Sixty-five million years ago, the middle of this continent was a shallow sea, and when this area was not underwater it formed a hot, humid subtropical coastline of marshes, rivers, and river deltas with dense vegetation near the watercourses and grassy plains farther to the west. Fossils show that turtles, crocodiles, lizards, toads, small primitive mammals, and dinosaurs lived on this coastal plain, including Tyrannosaurus, Triceratops, Edmontosaurus, Ankylosaurus, and Pachycephalosaurus. Many of the most complete specimens of these species displayed worldwide were collected in Garfield and McCone Counties. The first Tyrannosaurus rex skeleton was recovered from these hills in 1902, and about half of the 30 or so tyrannosaurs found in the world come from this area. These fossils came from the Hell Creek Formation, a rock unit extending across much of eastern Montana and the Dakotas.