Jurassic

The Jurassic (/dʒʊˈræsɪk/ juurr-ASS-ik) is a geologic period and stratigraphic system that lasted from the end of the Triassic Period 201.4 Ma (million years ago) to the beginning of the Cretaceous Period, 143.1 Ma. It was the second and middle period of the Mesozoic Era, named after the Jura Mountains, where scientists first found rocks from this time.

During the Jurassic, the huge landmass called Pangaea began to split into two parts: Laurasia to the north and Gondwana to the south. The climate was warmer than today, with no ice caps, and forests grew even near the poles.

This period was famous for its amazing dinosaurs. The first stem-group birds appeared, evolving from a branch of theropod dinosaurs. The oceans were home to fascinating marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs ruled the skies. Many modern groups of plants, like conifers, also began to appear during the Jurassic.

Etymology and history

The word "Jurassic" comes from the Jura Mountains, a mountain range between France and Switzerland. In 1795, a scientist named Alexander von Humboldt noticed special rocks there and called them Jura-Kalkstein, meaning "Jura limestone".

Later, in 1829, another scientist, Alexandre Brongniart, used the term terrains jurassiques to describe similar rocks in Britain, which means "Jurassic terrains". Other scientists also helped divide and name different parts of the Jurassic period based on fossils they found.

Geology

The Jurassic Period is divided into three epochs: Early, Middle, and Late. It is also split into the Lower Jurassic, Middle Jurassic, and Upper Jurassic series. Geologists study the rocks from this time and call them stages, which match up with time periods called ages.

Scientists use a special group of fossils called ammonites to help date these layers of rock. The first appearance of certain ammonite types marks the start of each stage and smaller time parts within stages, called "ammonite zones". These zones help experts compare rock layers from different places around the world.

The Jurassic had important resources like oil and coal. Large oil fields, such as the Ghawar Field, formed during this time. Coal reserves were found in places like north-west China, especially in the Turpan-Hami Basin and the Ordos Basin.

Some big impacts from space happened during the Jurassic. One was the Morokweng impact structure in South Africa, and another was the Puchezh-Katunki crater in Russia.

Paleogeography and tectonics

At the start of the Jurassic, all the major landmasses of the world were joined together in a huge supercontinent called Pangaea. During this time, Pangaea began to break apart into two smaller supercontinents: Laurasia in the north and Gondwana in the south. The first split happened between North America and Africa, creating the early Atlantic Ocean.

The Jurassic saw many changes in how the Earth’s landmasses and oceans were arranged. The Atlantic Ocean was still very narrow, while the Tethys Ocean lay between Gondwana and Asia. Europe became a chain of islands surrounded by shallow seas. Both the North and South Poles were covered by oceans. By the end of the Jurassic, Madagascar and Antarctica began to drift away from Africa, and North America and South America had separated to form a seaway connecting the Atlantic to the larger ocean, Panthalassa.

Climate

The climate during the Jurassic period was much warmer than today, about 5–10 °C warmer, with carbon dioxide levels in the atmosphere four times higher. Even areas near the poles had forests, experiencing warm summers and cold winters, though there were no large ice sheets. The oceans were also warmer, and coral reefs grew farther from the equator.

There were times when the climate cooled briefly, called "cold snaps," interrupting the overall warm conditions. One major event was the Toarcian Oceanic Anoxic Event, when a large increase in carbon dioxide caused very warm temperatures and changes in the oceans. This event affected many sea creatures, especially those that made shells or other calcium carbonate structures. By the end of the Jurassic, the climate began to cool again, leading into the Early Cretaceous period.

Flora

At the start of the Jurassic period, there was no mass extinction of plants. Some plant groups, like the Peltaspermaceae, went extinct, while others, like Dicroidium, declined but survived in some places.

Conifers were very important plants during the Jurassic. Many modern conifer groups, like Araucaria and Cupressaceae, first appeared during this time. Other conifer families, such as Cheirolepidiaceae and Pinaceae, were also common. Ginkgoales, which today include only the Ginkgo tree, were more diverse back then. Bennettitales, plants that looked like cycads but were different, were also common and had flower-like structures that might have been pollinated by insects. Cycads were around, but not as dominant as once thought.

Ferns and their relatives, such as the families Dipteridaceae and Osmundaceae, were widespread. The first modern horsetails of the genus Equisetum appeared in the Early Jurassic. Simple plants like mosses, liverworts, and Quillworts that are similar to modern species were also present during this time.

Fauna

Dinosaurs continued to thrive during the Jurassic period, becoming the dominant land animals. After the extinction event that marked the end of the Triassic, many new types of dinosaurs evolved. Theropods, such as coelophysoids and dilophosaurs, appeared early in this period, while later groups like ceratosaurs and tetanurans developed as the Jurassic progressed. Sauropods grew to enormous sizes, some becoming the largest land animals ever known.

Other reptiles also evolved diverse forms. Crocodylomorphs, the ancestors of modern crocodiles, showed a variety of lifestyles, including terrestrial and marine forms. Pterosaurs, with their wing membranes, flew through the skies, and marine reptiles like plesiosaurs and ichthyosaurs patrolled the oceans. These changes in animal life made the Jurassic a rich and dynamic time for Earth's ecosystems.