A LARGE SILESAURID SPECIMEN FROM PETRIFIED FOREST NATIONAL PARK, U.S.A., WITH COMMENTS ON LARGE BODY SIZES IN LATEST TRIASSIC ORNITHODIRANS

Compared to other regions of Pangea, what is now western North America is not known for preserving larger-bodied dinosauriforms from the Triassic Period, partly because sauropodomorphs were apparently climatically excluded from the region until the Early Jurassic. However, the presence of the Liliensternus-sized coelophysoid Gojirasaurus quayi in the middle Norian of New Mexico suggests that increased sampling will recover larger-sized silesaurids and theropods in the Chinle Formation and Dockum Group. This contribution describes a relatively large silesaurid from the Petrified Forest Member of the Chinle Formation at Petrified Forest National Park, Arizona, U.S.A., second only in estimated size to a Middle Triassic form from Tanzania. This site also preserves a large-bodied coelophysoid comparable in size to other specimens from the same or correlated strata from Arizona and New Mexico. These specimens add more support to the decoupling of an increase in body size of theropod dinosaurs with the end-Triassic mass extinction and suggests that silesaurids may have undergone a similar increase in body size prior to that biotic event.