The Middle Jurassic is the second

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided ...

of the

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

Period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

. It lasted from about 174.1 to 163.5

million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago ...

. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

s containing land animal fossils include the

Forest Marble Formation The Forest Marble is a geological formation in England. Part of the Great Oolite Group, it dates to the late Bathonian stage of the Middle Jurassic.Weishampel, David B; et al. (2004). "Dinosaur distribution (Middle Jurassic, Europe)." In: Weisha ...

in England, the

Kilmaluag Formation The Kilmaluag Formation is a Middle Jurassic geologic formation in Scotland. It was formerly known as the Ostracod Limestone for the abundance of fossil freshwater ostracods within it. The Kilmaluag Formation is very fossiliferous, with ostracod ...

in Scotland,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

Daohugou Beds The Haifanggou Formation (), also known as the Jiulongshan Formation (), is a fossil-bearing rock deposit located near Daohugou () village of Ningcheng County, in Inner Mongolia, northeastern China. The formation consists of coarse conglomerat ...

in China, the

Itat Formation The Itat Formation ( Russian: итатская свита) is a geologic formation in western Siberia. It was deposited in the Bajocian to Bathonian ages of the Middle Jurassic. Dinosaur remains are among the fossils that have been recovered from ...

in Russia, and the

Isalo III Formation The Isalo III Formation is a geological formation in Madagascar, off the eastern coast of Africa. It dates back to the Middle Jurassic.Weishampel et al., 2004, pp.542-543 The use of the term "Isalo III" is somewhat controversial as the two prior u ...

of western Madagascar.

Paleogeography

During the Middle Jurassic Epoch,

Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...

began to separate into

Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pa ...

and

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final sta ...

, and the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

formed. Eastern Laurasia was tectonically active as the

Cimmerian plate Cimmeria was an ancient continent, or, rather, a string of microcontinents or terranes, that rifted from Gondwana in the Southern Hemisphere and was accreted to Eurasia in the Northern Hemisphere. It consisted of parts of present-day Turkey, Ir ...

continued to collide with Laurasia's southern coast, completely closing the

Paleo-Tethys Ocean The Paleo-Tethys or Palaeo-Tethys Ocean was an ocean located along the northern margin of the paleocontinent Gondwana that started to open during the Middle Cambrian, grew throughout the Paleozoic, and finally closed during the Late Triassic; exi ...

. A

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

on the coast of western North America continued to create the

Ancestral Rocky Mountains The geology of the Rocky Mountains is that of a discontinuous series of mountain ranges with distinct geological origins. Collectively these make up the Rocky Mountains, a mountain system that stretches from Northern British Columbia through cen ...

Fauna

The Middle Jurassic is one of the key periods in the evolution 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 Marine life, sea life, or ocean life is the plants, animals and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. ...

(including

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...

s and

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...

s) flourished.

Ichthyosaur Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alt ...

s, although common, are reduced in diversity; while the top marine predators, the

pliosaur Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive toot ...

s, grew to the size of

killer whale The orca or killer whale (''Orcinus orca'') is a toothed whale belonging to the oceanic dolphin family, of which it is the largest member. It is the only extant species in the genus ''Orcinus'' and is recognizable by its black-and-white pat ...

s and larger (e.g. ''

Pliosaurus ''Pliosaurus'' (meaning 'more lizard') is an extinct genus of thalassophonean pliosaurid known from the Kimmeridgian and Tithonian stages (Late Jurassic) of Europe and South America. Their diet would have included fish, cephalopods, and marine ...

'', ''

Liopleurodon ''Liopleurodon'' (; meaning 'smooth-sided teeth') is an extinct genus of large, carnivorous marine reptile belonging to the Thalassophonea, a clade of short-necked pliosaurid plesiosaurs. ''Liopleurodon'' lived from the Callovian Stage of the M ...

'').

Plesiosaur The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appea ...

s became common at this time, and metriorhynchids first appeared.

Terrestrial life

Many of the major groups of

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s emerged during the Middle Jurassic, (including cetiosaurs, brachiosaurs, megalosaurs and primitive

ornithopod Ornithopoda () is a clade of ornithischian dinosaurs, called ornithopods (), that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous w ...

s). Descendants of the

therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...

s, the

cynodont The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide varie ...

s, were still flourishing along with the dinosaurs. These included the

tritylodonts Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and details o ...

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

Conifer Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All ext ...

s were dominant in the Middle Jurassic. Other plants, such as

ginkgo ''Ginkgo'' is a genus of non-flowering seed plants. The scientific name is also used as the English name. The order to which it belongs, Ginkgoales, first appeared in the Permian, 270 million years ago, and is now the only living genus with ...

es,

cycad Cycads are seed plants that typically have a stout and woody (ligneous) trunk with a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male o ...

s, and

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...

s were also common.

References

{{Geological history, p, m Jurassic geochronology Geological epochs