|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, Itat Formation in Russia, and the Isalo III Formation of western Madagascar.
During the Middle Jurassic epoch, Pangaea began to separate into Laurasia and Gondwana, and the Atlantic Ocean formed. Eastern Laurasia was tectonically active as the Cimmerian plate continued to collide with Laurasia's southern coast, completely closing the Paleo-Tethys Ocean. A subduction zone on the coast of western North America continued to create the Ancestral Rocky Mountains.
Middle Jurassic fauna
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.
- Benson RBJ, Campione NE, Carrano MT, Mannion PD, Sullivan C, Upchurch P, and Evans DC. 2014. Rates of dinosaur body mass evolution indicate 170 million years of sustained ecological innovation on the avian stem lineage. PLoS Biology 12, no. 5: e1001853.
- Close, Roger A.; Friedman, Matt; Lloyd, Graeme T.; Benson, Roger B.J. (2015). "Evidence for a mid-Jurassic adaptive radiation in mammals". Current Biology. 25 (16): 2137–2142. doi:10.1016/j.cub.2015.06.047. PMID 26190074.
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- Kemp, T 2005. The Origin and Evolution of Mammals. Oxford University Press.