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The Jurassic Period, from about 190 to 135 million years ago, is the middle division of the Mesozoic Era of geologic time. The name, introduced in 1829 by the French geologist Alexandre Brongniart, refers to the chalk sequence in the rock strata of the Swiss-French Jura Mountains. Stratigraphers have subdivided the Jurassic system into the following stages, 
in order of increasing age: (Upper Jurassic) Portlandian, Kimeridgian, Oxfordian, and Callovian; (Middle Jurassic) Bathonian and Bajocian; and (Lower Jurassic) Aalenian, Toarcian, Pliensbachian, Sinemurian, and Hettangian.
During this time of 55 million years, very large reptile life forms had evovlved and dominated land, sea and air. One of these life forms, probably the most commonly known today, were the dinosaurs.
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