New Examples ofTyrannosaurus rexfrom the Lance Formation of Wyoming, United States

Abstract

The dinosaur remains from the Upper Cretaceous Lance Formation of Wyoming, USA, have been known since 1870. Most of the fossils that are attributed to theropod dinosaurs are fragmentary skeletons consisting of cranial, axial and appendicular elements, and many isolated teeth are also known. The largest known theropod of the Lance Formation dinosaur fauna is Tyrannosaurus rex. Numerous specimens of this taxon are known from the Maastrichtian strata of the northwestern United States and southwestern Canada. In 1890 and 1892, two pedal phalanges of the left foot and several isolated teeth identified here as belonging to Tyrannosaurus were discovered near Lusk, Wyoming, by John Bell Hatcher and the prospecting party from Yale Univeristy's Peabody Museum of Natural History in New Haven, Connecticut, USA. The discovery of these fossils is significant. One of the first discoveries of Tyrannosaurus from Wyoming, it predates all other discoveries and naming of this taxon in the coeval Hell Creek Formation of Montana, North Dakota and South Dakota, USA, and the Willow Creek and Scollard formations of the province of Alberta, Canada. Additionally, a well-preserved anterior end of the right dentary with teeth discovered in 1947 of a large Tyrannosaurus from the Lance Formation of the Bighorn basin, Wyoming, is described. The discovery of this specimen is also significant, because it is the first occurrence of Tyrannosaurus in the upper coastal plain paleoenvironments located farther inland away from the coastline of the Western Interior Seaway. Previous discoveries of Tyrannosaurus fossil remains from the Lance Formation were from strata deposited near the coastline of the Western Interior Seaway. The discovery of these specimens and their lack of significant morphologic divergence from other specimens of T. rex suggest that this taxon is the only large-bodied theropod that is currently known from the Upper Cretaceous (late Maastrichtian) of northwestern North America.