Fig. 6.3. Dentitions of primitive Cretaceous eutherians: (A) Eomaia; (B) Montanalestes; (C) Daulestes; (D) Kennalestes; (E) Ukhaatherium. Key: ap, angular process; co, coronoid process; dc, dentary condyle; etd, entoconid; hyd, hypoconid; hyld, hypoconulid; iof, infraorbital foramen; med, metaconid; mf, mandibular foramen; mks, meckelian sulcus; ms, masseteric fossa; pad, paraconid; prd, protoconid; ptf, pterygoid fossa; sym, symphysis. (A from Ji et al., 2002; B from Cifelli, 1999; C from McKenna et al., 2000; D from Kielan-Jaworowska, 1969; E from Novacek et al., 1997.)
in other Cretaceous eutherians and has an elongated snout and a long diastema between the incisors and canine. There are only three incisors in each quadrant, the anterior ones enlarged and strongly procumbent. The last lower premolar (P4) is more molariform, the molar trigonids are lower and further compressed, the molar talonids are broader, and the upper molar stylar shelves are more reduced than in asio-ryctitheres. Kulbeckia, from deposits in Uzbekistan that are probably about 10 million years older than the Gobi Desert zalambdalestids, is the most primitive zalambdalestid (Archibald et al., 2001; Archibald and Averianov, 2003). It had at least three and possibly four or five upper incisors and lacked a significant diastema. The limbs of zalambdalestids (see Fig. 15.2) are long, the tibia and fibula fused, and the joints specialized for running and hopping, including a deeply grooved astragalus. In all these features zalambdalestids resemble rabbits and elephant shrews, and some recent studies suggest they could be the stem group of Glires (e.g., Archi bald et al., 2001; see Chapter 15 on Anagalida). Nonetheless, zalambdalestids, like asioryctitheres, are primitive in retaining epipubic bones. The coexistence in the Late Cretaceous of such differently adapted animals as zalambdalestids and asioryctitheres is evidence that the eutherian radiation was already well under way. But the presence of epipubic bones— perhaps related to either a pouch or external support of altricial young—suggests that these Cretaceous eutherians had not yet achieved the prolonged gestation characteristic of placentals (Novacek et al., 1997).
From the same strata in Uzbekistan that produced Kulbeckia come several genera with relatively broad, brachydont molars that seem to foreshadow conditions later developed in the hoofed mammals. Called zhelestids, these archaic eutherians have been interpreted as the stem group of the grandorder Ungulata (Archibald, 1996; Nessov et al., 1998; see Chapter 12).
The oldest uncontested eutherians from North America are Late Cretaceous (Campanian) Paranyctoides and the slightly younger Gallolestes and Gypsonictops (Lillegraven and McKenna, 1986; Cifelli, 2000; Fig. 6.5). Paranyctoides was recently reported from Asia (Uzbekistan) as well (Archibald and Averianov, 2001). Compared to other Cretaceous "insectivores" they have relatively low molar trigonids and broad talonid basins. The last premolar in all three is sub-
molariform, and both Paranyctoides (at least one species) and Gypsonictops are known to have had five lower premolars. (These genera are further discussed in Chapter 9.) Paranyctoides is usually considered a lipotyphlan insectivore, variously assigned to Nyctitheriidae or basal Soricomorpha
(shrews and related forms), whereas Gallolestes is usually assigned to Leptictida (McKenna and Bell, 1997); but Archibald and Averianov (2001) argue that they are basal eutheri-ans of uncertain ordinal affinity, possibly related to zheles-tids. Gypsonictops is a leptictidan.
SEVERAL CLADES OF PRIMITIVE EUTHERIANS are included in the Cimolesta, proposed as an order of the grandorder Ferae by McKenna (1975a). Some of the higher taxa that are discussed here were formerly assigned to the Insectivora sensu lato or to the Proteutheria, a "wastebasket taxon" that has been used to encompass various primitive placental groups of uncertain affinity (e.g., Didelphodonta, Pantolesta, Apatotheria; see, e.g., Romer, 1966). However, it is very unlikely that either Proteutheria or Insectivora is a natural group when these primitive clades are included. Other taxa included in McKenna's Cimolesta have traditionally been accorded ordinal rank by most researchers (e.g., Taeniodonta, Tillodontia, Pantodonta). Unfortunately, the evidence (mainly dental) that they constitute a monophyletic assemblage descended from or sharing a common ancestry with Cimolestidae is not much stronger and has not been tested by phylogenetic analysis. Hence their unification in Cimolesta should be regarded as tentative and is used here largely for convenience. Ordinal status for most of these groups is maintained here, and accordingly Cimolesta is considered a mirorder (Table 7.1).
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