Montana is famous for its dinosaur fossils, with seventy-five different dinosaur species discovered there, more than in any other state in America. The oldest fossils come from Jurassic Period rocks about 155 million years old and include Diplodocus, Stegosaurus, and Allosaurus. Most Montana dinosaurs come from Cretaceous Period rocks, including Deinonychus and Oryctodromeus. About 50-75 million years ago, nesting dinosaurs such as Maiasaura and Troodon lived on coastal plains beside an inland sea, and later Tyrannosaurus rex, herds of Triceratops, and Ankylosaurus lived in Montana. During the time dinosaurs ruled the region, Montana had a very different climate and geography, with inland bays of the Pacific Ocean in Jurassic times and the Atlantic Ocean in the Cretaceous providing a hot, humid subtropical climate. In eastern Montana in 1903, Barnum Brown of the American Museum of Natural History discovered the first Tyrannosaurus rex fossil, and since then at least 24 partial Tyrannosaurus skeletons have been found in the state. Deinonychus fossils discovered near Bridger in 1964 suggested that some dinosaurs were warm-blooded and are the ancestors of today's birds. About 65 million years ago, the Western Interior Seaway receded as the Rocky Mountains rose, pushing shorelines farther east, while great rivers crossed the coastal plain and deposited sediment that later became the Hell Creek Formation, where dinosaur fossils are frequently found.