The earliest-known dinosaurs appeared about 228 million years ago; their fossils have been found on every continent. They dominated the land until about 65 million years ago, when an episode of extinction eliminated the non-avian dinosaurs (but not the birds) as well as many other animals and plants, both on the land and in the seas. Possible causes for these extinctions include the impact of an extraterrestrial object or a major, global peak in volcanic activity.
The American Museum of Natural History is home to the single largest collection of dinosaur fossils in the world, with more than 100 specimens featured in its halls. The Hall of Saurischian Dinosaurs examines the branches of dinosaurs that possess the trait of a grasping hand, with fingers that differ in size and shape. This hall features some of the Museum's most beloved and terrifying specimens, including Tyrannosaurus rex and Apatosaurus. Both of the displays have been revised to represent new scientific thinking: T-rex, once presented upright, is now positioned in a low, stalking pose with its tail in the air, while Apatosaurus—previously known as Brontosaurus—has a new skull, additional neck bones, and a longer, elevated tail. Also featured in this hall is the group of dinosaurs—maniraptors—that includes on its evolutionary branch living birds.