Therian mammals from the Cenomanian (Upper Cretaceous) Dakota Formation, southwestern Utah

ABSTRACT

A diverse therian fauna is present in the Dakota Formation of Cenomanian age. Two pediomyid-like taxa are described on the basis of upper molars, but may be deciduous molars of nonpediomyid taxa. A new genus and species of marsupial-like tribothere is described and named (Dakotadens morrowi). A new family of marsupials (Alphadontidae) is established for Alphadon-like genera. The range of Alphadon is extended back into the Cenomanian, two new species are named (A. clemensi, A. lillegraveni), and a possible third new species is described. The range of Protalphadon is also extended back into the Cenomanian. Possible upper molars of Pariadens kirklandi are described and the diagnosis of the genus is revised based on unsheared lower molars; placement of this taxon in the Stagodontidae remains problematic. Upper molars of all therians recovered from the Dakota Formation have relatively broad stylar shelves; collectively, they include a wide range of stylar cusp configurations, although a cusp or cusps in the position of stylar cusp C is present on most upper molars. As such, the absence of cusps in the C position may not be primitive for marsupials. The marsupial-like tribothere described here, together with several other tribotheres, share derived characters with marsupials and may be part of an early diversification of the Metatheria. The divergence of metatherian and eutherian lineages may have occurred in the earliest Cretaceous (Neocomian).