Jurassic Period was a time in Earth’s history that lasted from about 201 million to 145 million years ago. The period’s name comes from the Jura Mountains on the border between France and Switzerland, where scientists first identified rocks of this age. The Jurassic Period was the second of the three periods that make up the Mesozoic Era, the time during which the dinosaurs lived.
Many well-known dinosaurs lived during the Jurassic Period. These include the giant plant-eater Apatosaurus, the armor-plated Stegosaurus, and Allosaurus, a large meat-eating dinosaur. Marine reptiles also flourished, including dolphinlike reptiles called ichthyosaurs and four-flippered, long-necked reptiles called plesiosaurs. Flying reptiles appeared in the Jurassic Period along with the first birds and the first true mammals.
At the beginning of the Jurassic Period, most of Earth’s land was arranged in a single giant land mass geologists call Pangaea. During the period, Pangaea split apart into a northern mass, now referred to as Laurasia, and a southern expanse, referred to as Gondwanaland. Laurasia consisted of what are now North America, Europe, and Asia. Gondwanaland would later break up to form South America, Africa, India, Australia, and Antarctica. As Laurasia and Gondwanaland separated, the present-day Atlantic Ocean began to form between them.
Between what are now North and South America, the split created basins of sea water that dried out, leaving behind great thicknesses of salt. Later, sediments containing the remains of microscopic sea life buried the salt. Over time, the remains turned to petroleum, resulting in the petroleum-bearing salt formations found along the coast of the modern Gulf of Mexico.
Sea levels around the world rose during the Jurassic Period. In what are now central Canada and the northern United States, Earth’s crust gently subsided (sank), allowing the sea to cover the region. This event, called a marine invasion, left numerous fossils of sea creatures on what is now dry land. Marine invasions also occurred as rising seas covered large portions of present-day Europe and Russia.
Unlike many periods in Earth’s history, the Jurassic Period did not end with widespread extinctions of living things. Rather, scientists generally identify the end of the Jurassic Period with a rise in global sea levels and widespread marine invasions in many parts of the world.
