The prairies - the Great Plains grasslands that spread out across the center of the North American continent from the great winding Mississippi River on the lowland plain to the towering wall of the massive western Rocky Mountains - support vast herds of grazing animals. The most widespread of these are the sprintosaurs, evolved from the North American varieties of duck-billed hadrosaurs of the Late Cretaceous. In the last 65 million years, as the forests gradually gave way to the grasslands, the Nearctic hadrosaurs evolved to adapt to the changing habitats. Like the particular hadrosaurs before them, there are two main groups of sprintosaur, the crested, and the non-crested.