The United States also contains rock outcrops spanning most of dinosaurian time. Often these occur in dry, eroded badlands where the odds are better of fossils being preserved — another reason North America is a great place for dinosaur discoveries. Even so, exposed layers of rock sample some slices of geologic time much better than others. Nearly half of all dinosaurs known worldwide lived during the Late Cretaceous, their final dominant phase ending 65 million years ago. In western North America, this period includes such household names — assuming your household contains young children — as Ankylosaurus, Triceratops and Tyrannosaurus.

The American West also harbors rich repositories of bones from further back in time, about 150 million years ago when super-sized sauropods shook the ground with every step. But the fact that we have so many fossils of Apatosaurus, Barosaurus, Camarasaurus and other Jurassic giants, as well as their smaller contemporaries like Allosaurus, Stegosaurus and tiny Dryosaurus, owes a lot to the existence of the Morrison Formation. This fossil-rich rock layer extends across 700,000 square miles of 11 states and has proven particularly productive for dinosaurs in Utah, Colorado and Wyoming.

Because no sediments from the middle part of the Jurassic, about 175 million years ago, have been uncovered, however, we know nothing about how the sauropods began to flourish in North America. Fossils documenting the earliest evolution of dinosaurs in the Late Triassic also are few and far between. And paleontologists are likewise just beginning to fill in a gaping hole of evidence with fossil sites from the Early Cretaceous, around 140 million years ago.

During the Cretaceous, the supercontinent of Pangea broke apart and the continents began drifting into their present positions. Evolving in greater isolation, some dinosaur groups became more diverse. At least one kind even began to get brainy. A carnivore named Troodon from Montana stood as tall as as an adult person, but possessed a brain six times larger than a crocodile's — as big as any mammal's of its day. One of the first North American dinosaurs to be named, Troodon hints at how dinosaur evolution might have continued. But except for those fortunate, fleet-footed dinosaurs that evolved into birds, Troodon and all the rest bit the dust by the close of the Cretaceous. Their terrestrial reign of 160 million years concluded, but the dinosaurs continue to inspire awe and fear today.

Use our Dino Finder to see which prehistoric beasts lived in your backyard.