The Jurassic is the second and middle geological period in the Mesozoic era and the eighth period in the Phanerozoic eon. It began 201.3 million years ago, and ended 145 million years ago. The Jurassic period happened between the Triassic and Cretaceous periods.
The Kimmeridge Clay of the Upper Jurassic was laid down in an environment which does not exist on the earth today. Much of Western Europe was covered by a high sea level. The supercontinent Pangaea was beginning to break up, causing a narrow Atlantic Ocean. Because of this, the United Kingdom was covered by a shallow and largely anoxic sea, perhaps less than 100 metres deep, with occasional landmasses.
This was shallower water than the Blue Lias of the Lower Jurassic. It was often low in oxygen, which caused its organic material to decompose, but only partially. The Jurassic's mudstones are organic-rich, and gave rise to most of the North Sea oil.
The world of the JurassicEdit
The Jurassic started with all the continents together, 201.3 million years ago (mya). This was the global supercontinent Pangaea. It ended about 145 mya. The boundary with the following Cretaceous period is not marked by any clear signs. This is the only boundary between geological periods which has no clear markers.
The Jurassic happened in two parts. The first was marked by widespread oceanic anoxia, ocean acidification, and relatively high temperatures. This was likely caused by the eruptions in what we now call South Africa. Already, Pangaea was beginning to break up. From now on it was Laurasia to the north, and Gondwana to the south. The break-up of Pangaea took a long time to complete. The process of pulling apart in geology is called rifting. 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.
On sea and land, evolutionary trends which started in the Upper Triassic continued through the Jurassic. Archosaurian reptiles dominated the land biota. Reptile groups radiated and filled many niches. Dinosaurs, pterosaurs, marine reptiles (ichthyosaurs, plesiosaurs, turtles) all flourished.
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 gradually lost their grip on the in-shore habitats; molluscan bivalves took their place.
Early mammals existed, but mostly as small nocturnal creatures on the margins of the reptilian world. The first fossils of small dinosaurs with feathers, such as Anchiornis, come from the Jurassic period. The famous fossil bird, Archaeopteryx, lived in the Upper Jurassic, though we now know it was not the ancestor of modern birds.
The dominant land plants were the gymnosperms (conifers). Ferns, large horsetails,'monkey puzzle' trees, ginkgos and cycads were common. These trees were not easy to digest, compared to modern flowering plant trees (Angiosperms). They must have spent longer in the gut than the food of modern herbivores. That would make increased size an advantage for sauropods, which did indeed become much larger in the Jurassic than any any animal land life had before.
- Chambers, Martin 2000. The Upper Jurassic Kimmeridge Clay. Hull Geological Society  Archived 2008-09-25 at the Wayback Machine
- Levin, Harold 2006. The Earth through time. Wiley, Hoboken N.J. Chapters 13 & 14.
- Benton M. 1990. The reign of the reptiles. Crescent, N.Y.
- Amos, Jonathan 2019. Mission Jurassic: searching for dinosaur bones. BBC News.