Early mammalian radiations

Dental evidence bearing on monotreme relationships finally emerged in 1985, with the discovery of Steropodon from the Early Cretaceous of Australia. An unquestioned monotreme, Steropodon has molars that are surprisingly similar to those of marsupials and placentals, suggesting a closer relationship to living therians than had generally been thought (Archer et al., 1985; Kielan-Jaworowska et al., 1987). Recent discoveries on southern landmasses have been equally or more iconoclastic: both Ambondro (Middle Jurassic, Madagascar) and Ausktribosphenos (Early Cretaceous, Australia) have fully advanced ("tribosphenic") molars like those of living therians (Rich et al., 1997; Flynn et al., 1999). Analyses reported by Luo et al. (2001, in press) suggest, however, that this pattern was independently achieved, and that the extinct taxa are related to monotremes, forming a clade that originated from a primitive "symmetrodont" and that is only remotely related to therians.

LATER JURASSIC RADIATIONS

Although one or more faunas and some individual mammal specimens are known from the Middle Jurassic (e.g., Evans and Milner, 1994; Clark et al., 1994; Flynn et al., 1999), a hiatus in the fossil record separates well-known taxa and assemblages from the Early and Late epochs of that period (Rowe, 1993). As noted above, the Late Triassic-Early Jurassic mammals have long been grouped with structurally similar, later-appearing taxa: haramiyids with multituberculates (Allotheria), morganucodonts with other "triconodonts," and Kuehneotherium with other "symmetrodonts." These allocations result in long stratigraphic ranges for groups of early mammals, and imply great antiquity for divergence of lineages (Fig. 1.2). Despite varied interpretations as to the relative placement of later "triconodonts," "symmetrodonts," and multituberculates, recent analyses suggest that they are more closely related to the crown group Mammalia than are the Late Triassic-Early Jurassic forms noted above (Fig. 1.3). Hence, at present there appears to be a cladistic basis for organizing treatment of these early mammals on a stratigraphic basis, as done here.

Multituberculates.-Discoveries through the twentieth century include a great wealth of multituberculate fossils, and as a result this group is incomparably the best known of all Mesozoic mammals. Late Cretaceous taxa from Mongolia are especially well represented, with the skull and skeleton being known in exquisite detail (Kielan-Jaworowska, 1971, 1986; Kielan-Jaworowska and Gambaryan, 1994; Hurum, 1994, 1998a, 19986; Gambaryan and Kielan-Jaworowska, 1995, 1997; Hurum et al., 1996; Rougier et al., 19966, 1997; Wible and Rougier, 2000). Well-represented dentitions and skulls are also known for some of the earliest and most primitive multituberculates, from the Late Jurassic of Portugal (Hahn, 1969, 1977a, 1977b, 1978, 1981, 1985, 1987, 1988; Hahn and Hahn, 1994). In view of the extraordinary breadth and detail of data now available, it is a great irony that multituberculate relationships, both of and within the order, remain elusive.