Jurassic

The Jurassic begins 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 the major Triassic – Jurassic extinction event. However, the end of the Jurassic Period did not witness any major extinction event. The start and end of the period are defined by carefully selected locations.

The Jurassic period of time is broken into Early, Middle, and Late Jurassic subdivisions, also known as Lias, Dogger and Malm in Europe. The faunal stages from youngest to oldest are:

• Upper/Late Jurassic
• Tithonian
• kimmeridgian
• Oxfordian


• Middle Jurassic
• Callovian
• Bathonian
• Bajocian
• Aalenian


• Lower/Early Jurassic
• Toarcian
• Pliensbachian
• Sinemurian
• Hettangian

Lower Jurassic Period - 208-188 Million Years Ago

The Lower Jurassic is the earliest of 3 time scales of the Jurassic period. The dinosaurs have attained dominance, while most of the other Triassic species of animals have died out in two major Triassic extinctions - the mid-Carnian and the terminal Rhaetic. Apart from one or two early species, the dinosaurs seem to have been unaffected by these extinction events.

As the Jurassic Period opened, Italy, Greece, Turkey and Iran were attached to the North African portion of Gondwanaland. The supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern-supercontinent Gondwana. The climate was warmer and moister than during the Triassic. Reptiles were the dominant form of animal life and experienced a great adaptive radiation. 

 

In the oceans various types of ichthyosaurs and plesiosaurs evolved. The Jurassic North Atlantic Ocean was relatively narrow, while the South Atlantic did not open until the following Cretaceous Period, when Gondwana itself rifted apart. The Tethys Sea closed and the Neotethys basin appeared. As in the Triassic, there was apparently no land near either pole, and no extensive ice caps existed.

In the air the pterosaurs began to diversify. On land many Triassic dinosaurs (prosauropod herbivores and coelophysid carnivores) continued, while a number of new forms (giant sauropods and armoured scelidosaurs) evolved. Under the feet of the dinosaurs rodent-like tritylodontid Therapsids co-existed with primitive shrew-like mammals and lizard-like sphenodont reptiles. Crocodiles also appeared, but they were mostly aquatic forms.

During the early Jurassic, evolution seems to have polarised, firstly, there were the ruling land animals, the great dinosaurs, which filled the ecological roles now taken up by medium-sized and large mammals. Secondly, the first mammals had appeared and together with the tritylodont therapsids they filled the small rodent and insectivore niche. The mammals were to remain small and individually insignificant - comparable to shrews, mice and rats of today - although doubtless very significant ecologically, for the 135 million years of the dinosaurs reign.

Dinosaurs of the Lower Jurassic include:

Barapasaurus | Ammosaurus | Vulcanodon | Kotasaurus

Middle Jurassic Period - 180-154 Million Years Ago

The Middle Jurassic, called the Dogger in the European system of classification, is the second epoch of the Jurassic Period lasting 180-154 million years ago. The second of the three divisions that make up the Mesozoic era saw warm tropical greenhouse conditions world wide, shallow continental seas, the break-up of Pangea, cosmopolitan flora and fauna, and the triumph of the majestic dinosaurs and the great sea reptiles. During the Jurassic the extent of the oceans was far more widespread then they had been in the Triassic. The Jurassic sea level rose and flooded large portions of the continents. During this time, marine life including ammonites and bivalves flourished and new types of dinosaurs evolved on land including Cetiosaurs, brachiosaurs, megalosaurs and hypsilophodonts.

Crocodiles were abundant and diverse, and included marine, semi-aquatic and even a few small lizard-like terrestrial forms. In the seas, Ichthyosaurs, although common, are reduced in diversity, while the top marine predators, the pliosaurs, grew to the size of killer whales and larger such as ferocious Liopleurodons. Modern shark groups also began to appear. The gigantic Leedsichthys, a huge, scaleless filter feeder who reached 10 or even 30 metres in length filled the same ecological role as modern baleen whales.

