|Subdivision of the Jurassic system
according to the ICS, as of 2017.
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174 to 163 million years ago. Fossil-bearing rocks from the Middle Jurassic are relatively rare, but some important formations include the Forest Marble Formation in England, the Kilmaluag Formation in Scotland , the Daohugou Beds in China, Berezovsk Quarry in Russia, and the Isalo Group of western Madagascar.
During the Middle Jurassic epoch, Pangaea began to separate into Laurasia and Gondwana, and the Atlantic Ocean formed. Tectonic activities are active on eastern Laurasia as the Cimmerian plate continues to collide with Laurasia's southern coast, completely closing the Paleo-Tethys Ocean. A subduction zone on the coast of western North America continues to create the Ancestral Rocky Mountains.
Middle Jurassic faunaEdit
During this time, marine life (including ammonites and bivalves) flourished. Ichthyosaurs, although common, are reduced in diversity; while the top marine predators, the pliosaurs, grew to the size of killer whales and larger (e.g. Pliosaurus, Liopleurodon). Plesiosaurs became common at this time, and metriorhynchid crocodilians first appeared.
Descendants of the therapsids, the cynodonts were still flourishing along with the dinosaurs. These included the tritylodonts and mammals. Mammals remained quite small, but were diverse and numerous in faunas from around the world . Tritylodonts were larger, and also had an almost global distribution. The first crown group mammals appeared in the Middle Jurassic. A group of cynodonts, the trithelodonts were becoming rare and eventually became extinct at the end of this epoch.
- British Geological Survey. 2011. Stratigraphic framework for the Middle Jurassic strata of Great Britain and the adjoining continental shelf: research report RR/11/06. British Geological Survey, Keyworth, Nottingham.
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