Only scattered dinosaur fossils have ever been discovered in North Dakota, compared to its neighbor in the south, but this state is rich in other prehistoric animals, as listed below. (See an interactive map of dinosaurs and prehistoric animals in the United States.)
Brontotherium--which sometimes goes by the names Brontops, Megacerops and Titanops--was one of the largest megafauna mammals of the late Eocene epoch, and was distantly ancestral to modern horses and other odd-toed ungulates. The lower jawbone of this two-ton beast was discovered in North Dakota's Chadron Formation. More about Brontotherium
One of the most common fossil animals of North Dakota, represented by entire skeletons, Champsosaurus was a late Cretaceous creature that closely resembled a prehistoric crocodile (but was, in fact, an obscure type of reptile known as a choristoderan). Like crocodiles, Champsosaurus prowled the ponds and lakes of North Dakota for tasty prehistoric fish.
North Dakota isn't usually known for its prehistoric birds, which is why it's remarkable that a Hesperornis leg bone was discovered in this state. The flightless, late Cretaceous Hesperornis is believed to have evolved from earlier flying ancestors, much like modern ostriches and penguins. More about Hesperornis
4. Mammoths and Mastodons
Mammoths and Mastodons roamed the northernmost reaches of North America during the Pleistocene epoch--and what state was located further north than North Dakota? Not only has this state yielded the remains of Mammuthus primigenius (the Woolly Mammoth) and Mammut americanum (the American Mastodon), but fossils of the distant elephant ancestor Amebelodon have been found here as well. More about prehistoric elephants
Megalonyx, the Giant Ground Sloth, is famous for having been first described by Thomas Jefferson. Somewhat surprisingly for a genus whose remains are usually found in the deep south, a Megalonyx claw was recently discovered in North Dakota, proof that this megafauna mammal had a wider range than was previously believed. More about Megalonyx
During the late Cretaceous period, much of North America (and much of North Dakota) was submerged under water. That explains the discovery, in 1995, of the nearly complete skull of Plioplatecarpus, an especially fierce type of marine reptile known as a mosasaur. This North Dakota specimen measured a scary 23 feet from head to tail!