wider talonid basin on m3; differs from D. europaeus in having lower hypoconids on p4 and m1–m3, a transversly wider and mesodistally shorter talonid on m3; differs from Yantanglestes feiganensis (originally assigned to Dissacus ) in having a double−rooted p2, the anterior edge of the orbit over M1 not M2; differs from D. indigenus in having a higher hypoconid and cristid obliqua on lower molars, less retroflexed p4 protoconid; differs from D. serratus in having higher and less recurved protoconids on p3 and p4, shorter talonid basin on p3; differs from D. magushensis in lacking an entoconid on m1; differs from D. navajovius in having better developed talonid basins on p3 and p4, molar protoconids more distal than metaconids, protocone more mesial than paracone on P4; differs from D. praenuntius in lacking ento− and ectocingula on m3 and a well−developed metastyle on M3; differs from D. rotundus in having a much wider talonid basin on m3; differs from D. willwoodensis in having a much wider m3, mandibular ramus that does not deepen posteriorly; differs from D. argenteus in having a wider talonid basin on m3; differs from D. zengi in being larger and in having a lower hypoconid on m2.

Figures

Fig. 1. Skull of Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene, in dorsal (A), lateral (B), and ventral (C) views. The antorbital and postorbital portions of the skull are completely separate and were positioned based on more complete mesonychid skulls.

Fig. 2. A. Stereopair of the right basicranium of Dissacus zanabazari sp. nov., holotype, MAE−BU− 97−13786, Tsagaan Khushuu, Gobi Desert, Mon− golia, early Eocene. B. Explanatory drawing of the same.

Fig. 3. A. Right P4 to M3 of Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene. B. Explanantory drawing of the same. Hatched areas in B indicate crushed and distorted enamel, and shading denotes missing enamel, with features reconstructed from left dentition.

Fig. 4. Mandible and lower dentition of Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene. A. Anterior view of the lower canines and incisors. Note the weakly trilobed morphology, most evident on i2. B. Stereopair of the left mandible in dorsal view. C. Medial view of the left mandible. D. Lingual view of the left p4 through m3.

Fig. 5. Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene. Left scapula shown in ventral (A) and lateral (B) views C. Stereopair of left manus, in anterior view. Abbreviations: ce, centrale; Mc, metacarpal; mg, magnum; td, trapezoid.

Fig. 6. Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene. A. Left radius and ulna in medial view. B. Right humerus in anterior view.

Fig. 7. A. Medial view of the left manus of Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene. B. Explanatory drawing of the same.

Fig. 8. Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene. A. Lateral view of pelvis with lumbar and sacral vertebrae. B. Right femur in distal view.

Fig. 9. Anterior views of right femur (A) and right tibia and fibula (B) of Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene.

Fig. 10. Proximal tarsal bones of Dissacus zanabazari sp. nov., holotype, MAE−BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene. Right calcaneus in lateral view (A) and stereopair in anterior view (B). Right astragalus in anterodorsal view (C) and stereopair in posterior view (D).

Fig. 11. Partial right tarsus of Dissacus zanabazari sp. nov., holotype, MAE− BU−97−13786, Tsagaan Khushuu, Gobi Desert, Mongolia, early Eocene. A1, anterior view, A2, explanatory drawing of the same; B1, medial view, B2, explanatory drawing of the same.