Early mammalian radiations

Journal of Paleontology, Nov 2001 by Cifelli, Richard L

Knowledge of Late Jurassic and younger "symmetrodonts," which are among the most poorly represented of Mesozoic mammals, has advanced significantly in recent years (Hu et al., 1997; Sigogneau-Russell and Ensom, 1998; Cifelli and Madsen, 1999). Despite dental similarity (presumably primitive) of some taxa to archaic "symmetrodonts" (Fox, 1985), the two best-known groups (tinodontids and spalacotheriids) have advanced features of the lower jaw and presumably had a characteristically mammalian three-boned middle ear (Cifelli and Madsen, 1999). This, together with postcranial evidence, suggests placement of the geologically younger "symmetrodonts" well within the crown group of living mammals, proximal to "eupantotherians" and living therians (Luo et al., in press), as envisaged in the middle 20th century (Patterson, 1956). "Eupantotheres," encompassing a variety of taxa such as dryolestids, amphitheriids, paurodontids, and peramurids, have long been interpreted to be structurally antecedent to tribosphenic therians (Simpson, 1928, 1929a; Butler, 1939; Patterson, 1956). This view has been upheld by detailed studies of the dentition (Mills, 1964; Clemens and Mills, 1971; Butler and Clemens, 2001). Cladistic analyses suggest that "eupantotheres" form a paraphyletic grade rather than a monophyletic assemblage (e.g., Prothero, 1981; Martin, in press), and relationships among them remain somewhat unstable. Nonetheless, new knowledge of cranial and postcranial anatomy in taxa such as Vincelestes and Henkelotherium (see Krebs, 1991; Rougier, 1993) has provided substantial support for a proximal relationship of "eupantotheres" to tribosphenic therians (Luo et al., in press and references therein). The diversity of the dentition among "eupantotheres" has also become better known as a result of discoveries in recent years (e.g., Bonaparte, 1990; Dashzeveg, 1994; Martin, 1997, 1999, in press; Ensom and Sigogneau-Russell, 1998; Sigogneau-Russell, 1999).

BOREOSPHENIDA

The living groups of therian mammals, Marsupialia and Placentalia, are readily distinguished on the basis of numerous anatomical features, both "soft" and "hard" (Lillegraven et al., 1987). Dental criteria have long proven useful in recognizing the affinities of many fossils of Late Cretaceous and younger age (Simpson, 1929a; Lillegraven et al., 1979). The water became muddied with the recovery of geologically older (Early Cretaceous) tribosphenic mammals, however: many of these, as well as problematic younger fossils, cannot be reliably placed on one side or the other of the marsupial-placental divide. A taxonomic netherworld, "Theria of metatherian-eutherian grade" (Patterson, 1956) or "Tribotheria" (Butler, 1978), was created to accommodate such mammals (Kielan-Jaworowska et al., 1979; Cifelli, 1993a). Taxa of this grade have proliferated as the Cretaceous record has improved (e.g., Clemens and Lillegraven, 1986; Fox, 1980 1982). Most celebrated of early tribosphenic mammals is Valanginian Aegialodon (see K. A. Kermack et al., 1965), though older (Berriasian) fossils are now known (Sigogneau-Russell, 1994; Sigogneau-Russell and Ensom, 1994). Luo et al. (2001) recognize these and other taxa as proximal relatives (Boreosphenida) of Marsupialia Placentalia, regarding them as distinct from early tribosphenic mammals of southern landmasses (see above).