Jurassic

Jurassic

The Jurassic period is the second of the three divisions of the Mesozoic era, "The Age of Reptiles." The Jurassic lasted for 64 million years, from about 208 to 144 million years ago. The period is named for rock strata found in the Jura Mountains on the border between Switzerland and France.

During the Jurassic, the supercontinent Pangaea began to break apart. This created two landmasses, a northern mass called Laurasia (North America, Europe, and Asia) and a southern mass called Gondwanaland (South America, Africa, Australia, Antarctica, and India). During the early Jurassic, North America separated from Africa and South America and moved northward, but still remained connected to Europe. By the late Jurassic, the North Atlantic was just beginning to appear between Europe and North America.

Widespread deposits of sand in western North America indicate that the region experienced a desert climate during the early Jurassic. Coral reefs and the remains of temperate and subtropical forests around the world provide evidence that the climate became moister and milder later in the period. Europe was covered with shallow seas throughout the Jurassic.

Jurassic vegetation consisted mainly of seed ferns, cycads, horsetails, conifers, and gingkoes. The Jurassic is sometimes called the "Age of Cycads" because of the variety and diversity of these seed-bearing, palmlike plants. Some cycads grew to be tall as trees; other forms were short and squat.

In the marine world, the great success story was that of the ammonites —the coiled, shelled relatives of modern squid. At the end of the Triassic (the period just before the Jurassic), nearly 47 percent of marine species went extinct, indicating a drastic rapid deterioration of the environment that results in a crisis for certain species and is known as an extinction event. Extinction events allow some species to adapt to different environmental conditions and fill new niches. This is known as adaptive radiation. Although only one family of ammonites survived an extinction event at the end of the Triassic, this family radiated into an astonishing array of forms, some of which attained sizes of 2 meters (6 feet) or more.

The Jurassic period is known for an increase in the numbers and diversity of dinosaurs. At the beginning of the period, dinosaurs such as the bipedal and carnivorous theropods were small and lightly boned, feeding on insects or other small dinosaurs. By the close of the period, massive predators like Allosaurus and Ceratosaurus had appeared. These dinosaurs had heavy bodies, powerful hind legs, front limbs used for grasping and holding prey, and long, sharp teeth for spearing and stabbing. The largest of all dinosaurs, the plant-eating sauropods, also developed during the Jurassic. The sauropods include Apatosaurus (formerly called Brontosaurus), Brachiosaurus, Diplodocus, Seismosaurus, and Suprasaurus. These animals were quadrupeds, with pillarlike legs (like the legs of an elephant) that supported their enormous body weight, which was often 18 metric tons (20 tons) or more. The large size of the sauropods may have helped them maintain a consistent body temperature. The Stegosaurus is known for a distinctive row of heavy, triangular, bony plates, known as scutes, which were arranged along its back. Paleontologists (scientists who study dinosaurs) believe these plates helped the Stegosaurus regulate its body temperature and protected it from being eaten. Several sharp, bony spikes on the end of the tail of Stegosaurus probably served as a weapon against attack.

The debate continues as to whether birds most likely evolved from small, bipedal dinosaurs or other ground-dwelling reptilian ancestors. Archaeopteryx is one of the earliest undisputed bird fossils. It exhibits features of both dinosaurs and birds, including a long, bony tail; small, sharp teeth; feathers; and a "wishbone" that allowed for the attachment of flight muscles.

Era	Period	Epoch	Million Years Before Present
Mesozoic	Cretaceous		144
	Jurassic		208
	Triassic		245

Mammals continued to diversify during the Jurassic, but remained small and nocturnal, possibly to avoid competition with the dinosaurs. These early mammals were almost all herbivores, insectivores, and frugivores (fruit eaters).

see also Geological Time Scale.

Leslie Hutchinson

Bibliography

Lane, Gary, A., and William Ausich. Life of the Past, 4th ed. Upper Saddle River, NJ: Prentice Hall, 1999.