On land gymnosperm plants were well represented. The superficially palm-like Cycadophyta (Cycads) were so abundant and diverse that the Jurassic period could well be called 'the Age of Cycads'. Some cycads were tall palm-like trees with rough branches marked by leaf scars, and pinnate (fern-like) leaf fronds. Other, unrelated forms, the equatorial flowering Bennettitales, were the most important group of shrubby trees, with short and stubby with squat bulbous trunks from the top of which the fronds grew. Conifers continued to be the most diverse large trees. The Middle Jurassic period was split into four sub divisions:

• Aalenian - a subdivision of the Middle Jurassic epoch of the geologic timescale that extends from about 175.6 million years ago to about 171.6 million years ago.
• Bajocian - this lasted from approximately 171.6 million years ago to around 167.7 million years ago.
• Bathonian - this lasted from approximately 167.7 million years ago to around 164.7 million years ago.
• Callovian - occurring from 164.7 to 161.2 million years ago.

It is the last stage of the Middle Jurassic. The stage takes its name from an old spelling of Kellaways Bridge, 3 kilometres north-east of Chippenham in Wiltshire in England. The name Jurassic comes from the Jura Mountains on the border of France and Switzerland, where rocks of this age were first studied. In 1795 Alexander von Humbolt described massive limestone formations of the Jura Mountains in Switzerland as the Calcaire de Jura. In 1839 Leopold von Buch formally named the rocks described by von Humbolt as the Jurassic System, the term has come into general use since.

Dinosaurs of the Middle Jurassic include:

Brachytrachelopan | Gasosaurus | Monolophosaurus | Omeisaurus

Upper Jurassic Period - 180-154 Million Years Ago

Upper Jurassic (also known as Malm) was an epoch of the Jurassic geologic period. It lasted from 161.2 million years ago to 145.5 million years ago. It is divided into 3 ages:

Tithonian - The Tithonian is the final stage of the Late Jurassic Epoch. It spans the time between 150.8 million years ago and 145.5 million years ago. It is followed by the Berriasian stage of the Early Cretaceous Epoch. Kimmeridgian – The Kimmeridgian is a stage of the Late Jurassic Epoch. It spans the time between 155.7 million years ago and 150.8 million years ago. The stage takes its name from the town of Kimmeridge on the Dorset coast, England. The beach at Kimmeridge Bay is a good place for looking for fossils — there are specimens on the beach washed in by the tide. The Kimmeridge Clay formation is the source for about 95% of the petroleum in the North Sea. Oxfordian - The Oxfordian stage is the first stage of the Late Jurassic Epoch. It spans the time between 161.2 million years ago and 155.7 million years ago. The stage takes its name from the city of Oxford in England.

This period was well known for many famous types of dinosaurs:

Sauropods:

• Camarasaurs
• Brachiosaurs
• Diplodocids

Many smaller animals were flourishing:

• Reptiles
• Lizards
• Early mammals
• Very early birds

The late Jurassic period sees the evolution of some of the greatest dinosaurs of all. The sauropods continue to flourish and to diversify, as the older Mid-Jurassic cetiosaurids are replaced by a diverse Late Jurassic fauna of Camarasaurs, Brachiosaurs and Diplodocids. Some of these creatures attained tremendous size. The giraffe-like Brachiosaurus reached 22 metres and weighed 15 tons and more. The Seismosaurus, a more slender dinosaur, exceeded 40 metres in length. More modest sized dinosaurs appeared such as Scelidosauridae, Stegosauridae and the camptosaurid iguanodonts. Along with the giant herbivores there were also the small fleet-footed 'fabrosaurs', scutellosaurs and hypsilophodontids, the 'gazelles' of the dinosaur world. These plant eaters were kept in check by a variety of carnivorous (theropod) dinosaurs, including small lightly built coelophysids, compsognathids and ornitholestids, and larger (from several hundred kilos to several tons in weight) dilophosaurids, Ceratosauria, Torvosauroidea and Allosauridae.

In addition there were many other types of animals around at this time: a number of different types of mammals, crocodiles, turtles, lizards, frogs, flying reptiles (Pterosaurs), marine reptiles and the first birds (Archaeopteryx).

There was a minor mass extinction toward the end of the Jurassic period (roughly 190-183 million years ago) in which more than 80% of marine bivalve species and many other shallow-water species died out. The cause of this extinction is unknown, but there is some speculation that it was triggered by the release of huge methane deposits from within the Earth (these deposits formed beneath the seabed as surface algae dies and sinks to the sea floor).

Dinosaurs of the Upper Jurassic include:

Apatosaurus | Archaeopteryx | Brachiosaurus | Diplodocus | Kentrosaurus