New Tribosphenic Mammals from the Mussentuchit Local Fauna (cedar Mountain Formation, Cenomanian), Utah, Usa

Herein we describe two new tribosphenic mammals from the Cedar Mountain Formation of Emery County, Utah, USA. The fossils occur in a narrow stratigraphic interval near the top of the unit (Mussentuchit Member) and date to approximately the Early–Late Cretaceous boundary (early Cenomanian, ~97 Mya). The specimens were recovered from screen-washing bulk rock samples. Both taxa are based on isolated teeth; from the limited morphology represented, both lack obvious specializations of the two major therian groups, Metatheria and Eutheria, and for present purposes they are regarded as basal tribosphenidans. Dakotadens pertritus sp. n. is the more abundant of the two and is closely related to a previously known species from the overlying (upper Cenomanian) Naturita Formation. Culicolestes kielanae gen. et sp. n. is notable in its large size (similar to Pariadens mckennai from the same fauna) relative to most other pre-Campanian tribosphenidans. The mammals of the Mussentuchit Local Fauna, now fully described on the basis of available fossils, represent one of the most diverse assemblages known from the Mesozoic, including some 22 taxa. Tribosphenidans, however, are neither abundant nor diverse, and include only six recognized species; most Late Cretaceous assemblages of North America include twice as many. Four of these are marsupialiforms and the remaining two, described herein, are considered basal tribosphenidans. Both eutherians and deltatheroidans, known from earlier and later faunas on the continent, are lacking. Triconodontids (Eutriconodonta) and spalacotheriids (basal Trechnotheria) make up nearly a third of mammalian diversity in the Mussentuchit Local Fauna, and account for about 40% of mammalian fossils identified to species level. Spalacotheriids (four taxa) are particularly abundant relative to both older (Albian) and younger (late Cenomanian onward) assemblages, suggesting some taphonomic or paleoecologic bias.