Series: Geologic Time Periods in the Mesozoic Era

The Mesozoic Era (251.9 to 66 million years ago) was the "Age of Reptiles." During the Mesozoic, Pangaea began separating into the modern continents, and the modern Rocky Mountains rose. Dinosaurs, crocodiles, and pterosaurs ruled the land and air. As climate changed and rapid plate tectonics resulted in shallow ocean basins, sea levels rose world-wide and seas expanded across the center of North America.

  • Article 1: Cretaceous Period—145.0 to 66.0 MYA

    dinosaur footprint in stone

    Many now-arid western parks, including Chaco Culture National Historical Park and Mesa Verde National Park, were inundated by the Cretaceous Interior Seaway that bisected North America. Massive dinosaur and other reptile fossils are found in Cretaceous rocks of Big Bend National Park. Read more

  • Article 2: Jurassic Period—201.3 to 145.0 MYA

    dinosaur skull in rock face

    Dinosaur National Monument is home to thousands of dinosaur fossils making it a true “Jurassic Park.” A vast desert covered Southwest North America in the Jurassic, and ancient sand dunes now form tall cliffs in many parks including Glen Canyon National Recreation Area. Read more

  • Article 3: Triassic Period—251.9 to 201.3 MYA

    fossil footprint on stone

    The brightly colored Triassic rocks of Petrified Forest National Park yield not only the petrified trees but many other plant and animal fossils. Read more