Middle Jurassic

Middle Jurassic
174.1 ± 1.0 – 163.5 ± 1.0 Ma
MiddleJurassicMap.jpg
Map of the world during the Middle Jurassic
Chronology
Etymology
Name formalityFormal
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitEpoch
Stratigraphic unitSeries
Time span formalityFormal
Lower boundary definitionFAD of the Ammonites Leioceras opalinum and Leioceras lineatum
Lower boundary GSSPFuentelsaz, Spain
41°10′15″N 1°50′00″W / 41.1708°N 1.8333°W / 41.1708; -1.8333
Lower GSSP ratified2000
Upper boundary definitionNot formally defined
Upper boundary definition candidatesHorizon of Ammonite Cardioceras redcliffense.
Upper boundary GSSP candidate section(s)
Middle Jurassic strata of the San Rafael Group, Colorado Plateau.

The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations containing land animal fossils include the Forest Marble Formation in England, the Kilmaluag Formation in Scotland, the Daohugou Beds in China, the Itat Formation in Russia, and the Isalo III Formation of western Madagascar.

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.

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.

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 metriorhynchids first appeared.

Terrestrial life

Many of the major groups of dinosaurs emerged during the Middle Jurassic, (including cetiosaurs, brachiosaurs, megalosaurs and primitive ornithopods).

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 late Early Jurassic. A group of cynodonts, the trithelodonts, were becoming rare and eventually became extinct at the end of this epoch.[citation needed]

Flora

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


This page was last updated at 2023-01-18 02:15 UTC. Update now. View original page.

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