Didymoconidae

Didymoconids (Fig. 7.5) are an enigmatic group of carnivorous Early Tertiary mammals probably endemic to Asia. The half dozen or so genera, most of which are known only from teeth and jaws, are characterized by loss of P1 and M3, and presence of simple premolars; transverse tritubercular upper molars with connate paracone and metacone and a somewhat reduced stylar shelf, and usually lacking a hypo-cone or postcingulum; and lower molars with tall trigonids and narrow talonids (Gingerich, 1981b). The oldest known didymoconid skulls, belonging to latest Paleocene Archaeo-ryctes from Mongolia and late early Eocene ?Hunanictis from China, are posteriorly broad, with small orbits, frontal-maxillary contact in the orbit, a reduced jugal, an ossified auditory bulla composed mainly of the entotympanic (?Hunanictis), and various other specialized cranial features (Meng, Ting, and Schiebout, 1994; Lopatin, 2001). Oligocene Didymoconus had a short, broad skull, with flaring zy-gomae, a broad occiput, and a prominent sagittal crest (Fig. 7.5B). The mandible was deep and the canines large. Post-crania of didymoconids are rare and poorly known. Best represented is Didymoconus, which had moderately robust forelimb elements, including short metacarpals and phalanges suggestive of fossorial habits (X. Wang et al., 2001). The humerus of middle Eocene Ardynictis was short and very robust with a prominent supinator crest, as in diggers, but the deltopectoral crest (typically well developed in diggers) seems to have been low and weakly developed (Lopa-tin, 2003b).

The phylogenetic position of didymoconids is uncertain. The dentition is consistent with relationship to Cimolesta or Leptictida, but this observation seems to be based primarily on primitive similarity. Didymoconidae most recently has been placed in Leptictida (McKenna and Bell, 1997) or Insectivora (Meng, Ting, and Schiebout, 1994; Lopatin, 2001; X. Wang et al., 2001), the latter used in the broadest sense. Cranial features have been cited as evidence of insec-tivoran affinity (Meng, Ting, and Schiebout, 1994), but the entotympanic bulla indicates that didymoconids are not lipotyphlans.

Possibly related to the didymoconids are the Wyolestidae, comprising three early to middle Eocene genera, two from

Fig. 7.4. Calcaneus and astragalus attributed to the cimolestid Procerberus compared with those of the basal ungulate Protungulatum. Protungulatum: (A) calcaneus; (B) astragalus. Procerberus: (C) calcaneus; (D) astragalus. Key: 1, posterior calcaneoastragalar facet (ectal facet); 2, fibular facet; 3, sustent-acular facet; 4, peroneal tubercle; 5, astragalar foramen or canal; 6, navicular facet; 7, astragalar trochlea; 8, astragalar neck. Scale bars = 1 mm. (From Szalay and Drawhorn, 1980.)

Fig. 7.4. Calcaneus and astragalus attributed to the cimolestid Procerberus compared with those of the basal ungulate Protungulatum. Protungulatum: (A) calcaneus; (B) astragalus. Procerberus: (C) calcaneus; (D) astragalus. Key: 1, posterior calcaneoastragalar facet (ectal facet); 2, fibular facet; 3, sustent-acular facet; 4, peroneal tubercle; 5, astragalar foramen or canal; 6, navicular facet; 7, astragalar trochlea; 8, astragalar neck. Scale bars = 1 mm. (From Szalay and Drawhorn, 1980.)

Age Mammals

Asia and one—Wyolestes (Fig. 7.6), the best known—from western North America (Gingerich, 1981b; Novacek et al., 1991). Wyolestids are known from relatively complete dentitions, which are very generalized and similar to those of didymoconids (again mainly in primitive features), except for retaining the first premolar and last molar. The upper molars are essentially tritubercular and triangular with more separated paracone and metacone than in didymo-conids, and the lowers have larger talonids. Relationship to hyaenodontid creodonts has also been suggested, but the wyolestids have not yet been linked with any other group based on demonstrably synapomorphic traits. Gingerich (1981b) assigned wyolestids to the Didymoconidae, which he allied with Mesonychia. Subsequent authors have left Wyolestidae unassigned (Eutheria incertae sedis: Novacek et al., 1991) or placed them in the Cimolesta (McKenna and Bell, 1997).

The primitive anatomy and diverse opinions about didy-moconids and wyolestids suggest that they may represent early offshoots of the eutherian stem.

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