Middle Jurassic
Encyclopedia
The Middle Jurassic is the second epoch
of the Jurassic
Period. It lasted from 176-161 million years ago. In European lithostratigraphy
, rocks of this Middle Jurassic age are called the Dogger.
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.
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.
s and bivalves) flourished. Ichthyosaur
s, although common, are reduced in diversity; while the top marine predators, the pliosaur
s, grew to the size of killer whale
s and larger (e.g. Pliosaurus
, Liopleurodon
). Plesiosaur
s became common at this time, and metriorhynchid crocodilia
ns first appeared.
s evolved on land (including Cetiosaurs
, brachiosaurs
, megalosaurs and hypsilophodonts).
Descendants of the therapsids, the cynodont
s 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" mammal
s evolved.
es, cycad
s, and fern
s were also common.
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...
of the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
Period. It lasted from 176-161 million years ago. In European lithostratigraphy
Lithostratigraphy
Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology...
, rocks of this Middle Jurassic age are called the Dogger.
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, PangaeaPangaea
Pangaea, Pangæa, or Pangea is hypothesized as a supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration....
began to separate into Laurasia
Laurasia
In paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...
and Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
, and the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
formed. Tectonic activities are active on eastern Laurasia as the Cimmerian plate
Cimmerian Plate
The Cimmerian Plate is an ancient tectonic plate that comprises parts of present-day Anatolia, Iran, Afghanistan, Tibet, Indochina and Malaya regions. The Cimmerian Plate was formerly part of the ancient supercontinent of Pangaea. Pangaea was shaped like a vast "C", facing east, and inside of the...
continues to collide with Laurasia's southern coast, completely closing the Paleo-Tethys Ocean
Paleo-Tethys Ocean
The Paleo-Tethys Ocean was an ancient Paleozoic ocean. It was located between the paleocontinent Gondwana and the so called Hunic terranes. These are divided into the European Hunic and Asiatic Hunic...
. A subduction zone on the coast of western North America continues to create the Ancestral Rocky Mountains.
Marine life
During this time, marine life (including ammoniteAmmonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
s and bivalves) flourished. Ichthyosaur
Ichthyosaur
Ichthyosaurs were giant marine reptiles that resembled fish and dolphins...
s, although common, are reduced in diversity; while the top marine predators, the pliosaur
Pliosaur
Pliosauroidea is an extinct clade of marine reptiles. Pliosauroids, also commonly known as pliosaurs, are known from the Jurassic and Cretaceous Periods. The pliosauroids were short-necked plesiosaurs with large heads and massive toothed jaws. These swimming reptiles were not dinosaurs but distant...
s, grew to the size of killer whale
Orca
The killer whale , commonly referred to as the orca, and less commonly as the blackfish, is a toothed whale belonging to the oceanic dolphin family. Killer whales are found in all oceans, from the frigid Arctic and Antarctic regions to tropical seas...
s and larger (e.g. Pliosaurus
Pliosaurus
Pliosaurus is a genus of extinct marine reptile. It is included in the family Pliosauridae. Its diet would have included fish, squid and other marine reptiles. This genus has contained several species in the past but it currently consists of the type species P. brachydeirus, P. macromerus and P...
, Liopleurodon
Liopleurodon
Liopleurodon is a genus of large, carnivorous marine reptile belonging to the Pliosauroidea, a clade of short-necked plesiosaurs. Two species of Liopleurodon lived during the Callovian stage of the Middle Jurassic Period , while the third, L. rossicus, lived during the Late Jurassic...
). Plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...
s became common at this time, and metriorhynchid crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
ns first appeared.
Terrestrial life
New types of dinosaurDinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s evolved on land (including Cetiosaurs
Cetiosaurus
Cetiosaurus meaning 'whale lizard', from the Greek cetus/κητος meaning 'sea monster' and saurus/σαυρος meaning 'lizard', was a sauropod dinosaur from the Mid to Late Jurassic Period in what are now Europe and Africa. It is estimated to have been about long and to have weighed roughly...
, brachiosaurs
Brachiosauridae
Brachiosauridae are a family of dinosaurs, whose members are known as brachiosaurids. They were herbivorous quadrupeds with longer forelegs than hind legs - the name derives from the Greek for arm lizard - and long necks...
, megalosaurs and hypsilophodonts).
Descendants of the therapsids, the cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
s were still flourishing along with the dinosaurs even though they were shrew
Shrew
A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...
-sized; none exceeded the size of a badger
Badger
Badgers are short-legged omnivores in the weasel family, Mustelidae. There are nine species of badger, in three subfamilies : Melinae , Mellivorinae , and Taxideinae...
. 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" mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s evolved.
Flora
Conifers were dominant in the Middle Jurassic. Other plants, such as ginkgoGinkgo
Ginkgo , also spelled gingko and known as the Maidenhair Tree, is a unique species of tree with no close living relatives...
es, cycad
Cycad
Cycads are seed plants typically characterized by a stout and woody trunk with a crown of large, hard and stiff, evergreen leaves. They usually have pinnate leaves. The individual plants are either all male or all female . Cycads vary in size from having a trunk that is only a few centimeters...
s, and fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s were also common.