Mammals from the end of the age of dinosaurs in North Dakota and southeastern Montana, with a reappraisal of geographic differentiation among Lancian mammals

Abstract

An end-Cretaceous nonavian dinosaur extinction and an early Paleocene mammalian radiation is documented primarily in stratigraphic sequences in eastern Montana. To determine how representative these sequences are, we extended investigation of this Cretaceous-Tertiary (K-T) transition to new areas. Studies in southwestern North Dakota and southeastern Montana provide new records of mammals through the last 1.32-1.68 million years of the Cretaceous and extending into the Paleocene, allowing us to evaluate mammalian faunal differentiation across the geographic landscape of the North American Western Interior in the latest Cretaceous. In North Dakota, mammals occur at 15 horizons in the Hell Creek Formation from ∼80 to 3 m below the Hell Creek-Fort Union formational contact. In southeastern Montana, where the Hell Creek Formation is ∼150 m thick, mammals occur from 85 to 15 m below the formational contact, with well-sampled local faunas at 65 and 61 m. In faunal comparisons, the new study areas are closely similar to contemporaneous local faunas, but differ from those at higher latitudes in several ways. Although mammalian faunas of the northwestern coastal plain of the Western Interior Seaway (i.e., the Hell Creek Formation and its lateral equivalents) show little differentiation, differences that exist are strongly associated with geographic distance rather than latitude per se. The overlying Fort Union Formation in North Dakota and southeastern Montana has produced a few early Paleocene mammalian specimens, promising that future work in the area will contribute to knowledge of mammalian radiation after the K-T boundary.