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File:Blakey 170moll.jpg
170 million years ago during the Middle Jurassic

The Jurassic is the second geological period in the Mesozoic that began 201.3 million years ago, and ended at 145 million years ago. Before it was the Triassic. It was followed by the Cretaceous.


Climatically, the Jurassic had higher temperatures, carbon dioxide levels and sea-levels compared to today. The Kimmeridge Clay of the Upper Jurassic was laid down in an environment which is not present on the earth today,[1] with much of Western Europe being covered by a high sea-level which has been related to opening of the Atlantic. The consequence of this is that the UK was covered by a shallow and largely anoxic sea, perhaps less than 100m deep, with occasional landmasses.

This was shallower water than the Blue Lias of the Lower Jurassic. It was often low in oxygen, which in turn led to only partially decomposition of its organic material. The mudstones are organic-rich, and gave rise to most of the North Sea oil.[1]

Plate tectonics

There were forces of tension and rifting (breaking apart) to make the supercontinents Laurasia and Gondwana. This was the start of the break-up of Pangaea, a process which took a long time to complete.

Floods of lava flowed from fissures (splits) and volcanos. By the end of the Jurassic, South America had begun to part from Africa. In the western part of North America, mountain ranges began to form. This continued as the American tectonic plates gradually moved west. The westward moving North American plates gradually rode over the Pacific Ocean plates to form the Rocky Mountains.[2]


On sea and land, evolutionary trends which started in the Upper Triassic kept going through the Jurassic. The land biota was dominated by Archosaurian reptiles. In general, the climate was hotter and wetter than today. Reptile groups radiated and filled many niches. Dinosaurs, pterosaurs, marine reptiles (Ichthyosaurs, Plesiosaurs, turtles) all flourished.[3] Amongst invertebrates there was much change. Modern predators like starfish, crabs and hole-boring gastropods took over the sea-floor, eating the benthic fauna in huge numbers. Brachiopods lost their grip on the in-shore habitats; molluscan bivalves took their place.

Early mammals existed, but mostly as small creatures living in burrows, on the margins of a reptilian world. The first fossils which show small dinosaurs with feathers are found: Anchiornis. The first fossil bird, Archaeopteryx, is found in the Upper Jurassic. The dominant land plants were the gymnosperms.

Related pages


  1. 1.0 1.1 Chambers, Martin 2000. The Upper Jurassic Kimmeridge Clay. Hull Geological Society [1]
  2. Levin, Harold 2006. The Earth through time. Wiley, Hoboken N.J. Chapters 13 & 14.
  3. Benton M. 1990. The reign of the reptiles. Crescent, N.Y.