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Early mammalian radiations
Journal of Paleontology, Nov 2001 by Cifelli, Richard L
Later "Triconodonts" and "Symmetrodonts"; "Eupantotheres".-Molar pattern has always figured prominently in interpretation of mammalian relationships. The elaboration of molar pattern through triconodont, symmetrodont, and eupantotheres stages, ultimately leading to the complex, multifunctional (tribosphenic) molars of marsupials and placentals, has traditionally been central to understanding early mammal phylogeny (Patterson, 1956; Crompton, 1971). This morphological transition remains unchallenged, and molar pattern justifiably continues to figure prominently in debates about the relationships of early mammals.
However, the fossil record has been significantly augmented in recent decades by discovery of relatively complete specimens, permitting the extension of comprehensive comparisons to the skull and skeleton. In addition, the application of cladistic methodology has provided tests of traditional hypotheses of relationships and generation of new hypotheses. Several important points emerge: 1) some formal groups, such as "Pantotheria" or "Eupantotheria" represent paraphyletic grades rather than monophyletic groups; 2) other groups, such as "Symmetrodonta" and "Triconodonta," are likely polyphyletic if Late Triassic-Early Jurassic taxa are included in them; and 3) simple, triconodont and symmetrodont molar patterns probably appeared independently, either through homoplasy or reversal.
The core of "Triconodonta," Amphilestidae and Triconodontidae (Eutriconodonta of K. A. Kermack et al., 1973), collectively range from Middle Jurassic through Late Cretaceous. Important, recent discoveries bearing on eutriconodont relationships include specimens of Priacodon (Late Jurassic), an unnamed triconodontid, Gobiconodon, and Jeholodens, the last three all of Early Cretaceous age (Jenkins and Crompton, 1979; Jenkins and Schaff, 1988; Rasmussen and Callison, 1981; Rougier et al., 1996a; Kielan-Jaworowska and Dashzeveg, 1998; Ji et al., 1999). Despite some primitive retentions in the axial skeleton and hind limb, these taxa share a number of advanced characters of the skull and fore limb with therians, to the exclusion of morganucodontsthus belying the simple, serially tricuspate molar pattern common to "triconodonts" (Jenkins and Crompton, 1979; Jenkins and Schaff, 1988; Wible et al., 1995; Rougier et al., 1996a; Ji et al., 1999), and contradicting earlier hypotheses of a monophyletic "Triconodonta," noted above. Notwithstanding this apparent advance in understanding, the monophyly of eutriconodonts remains uncertain, as does their placement (collectively or individually) with respect to crown mammals and later "symmetrodonts" discussed in this section (see Luo et al., in press). As intimated earlier, the consensus view that these "triconodonts" occupy a more proximal place to therians than do the archaic "symmetrodonts" implies either reversal of the reversed triangle molar pattern (Luo et al., 2001), or independent origin of the symmetrodont cusp configuration (Rougier et al., 19966).
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