Late Maastrichtian paleoenvironments and dinosaur biogeography in the western interior of North America

During Late Maastrichtian time, three major sedimentary depositional provinces existed in continental environments of the western interior region of North America. The sediments accumulated within these provinces are herein termed the piedmont, alluvial plain, and coastal lowland lithosomes. These sedimentary lithosomes correspond to broad paleoenvironmental and paleophysiographic provinces that resulted from the westward shift of the Pacific coast, the bisection and retreat of the interior epeiric sea, and the development of an intermontane basin system during Maastrichtian time. These dramatic physiographic changes created environments markedly different from those that had prevailed during most of the Late Cretaceous, when western North America consisted of a narrow peninsula flanked on the east by a remarkably stable and monotonous coastal plain. In conjunction with these physiographic and environmental changes, three diverse dinosaur-dominated continental vertebrate faunas arose, herein termed the Leptoceratops, Triceratops, and Alamosaurus faunas. Each was peculiar to a sedimentary/environmental province and among them there was little interchange. The separation of these faunas indicates that the dinosaur-dominated communities had adapted to changing or "deteriorating" environmental conditions at the close of Cretaceous time. The Alamosaurus fauna characterized markedly seasonal, semi-arid, environments of the intermontane basins south of about 35°N latitude. The Triceratops fauna inhabited humid coastal floodplains and swamps bordering the retreating northern embayment of the epeiric sea, north of latitude 35°N. The Leptoceratops fauna inhabited cool piedmont environments flanking the mountainous Cordilleran Overthrust Belt north of 35°N latitude. Recognition of this biogeographic zonation indicates that the Triceratops biozone is only of local utility, and that the biostratigraphy of Upper Cretaceous continental deposits in North America must reflect the provincialism now recognized. Moreover, the evident adaptability of dinosaur faunas to varied and changing environmental conditions suggests that climatic change alone may not have been responsible for their demise.