Nanotyrannidae

Nanotyrannidae is a group of tyrannosauroid dinosaurs from the Late Cretaceous of western North America. Its only confirmed member is the genus *Nanotyrannus*. One proposed evolutionary tree places *Appalachiosaurus* and *Dryptosaurus* as close relatives within this clade, suggesting that *Nanotyrannus* evolved from a line of tyrannosaurs that originally lived on the Appalachian landmass and later moved to Laramidia as the Western Interior Seaway shrank.

*Nanotyrannus* itself lived between 67 and 66 million years ago, during the Maastrichtian age, in what is now the Hell Creek Formation. It was among the last non-avian dinosaurs, surviving until the Cretaceous–Paleogene extinction event. The first species, *Gorgosaurus lancensis*, was originally described in 1946 by Charles W. Gilmore as a new species of *Gorgosaurus* based on a single skull. In 1988, Robert T. Bakker, Michael Williams, and Philip J. Currie re-examined that skull and placed it in a new genus, *Nanotyrannus*, naming it *Nanotyrannus lancensis* for its small size relative to other tyrannosaurids.

For decades, many researchers argued the skull belonged to a juvenile *Tyrannosaurus rex*, sparking intense debate. As of the current widely-known canon, no subsequent species has been formally described within *Nanotyrannus*, and the genus remains monospecific.

*Nanotyrannus* was a mid-sized tyrannosauroid, about 5.2 meters (17 feet) long. The adult *N. lancensis* likely weighed around 704 kilograms (1,552 pounds), based on the "Bloody Mary" specimen. Its body combined features of more basal tyrannosauroids like *Moros* and *Xiongguanlong* with those of more advanced tyrannosaurids. Unlike *Tyrannosaurus*, *Nanotyrannus* had a slender, low skull with small crests or horns just in front of its eyes. The muscle attachment sites at the back of its skull were large, suggesting it could strongly flex its head side to side.