Faunal implications of an environmental change before the Cretaceous-Tertiary (K-T) transition in central Utah

Abstract

The North Horn Formation at North Horn Mountain in central Utah records a shift in the composition of the upper Maastrichtian vertebrate fossil assemblage below the Cretaceous-Tertiary (K-T) boundary. Dinosaurs (Alamosaurus,Tyrannosaurus , Torosaurus, and hadrosaurs), turtles (Basilemys), mammals, and lizards(Polyglyphanodon ) occur in the lowest unit of the formation. The environment changed from semi-arid to wetter conditions, and several of the mostly terrestrial vertebrate taxa no longer appeared in the type area. Several taxa that dominated the vertebrate assemblage low in the formation are replaced in the succeeding unit below the K-T boundary by different dinosaur taxa that are represented by eggshells. This altered vertebrate assemblage was more tolerant of, or was adapted to, wetter conditions, but it, too, disappeared at the type locality by the end of the Cretaceous. This wetland dinosaur assemblage is not identical to that found in semi-arid dinosaur assemblages or to dinosaur assemblages in other K-T boundary sites that represent different paleolatitudes. Most of the faunas in the succeeding unit are represented by aquatic forms (crocodilians, turtles, fish, mollusks, and ostracods) which persisted after the dinosaurs disappeared. There appears to be no current tectonic or biostratigraphic reason to conclude that a significant stratigraphic gap exists immediately below the K-T boundary in the North Horn Formation in central Utah.