New Late Cretaceous, North American advanced therian mammals that fit neither the marsupial nor eutherian molds

Abstract
Upper molars of two new genera and species of mammals (Falepetrus barwini and Bistius bondi) are described from rocks of Late Cretaceous age of Montana, Wyoming, and New Mexico. Although these teeth are evolutionarily advanced in being fully tribosphenic, they have combinations of characters that preclude identification of the animals that bore them as either marsupials or eutherians. A review of available dental features useful in classification of known, fully-tribosphenic mammals from the Late Cretaceous suggests the presence of four principal groups: (1) Marsupialia; (2) Eutheria; (3) “deltatheridians;” and (4) others. Groups “ 1 ” and “2” are recognized as formal taxonomic units, defined by anatomically diverse suites of derived characters. Groups “3” and “4” are recognized informally and, as a grade, dubbed “tribotheres,” mammals with tribosphenic dentitions lacking documented specializations characteristic of either marsupials or eutherians. Although group “3” may represent an evolutionary clade equivalent in taxonomic rank to marsupials or eutherians, members of group“4” (including Falepetrus and Bistius) comprise a heterogeneous conglomeration whose members have uncertain relationships to members of groups “1-3.” In addition to the evolutionary radiations of contemporary marsupials and eutherians, the tribotheres provide evidence of at least a third, if not several, broad mammalian radiations during the Cretaceous. However, available dental criteria are inadequate to allow development of a useful, phylogenetically-based classification of the tribotheres.