FOSSIL MAMMALS OF THE SENTINEL BUTTE FORMATION (LATE PALEOCENE) OF NORTH DAKOTA

The fossil mammals of the Tongue River Formation in North Dakota have been described or discussed in a number of papers, whereas those of the overlying Sentinel Butte Formation have received little attention. Recent work has located several new localities and reinterpreted previously known localities as occurring in this formation. The Grassy Butte Locality is in a channel sandstone at the Tongue River–Sentinel Butte formational contact and has produced four taxa (one multituberculate, three eutherians). The River Basin 3 Locality (2–19 m above the base) and the Cross Locality (10–19 m above the base) have each produced only Titanoides primaevus. The Riverdale Locality is approximately 23 m above the base of the formation and contains five taxa (one multituberculate, four eutherians). L5500b is approximately 26 m above the base and contains 10 taxa (two multituberculates, one marsupial, seven eutherians). The Red Spring Locality contains nine taxa (four multituberculates, five eutherians) and is approximately 36 m above the base of the formation. The highest occurrence of mammals is at the type locality of Titanoides primaevus (Witter Locality), the first Paleocene fossil mammal reported from North Dakota, 48 m above the base of the formation. All of the localities probably represent the Tiffanian-4 biochron, although not all localities contain time diagnostic taxa. Titanoides primaevus has the greatest stratigraphic distribution, occurring in the lowest and highest localities, as well as in all other localities except L5500b and Red Spring. Ptilodus and Plesiadapis occur from the base of the formation through the Red Spring Locality. Other taxa are more restricted, occurring in only one or two localities. The Sentinel Butte Formation is between 115 and 189 m thick in North Dakota, The overlying Bear Den Member of the Golden Valley Formation has been determined to be Clarkforkian in age (based upon fossil plants). Therefore, the upper Sentinel Butte Formation may represent a significant portion of the late Tiffanian and/or the Clarkforkian.