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 Calcaire de Caen of France,[5] the Daohugou Beds in China, the Itat Formation in Russia, the Tiouraren Formation of Niger,[6] and the Isalo III Formation of western Madagascar.
| Middle Jurassic | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 Map of the world during the Middle Jurassic | |||||||||
| Chronology | |||||||||
| |||||||||
| 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 41.1708°N 1.8333°W | ||||||||
| 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) |
| ||||||||
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.[7][8]
Marine life
During this time, marine life (including ammonites and bivalves) flourished. Ichthyosaurs, although common, are reduced in diversity; 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. 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 dinosaurs emerged during the Middle Jurassic, (including cetiosaurs, brachiosaurs, megalosaurs and primitive ornithopods).[7]
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.[9][10] Tritylodonts were larger, and also had an almost global distribution.[11] 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.
Flora
Conifers were dominant in the Middle Jurassic. Other plants, such as ginkgoes, cycads, and ferns were also common.
References
- "International Chronostratigraphic Chart" (PDF). International Commission on Stratigraphy.
- 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 (3): 166–175. doi:10.18814/epiiugs/2001/v24i3/003. Retrieved 13 December 2020.
- 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.
- 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.
- Allain, Ronan (24 August 2010). "Discovery of megalosaur (Dinosauria, Theropoda) in the middle Bathonian of Normandy (France) and its implications for the phylogeny of basal Tetanurae". Journal of Vertebrate Paleontology. 22 (3): 548–563. doi:10.1671/0272-4634(2002)022[0548:DOMDTI]2.0.CO;2. S2CID 85751613. Retrieved 10 April 2023.
- Rauhut; Lopez-Arbarello (15 January 2009). "Considerations on the age of the Tiouaren Formation (Iullemmeden Basin, Niger, Africa): Implications for Gondwanan Mesozoic terrestrial vertebrate faunas". Palaeogeography, Palaeoclimatology, Palaeoecology. 271 (3–4): 259–267. Bibcode:2009PPP...271..259R. doi:10.1016/j.palaeo.2008.10.019. Retrieved 12 April 2023.
- 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.
- 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 Archived 2020-08-03 at the Wayback Machine Palaeontology Online.
- Kemp, T 2005. The Origin and Evolution of Mammals. Oxford University Press.
