If the Paleozoic sounds a little dull, it’s because we haven’t told the whole story. During the Paleozoic Era, all the major groups of organisms evolved. The seas swarmed with trilobites and shellfish of all kinds, some weird and fantastic and some very like those alive today. With them coexisted fish and sea-lilies, seaweeds and giant swimming “scorpions.” For the first time, plants and then animals came out of the sea to live on the land. These events and creatures are preserved in Paleozoic rocks. The Mesozoic Era saw the development of mammals from reptiles, the rule of giant dinosaurs, and the beginnings of flight. Strange reptiles evolved and returned to the sea, or glided through the air on motionless wings. The Sundance Formation, deposited in the northern Rocky Mountains about the middle of the Mesozoic Era, is noted for the masses of bullet-shaped squid shells found in it. Water, land, and air were full of life.
In the Middle Mesozoic, the trough drained and much of it became an area of swamp and tropical forest extending from Montana to southern Utah. This was the domain of our largest dinosaurs, giant reptiles whose bones were preserved in impressive numbers. Como Bluff and Bone Cabin, Wyoming; Morrison, Colorado; and Dinosaur National Monument and Cleveland, Utah, are the sites of quarries where many fine specimens of _Apatosaurus_, _Diplodocus_, _Stegosaurus_, and _Allosaurus_ have been collected. These caches of bones have made our large museums showplaces known throughout the world.
During the last 65 million years of the Mesozoic, the trough fluctuated up and down. During much of that time a broad, shallow sea covered central North America from the Gulf of Mexico to the Arctic Ocean. The sea was filled with fish like the giant _Portheus_ (3.5 meters/11.5 feet long), with squid-like animals floating about in elaborate chambered shells, and with reptiles which had gone back to the water. Where there was a broad coastal plain, it swarmed with dinosaurs, the ones that make Lance Creek, Wyoming and Hell Creek, Montana famous collecting grounds for museum field parties. Yet at the end of the era the trough was a seaway again, and in the Agate area a final blanket of black mud, the Pierre Shale, was deposited in the sea.