Toward the close of the Paleozoic era, changing climatic conditions caused the death of many forms of life upon the earth, and the development of new and hardier types. With these, in the Mesozoic era, came the new lords of the earth, the reptiles. Much of the globe from the Arctic Ocean to New Mexico was covered by a great sea on whose swampy shores huge dinosaurs, alligators, and crocodiles made their homes. Largest of these grotesque creatures was the brontosaurus, with his long snakelike neck and face and huge body. Struggling with him for supremacy of the swamps was the armored stegosaurus, whose row of vertical plates along his backbone from head to tip of tail made him a formidable enemy. Among the plant and animal life that throve on the sea bottom were shellfish three feet or more in diameter. The earliest seas of the Mesozoic era deposited the Dakota sandstone that underlies all of the State except the Red River Valley. It is a soft white or gray stone, containing many marine fossils. Although it does not appear at the surface anywhere in the State, it has been studied from specimens obtained from deep wells here or from outcroppings in other States. Many rivers flowing into the prehistoric seas brought mud and clay to mix with the soils of the sea bottom, forming the shales that today underlie most of the Great Plains, including North Dakota. The lowest of these, Benton shale, is dark gray, almost jet black in places, and contains bits of pyrite (fool's gold) and gypsum. Over it lies the bluish-gray Niobrara shale, in which natural cement is found. In the Pembina Mountains and the Sheyenne Valley, where these rocks appear at the surface, they have yielded shells of lamellibranchs (ancestors of today's clams and oysters) and bones of great sharklike fish. Over these two strata lies the Pierre shale deposit, dark bluish-gray in color. It is frequently seen in the valleys of streams east of the Missouri Plateau, but outcrops in only two places west of the Missouri Escarpment--where the Missouri leaves the State, and in the valley of the Little Beaver Creek in southwestern North Dakota. In it have been found fossils of the chambered nautilus, the oyster, and other marine animals, the crocodile, and the plesiosaur--that ungainly reptile which had "the body of a turtle strung on a snake." Once again the sea covered the State and left the rust-colored Fox Hills sandstone, which is particularly conspicuous along the Cannonball River where action of underground water has formed it into the rusty-looking spheres which give the river its name. Similarly formed cylinders of this sandstone in concretionary form are also found along the stream, and large cylinders protrude from the top of Cannon Butte in the Badlands like the barrels of cannon from the turret of a huge battleship.