The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as Dinosaur, from the Middle Jurassic are relatively rare,[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.] the Calcaire de Caen of France,
Paleogeography
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.
Significant subduction zones were active along practically all of the continental edges surrounding Pangea, as well as in southern Tibet, southeastern Europe, and other locations, to allow the formation of fresh seabed in the proto-Atlantic Ocean. Plate tectonic activity in subduction zones caused the construction of north-south mountain ranges such as the Rocky Mountains and the Andes all along the west coast of North, Central, and South America.
Fauna
The Middle Jurassic is one of the key periods in the history of life on Earth. Many groups, including dinosaurs and
mammals, diversified during this time.
[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.]
Marine life
During this time,
marine life (including
and
) flourished.
Ichthyosauria, although common, are reduced in diversity; the top marine predators, the
, grew to the size of
orca and larger (e.g.
Pliosaurus,
Liopleurodon).
became common at this time, and
first appeared. In the Jurassic seas, a wide range of animals swam. Cartilaginous and bony fish were plentiful. Large fish and marine reptiles were plentiful.
Terrestrial life
Many of the major groups of
emerged during the Middle Jurassic, (including
Cetiosaurus,
Brachiosauridae,
Megalosauridae and primitive
).
Descendants of the , the , 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.[Kielan-Jaworowska, Z., Cifelli, R.L., and Luo, Z.-X. 2004. Mammals from the age of dinosaurs: origins evolution and structure. 630 pp. Columbia University Press, New York.][Panciroli, E. 2017. The First Mammals Palaeontology Online.] Tritylodonts were larger, and also had an almost global distribution.[Kemp, T 2005. The Origin and Evolution of Mammals. Oxford University Press.]
Flora
were dominant in the Middle Jurassic. Other
plants, such as
,
, and
were also common.
See also
