INTRODUCTION

One of the most interesting events in mammalian evolution is the divergence between marsupials and placentals. Putative earliest eutherian and metatherian taxa have been named from the Early Cretaceous Liaoning Beds of China (Ji et al. 2002; Luo et al. 2003). In North America, Early Cretaceous mammals are known mainly from fragmentary dental remains. Among these, an apparent diversity of tribosphenidan (sensu McKenna and Bell 1997; or boreosphenidan sensu Luo et al. 2001) mammals provides provocative glimpses into the morphotypes that, it is argued, preceded the origin of the modern marsupial and placental clades (Jacobs et al. 1989; Kobayashi et al. 2002). Nine tribosphenidan mammals (Pappotherium pattersoni, Holoclemensia texana, Kermackia texana, Slaughteria eruptens, Trinititherium slaughteri, Comanchea hilli, Adinodon pattersoni, Atokatheridium boreni, Oklatheridium szalayi) have been named from the Late Aptian-Early Albian Trinity Group of Texas and Oklahoma, each based on one or a few upper or lower teeth with equivocal association of teeth from the opposing jaw (Slaughter 1965, 1968a, 1971; Butler 1978; Jacobs et al. 1989; Hershkovitz 1995; Kielan-Jaworowska and Cifelli 2001; Davis et al. 2008). A fresh opportunity to examine relationships among these taxa is provided by new information about one of them, Slaughteria eruptens. Analysis of high resolution X-ray CT data revealed that the holotype (a lower jaw) of S. eruptens (SMU 61992) houses an unerupted replacement premolar below what had been presumed to be a molar (Figure 1 and Figure 2; Kobayashi et al. 2002). The four erupted teeth in S. eruptens are now interpreted as two permanent premolars, plus dp4 and m1 (Kobayashi et al. 2002). Slaughteria eruptens demonstrates that the premolar eruption sequence for this primitive tribosphenic mammal was p3->p2->p4 (Kobayashi et al. 2002; Kielan-Jaworowska et al. 2004; Luo et al. 2004).

We utilize current visualization techniques to document the anatomy of S. eruptens in high-resolution interactive 3D imagery and reconstructions based on CT data and new digital SEM photography. Using molar morphology and size relationships developed from other Cretaceous tribosphenidan mammals, we quantitatively evaluate this taxon's relationship to other Trinity tribosphenidan taxa that are named from upper molars. Of the well-represented Trinity Group mammal taxa that are based upon upper molars, those that are compatible with the tooth structure of S. eruptens are Pappotherium pattersoni and Holoclemensia texana. These three are the Trinity Group therian mammals most commonly cited in phylogeny reconstructions and studies of molar occlusal function (e.g., Crompton 1971; Flynn et al. 1999; Luo et al. 2002)