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Middle Jurassic

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Middle Jurassic

System/
Period
Series/
Epoch
Stage/
Age
Age (Ma)
Cretaceous Lower/
Early
Berriasian younger
Jurassic Upper/
Late
Tithonian 145.0–152.1
Kimmeridgian 152.1–157.3
Oxfordian 157.3–163.5
Mid/
Middle
Callovian 163.5–166.1
Bathonian 166.1–168.3
Bajocian 168.3–170.3
Aalenian 170.3–174.1
Lower/
Early
Toarcian 174.1–182.7
Pliensbachian 182.7–190.8
Sinemurian 190.8–199.3
Hettangian 199.3–201.3
Triassic Upper/
Late
Rhaetian older
Subdivision of the Jurassic system
according to the IUGS, as of July 2012.

The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176 to 161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger.[1][2] This name in the past was also used to indicate the Middle Jurassic epoch itself, but is discouraged by the IUGS, to distinguish between rock units and units of geological time.

Paleogeography

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.

Life forms of the epoch

Marine life

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.

Terrestrial life

New types of dinosaurs evolved on land (including cetiosaurs, brachiosaurs, megalosaurs and hypsilophodonts).

Descendants of the therapsids, the cynodonts were still flourishing along with the dinosaurs even though they were shrew-sized; none exceeded the size of a badger. A group of cynodonts, the trithelodonts were becoming rare and eventually became extinct at the end of this epoch. The Tritylodonts were still common though. Mammaliformes, who evolved from a group of cynodonts were also rare and less significant at this time. It was at this epoch that the "true" mammals evolved.

Flora

Conifers were dominant in the Middle Jurassic. Other plants, such as ginkgoes, cycads, and ferns were also common.

References

  1. ^ A Geologic time scale 1989, Walter Brian Harland, 1990, p.53, webpage: Books-Google-eggC.
  2. ^ "A palynological investigation of the Dogger Formation (Middle Jurassic)", JB Riding, NERC.ac.uk, 2007, webpage: nerc-65.
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