EUTHERIA ARE DIFFERENTIATED FROM METATHERIA on the basis of reproductive anatomy and biology and the presence of a trophoblast during development (an extra-embryonic layer that surrounds the inner cell mass of the embryo; Novacek, 1986a; Lillegraven et al., 1987). Such a characterization, however, cannot be applied to fossils. Consequently, morphologists and paleontologists have sought reliable features of the hard tissues that can be used to recognize each group. As detailed in Chapter 5, there are several dental and skeletal traits that differ more or less consistently between the two groups.
Especially distinctive is the dental formula. The primitive eutherian dental formula was long considered to be 188.8.131.52/184.108.40.206, in contrast to the primitive metatherian formula 220.127.116.11/18.104.22.168. Evidence that many archaic Cretaceous eutherians had more than three incisors and that some had five premolars (McKenna, 1975 a) led to reconsideration of the primitive eutherian dental formula (Novacek, 1986b), but the problem is not yet resolved. Some experts believe the "extra" premolar is a retained milk tooth (dP2: Luckett, 1993), an interpretation supported by the apparent retention of dP2 in dryolestids (Martin, 1997b). The presence of five premolars in sirenians, previously thought to support a primitive premolar number of five, has more recently been interpreted as a synapomorphy of Sirenia (Domning, 1994). Discoveries of several well-preserved basal eutherians in the past decade or so have contributed new information on the primitive eutherian dental formula.
Surprisingly, the recently improved fossil record of the most primitive eutherians has contributed little to the understanding of placental interrelationships, because new discoveries have led to the realization that there is little convincing evidence for assigning them to modern orders. Nonetheless, some taxa appear to be distinctly
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