Evolution of the mammalian fauna across the Cretaceous-Tertiary boundary in northeastern Montana and other areas of the Western Interior

Abstract

Paleontological and geological studies of the Hell Creek Formation and Tullock Member of the Fort Union Formation in the northern Western Interior of North America provide a globally unique, detailed record of the evolution of terrestrial vertebrates during the transition from the latest Cretaceous into the earliest Paleocene. In the area south of the Fort Peck Reservoir, northeastern Montana, high-resolution lithostratigraphic and biostratigraphic studies, abetted by 40Ar/39Ar radiometric age determinations and paleomagnetic correlations, have clarified the local record of faunal evolution. This research demonstrates that the lithostratigraphic boundary between the Hell Creek and Tullock in northeastern Montana usually is coincident with, but can be younger than, the Cretaceous-Tertiary boundary. A nonrandom pattern of survival and extinction of mammals and other terrestrial vertebrates characterizes faunal evolution across the Cretaceous-Tertiary boundary in northeastern Montana. Paleontological and geological data support hypotheses that a variety of long- and short-term factors influenced the course of vertebrate evolution. Recovery of the terrestrial fauna of northeastern Montana during the first million years of the Paleocene was not simply the product of evolutionary radiations stemming from some local survivors. In addition, waves of immigration of mammals from outside the northern Western Interior are recorded in the early (Pul) and undifferentiated middle and late (Pu2-Pu3) Puercan local faunas of the area. They document both the important role of dispersal in recovery of the mammalian fauna of northeastern Montana during the Puercan as well as the biogeographic differentiation of Lancian and Puercan local faunas in more distant areas.