Middle Jurassic

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,^[3] but geological formations containing land animal fossils include the Forest Marble Formation in England, the Kilmaluag Formation in Scotland,^[4] the Daohugou Beds in China, the Itat Formation in Russia, and the Isalo III Formation of western Madagascar.

Middle Jurassic
174.1 ± 1.0 – 163.5 ± 1.0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Map of the world during the Middle Jurassic
Chronology
−200 — – −195 — – −190 — – −185 — – −180 — – −175 — – −170 — – −165 — – −160 — – −155 — – −150 — – −145 — – Mesozoic T Jurassic K Late T Early Middle Late Early K Hettangian Sinemurian Pliensbachian Toarcian Aalenian Bajocian Bathonian Callovian Oxfordian Kimmeridgian Tithonian       ← Triassic–Jurassic extinction event Subdivision of the Jurassic according to the ICS, as of 2022.^[1] Vertical axis scale: millions of years ago.
Etymology
Name formality	Formal
Usage information
Celestial body	Earth
Regional usage	Global (ICS)
Time scale(s) used	ICS Time Scale
Definition
Chronological unit	Epoch
Stratigraphic unit	Series
Time span formality	Formal
Lower boundary definition	FAD of the Ammonites Leioceras opalinum and Leioceras lineatum
Lower boundary GSSP	Fuentelsaz, Spain 41°10′15″N 1°50′00″W / 41.1708°N 1.8333°W / 41.1708; -1.8333
Lower GSSP ratified	2000^[2]
Upper boundary definition	Not formally defined
Upper boundary definition candidates	Horizon of Ammonite Cardioceras redcliffense.
Upper boundary GSSP candidate section(s)	Redcliff Point, Dorset, UK Savouron, Provence, France

Middle Jurassic strata of the San Rafael Group, Colorado Plateau.

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.^[5][6]

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).^[5]

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.^[7][8] Tritylodonts were larger, and also had an almost global distribution.^[9] 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.

References

1. "International Chronostratigraphic Chart" (PDF). International Commission on Stratigraphy.
2. Cresta, S.; Goy, A.; Arias, C.; Barrón, E.; Bernad, J.; Canales, M.; García-Joral, F.; García-Romero, E; Gialanella, P.; Gómez, J.; González, J.; Herrero, C.; Martínez2, G.; Osete, M.; Perilli, N.; Villalaín, J. (September 2001). "The Global Boundary Stratotype Section and Point (GSSP) of the Toarcian-Aalenian Boundary (Lower-Middle Jurassic)" (PDF). Episodes. 24: 166–175. Retrieved 13 December 2020.
3. Clark, James (June 2009). "Evolutionary Transitions Among Dinosaurs: Examples from the Jurassic of China". Evolution: Education and Outreach. 2 (2): 243–244. doi:10.1007/s12052-009-0137-0.
4. 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.
5. 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.
6. 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.
7. 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.
8. Panciroli, E. 2017. The First Mammals 2020-08-03 at the Wayback Machine Palaeontology Online.
9. Kemp, T 2005. The Origin and Evolution of Mammals. Oxford University Press.

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