DEFINITION OF JURASSIC

The second of three periods of the Mesozoic, lasting

from 206 to 144 million years ago. The name Jurassic

comes from studies of the Jura Mountains of Switzerland by

Alexander von Humboldt, published in 1795, and compared

with strata of England and Germany by Leopold von Buch in

1839. Rocks of the Jurassic are well preserved throughout

western Europe, and many of the subdivisions of Jurassic

time are based on studies in that region. Ammonites evolved

rapidly in this time and are widely used as index fossils for

the Jurassic, which includes more than 70 ammonite zones.

Pangea was breaking up and dispersing in the Jurassic

and had already broken into the northern Laurentian and

southern Gondwanan landmasses, separated by the growing

Tethys Ocean. North America separated from Africa, forming

the incipient Atlantic Ocean. Rifting of Antarctica, India,

and Australia also contributed to the pattern of global

breakup and separation, contributing to global sea-level rise.

At the same time, convergent margin tectonism was affecting

the western Cordilleran margins of North and South America.

Massive volcanism associated with plate breakup and

eruption of the Karoo flood basalt volcanism contributed to

climate warming.

The Jurassic is famous as a time when dinosaurs and

pterosaurs roamed freely on the land, with many herbivores

and giant bipedal carnivores. Gymnosperms including conifers

and ginkgos, along with ferns and horsetails, covered much of

the land, creating great forests. Marine life also flourished in

the Jurassic, with coral reefs, algae, sponges, and bryozoans

common, as were free-moving benthic forms including

bivalves, gastropods, belemnites, and foraminifera. Fish were

generally large with enameled scales, and several types of

sharks roaming the seas. Marine vertebrates included the longnecked

plesiosaurs, the large ichthyosaurs, giant crocodiles,

turtles, and other reptiles.

See also MESOZOIC; SUPERCONTINENT CYCLE.

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