Fig. 6.1. Proposed relationships of the most primitive known eutherians. (Simplified after Archibald et al., 2001, and Archibald, 2003.)
more primitive or more derived than others, permitting a first-order approximation of relationships among these primitive eutherians, which is shown in Figure 6.1.
The terms Eutheria and Placentalia are sometimes used interchangeably, but there is a growing convention to restrict Placentalia to the last common ancestor of the extant orders and all its descendants, whereas the broader Eutheria includes additional stem taxa that are closer to placentals than to marsupials. In practice, it is not always clear which primitive eutherian taxa should be excluded from Placentalia, and in this book all eutherians except the Cretaceous forms discussed in this chapter are considered to be placentals. Indeed, some of these Cretaceous taxa could also turn out to be placentals.
Fossils that depart from the metatherian pattern and display features and dental formulae like those of later placen-tals are first known from the Early Cretaceous of Asia and western North America. Until recently these earliest euthe-rians were known only from dentitions, the oldest of which dated from the Aptian-Albian boundary on both continents, thus raising doubt about where they originated.
The recent discovery of a complete skeleton of a probable eutherian, older than any known previously, provides evidence of the anatomy of the earliest eutherians and suggests that they arose in Asia. Eomaia scansoria (Fig. 6.2) comes from the Early Cretaceous Yixian Formation of China and is dated to about 125 million years ago (Ji et al., 2002). It was a shrew-sized mammal, estimated to have weighed only 20-25 g—less than an ounce. Eomaia is more primitive than other eutherians in having a slightly inflected mandibular angle and retaining Meckel's groove on the mandible. It also retains epipubic (marsupial) bones. Unlike metatherians, however, the dental formula is 184.108.40.206/220.127.116.11, the premolars are simple and not molarized, and the molar hypoconulid and entoconid are not twinned (Fig. 6.3A). The skeleton was adapted for scansorial or arboreal habits, which is particularly apparent in the anatomy of the phalanges.
About 10 million years younger is Prokennalestes, another shrew-sized form with five premolars and three molars, from the ?Aptian or Albian of Khovboor, Mongolia (Kielan-Jaworowska and Dashzeveg, 1989). It is sometimes assigned to the leptictidan family Gypsonictopidae (McKenna and Bell, 1997). As in metatherians, the upper molars of Prokennalestes have a moderately wide stylar shelf with cuspules and lack a hypocone, but the paracone is higher than the meta-cone and the lower molars have a low paraconid and a centrally placed hypoconulid rather than a twinned entoconid-hypoconulid, characteristics of eutherians. A petrosal recently referred to Prokennalestes shows a combination of primitive mammalian and derived therian traits, the latter including a cochlea showing one full coil, which is the oldest known occurrence of this feature (Wible et al., 2001). Murtoilestes is a closely related form, based on a few isolated teeth from Russia that may be a little older than Prokennalestes but younger than Eomaia (Averianov and Skutschas, 2001).
North American Montanalestes (Fig. 6.3B), from the Albian (about 110 Ma) of Montana, is the oldest North American eutherian-like form. It is known from a lower jaw with four or five premolars (the precise number is uncertain) and three molars. The last premolar has a metaconid and an
anterobasal cusp, indicating a gradation from simple pre-molars to complex molars, a eutherian hallmark (Cifelli, 1999).
Whether these Early Cretaceous mammals are the earliest eutherians or are better considered as "proto-eutherian" will be decided by further evidence. It is notable that the traits that distinguish the earliest eutherians from meta-therians are, for the most part, primitive features.
Apart from Eomaia, the oldest well-preserved eutherian mammals come from the Late Cretaceous of central Asia (Figs. 6.3, 6.4; Plates 1.3, 1.4). Kennalestes, Asioryctes, Ukhaa-therium, Zalambdalestes, and Barunlestes, from ?Campanian strata (about 75 Ma) of the Gobi Desert of Mongolia, and Daulestes, from the Coniacian of Uzbekistan, are each known from skulls, and all but Daulestes and Kennalestes from partial or complete skeletons as well. The first three genera are now united in the higher taxon Asioryctitheria (Novacek et al., 1997), based on details of cranial anatomy, although Kennalestes was earlier placed in Leptictidae or Gypsonic-topidae. Daulestes is probably also an asioryctithere. It has a cochlea with one full turn—more than in multituberculates or monotremes, but less than in other known placentals (or marsupials) except for Prokennalestes and Zalambdalestes
(McKenna et al., 2000). This and other primitive features of the ear region suggest that marsupials and placentals con-vergently acquired many of their derived auditory traits (Wible et al., 2001).
Not unexpectedly, asioryctitheres have many plesiomor-phic traits, including more incisors than in other eutherians (5/4 in Asioryctes and Ukhaatherium, as in primitive marsupials; 4/3 in Kennalestes; the incisor count in Daulestes is uncertain); primitive skull structure, including such features as a large facial exposure of the lacrimal bone; generalized limbs with separate radius-ulna and tibia-fibula; a primitive ankle with an ungrooved astragalar trochlea and ventrally curved calcaneal tuber; and epipubic bones (Novacek et al., 1997; Horovitz, 2000, 2003). The four premolars are simple except the last one (semimolariform), the lower molars have very tall, mesiodistally compressed trigonids with reduced paraconids and low, basined talonids, and the upper molars are transversely wide, with well-developed parastyle and/or metastyle and no hypocone. These features suggest that asioryctitheres lie near the base of the eutherian radiation.
Zalambdalestes and Barunlestes, assigned to the family Za-lambdalestidae, are considerably more specialized (Kielan-Jaworowska, 1978, 1984). The skull (Fig. 6.4D) is larger than
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