Jurassic

The Jurassic is the second and middle geological period in the Mesozoic era and the eighth period in the Phanerozoic eon.

It began 201.3 million years ago, and ended 145 million years ago. The Jurassic period happened between the Triassic and Cretaceous periods.

During the Jurassic period, the climate was hotter and wetter than it is today. Carbon dioxide levels and sea levels were also higher than today.

The Kimmeridge Clay of the Upper Jurassic was laid down in an environment which does not exist on the earth today.^[1] Much of Western Europe was covered by a high sea level. The supercontinent Pangaea was beginning to break up, causing a narrow Atlantic Ocean. Because of this, the United Kingdom was covered by a shallow and largely anoxic sea, perhaps less than 100 metres deep, with occasional landmasses.

This was shallower water than the Blue Lias of the Lower Jurassic. It was often low in oxygen, which caused its organic material to decompose, but only partially. The Jurassic's mudstones are organic-rich, and gave rise to most of the North Sea oil.^[1]

The Jurassic started with all the continents together, 201.3 million years ago (mya). This was the global supercontinent Pangaea. It ended about 145 mya. The boundary with the following Cretaceous period is not marked by any clear signs. This is the only boundary between geological periods which has no clear markers.

The Jurassic happened in two parts. The first was marked by widespread oceanic anoxia, ocean acidification, and relatively high temperatures. This was likely caused by the eruptions in what we now call South Africa. Already, Pangaea was beginning to break up. From now on it was Laurasia to the north, and Gondwana to the south. The break-up of Pangaea took a long time to complete. The process of pulling apart in geology is called rifting. Floods of lava flowed from fissures (splits) and volcanos.

By the end of the Jurassic, South America had begun to part from Africa. In the western part of North America, mountain ranges began to form. This continued as the American tectonic plates gradually moved west. The westward-moving North American plates gradually rode over the Pacific Ocean plates to form the Rocky Mountains.^[2]

On sea and land, evolutionary trends which started in the Upper Triassic continued through the Jurassic. Archosaurian reptiles dominated the land biota. Reptile groups radiated and filled many niches. Dinosaurs, pterosaurs, marine reptiles (ichthyosaurs, plesiosaurs, turtles) all flourished.^[3] A recent discovery has shown that pliosaurs (a clade of large plesiosaurs) may have been top predators in the Upper Jurassic.

Amongst invertebrates, there was much change. Modern predators like starfish, crabs, and hole-boring gastropods took over the sea floor, eating the benthic fauna in huge numbers. Brachiopods gradually lost their grip on the in-shore habitats; molluscan bivalves took their place.

Early mammals existed, but mostly as small nocturnal creatures on the margins of the reptilian world. The first fossils of small dinosaurs with feathers, such as Anchiornis, come from the Jurassic period. The famous fossil bird, Archaeopteryx, lived in the Upper Jurassic, though we now know it was not the ancestor of modern birds.

In the last 20 million years of the Cretaceous period (Turonian–Maastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators.

The dominant land plants were the ferns (especially tree ferns), gymnosperms (conifers), large horsetails, 'monkey puzzle' trees, ginkgos and cycads. The coalfields laid down in the first part of the Jurassic were formed mainly from tree ferns, which grew thickly. They were for a time the most common plant type.

Ginkgoales, of which the only living species is Ginkgo biloba, were more diverse during the Jurassic.^[4] These plants were not easy to digest, compared to modern flowering plants (Angiosperms).^[5] They must have spent longer in the gut than the food of modern herbivores. That would make increased size an advantage for ankylosaurs and sauropods. Sauropods did indeed become much larger in the Jurassic than any animal land life had before.

The Jurassic is the second period notable for large coal seams (the first being the Upper Carboniferous). Once again, plants which grew and died in stagnant water did not decompose well, and formed coal as a result. The coalfields laid down in the first part of the Jurassic were formed mainly from tree ferns, which grew thickly. They were for a time the most common plant type.

• Blue Lias
• Lower Jurassic
• Middle Jurassic
• Upper Jurassic