Jurassic Period, the geologic name given to the middle period of the Mesozoic Era, between the Triassic and the Cretaceous periods. The Jurassic period began about 200,000,000 years ago and lasted about 65,000,000 years. The name Jurassic is derived from the Jura Mountains, where rocks formed during this period are found. In the United States, outcrops of rocks of the Jurassic period are found only west of the Mississippi River. Most of the rocks of this period are marine deposits (shale, sandstone, and limestone) containing a large number and variety of fossils. Dinosaurs and flying reptiles lived during this period and the first known birds appeared.