Jurassic Period

Jurassic Period, middle of the three periods of the Mesozoic Era of the geological timescale. It lies between the Triassic Period, which ended 206 million years ago, and the Cretaceous Period, which started 142 million years ago. The name Jurassic is taken from the Jura Mountains, which lie on the border of France and Switzerland. Rocks of this age, the “Jura-Kalkstein”, were first recognized as a distinct formation by Alexander von Humboldt in 1795. In 1829 the French geologist Alexandre Brongniart used “Terrains Jurassiques” to describe that part of the sequence of rocks which we now call the Jurassic.

During the Jurassic’s 64-million-year duration, the supercontinent of Pangaea fragmented. Pangaea had formed from the pieces of another supercontinent at the end of the Carboniferous Period, and was the dominant landmass on the Earth for 100 million years. The break-up led to changes in the arid environments over the core of this enormous landmass, and to the faunas and floras it, and the surrounding oceans, had supported. The first signs of the break-up of Pangaea were seen during the Triassic Period, when rifting divided the supercontinent into two still very substantial continents. One, Laurasia, consisted of what is now North America, Europe, and most of Asia, and the other, Gondwana, was made up of what is today South America, Africa, India, Madagascar, Australia, and Antarctica. They were separated by the Tethys Sea, which is now represented, in much diminished form, by the Mediterranean Sea. The rifting of the continents, driven by processes in the Earth’s upper mantle, continued and even accelerated during the Jurassic, with Laurasia breaking up into North America and Europe as the North Atlantic Ocean formed and widened, and Gondwana fragmenting into three landmasses composed of what today are South America plus Africa, Australia plus Antarctica, and India.

The rifting of the continents formed new oceans and coastlines. Within the oceans, plate tectonic processes led to the formation of mid-ocean ridges. The view that the upwelling of magma was responsible for driving the continents apart is now questioned. Another explanation for sea-floor spreading is that oceanic crust is drawn down beneath the continental margins by its own weight at subduction zones, with the oceanic slabs being pulled apart along the mid-ocean ridge. Whatever the mechanism, the result in the Jurassic was a significant rise in sea level as water in the oceans was displaced by the developing ridges. The edges of the newly formed continents were readily inundated, providing shallow waters and extensive new habitats for a diversifying littoral (shore-living) fauna and flora. After this initial Jurassic transgression, sea levels remained relatively high until a regression (lowering of sea level) took place towards the end of the Jurassic.

A consequence of the smaller continental masses and new oceans was the replacement of the arid Triassic climate by more humid conditions, though there is some evidence for increasing aridity towards the close of the Jurassic. Our understanding of Jurassic climates is based on studies of sedimentary rocks and the fossils they contain. Deposits of coal form in humid conditions, bauxites and ironstones in warm and humid conditions, evaporites (for example, salt and gypsum) in warm arid environments, and oolitic limestones in warm waters. No glacial deposits of Jurassic age have yet been recognized. Coral reefs in the Jurassic occur 30° latitude higher than today, providing further evidence for a warm, “greenhouse” climate, with a much gentler temperature gradient from the tropics to the poles.

The fossil faunas and floras contained in Jurassic rocks are rich and varied. Tiny fossils such as the single-celled foraminifera, the more complex ostracods, and a variety of plant spores and pollen are abundant. The ammonites, which had originated earlier, flourished. Their rapid evolution and wide distribution in marine sediments have made them especially valuable for stratigraphical correlation, allowing the identification of rocks of the same age in different places. Other molluscs, corals, sponges, brachiopods (lampshells), and bryozoans (moss animals) are also present. The earliest known moths and crabs have been found in Early Jurassic rocks in Britain. The Jurassic and the succeeding Cretaceous Periods were truly the age of the reptile. The marine reptiles, such as plesiosaurs and ichthyosaurs, first recorded in the Triassic, thrived in the expanding range of ecological niches furnished by the huge areas of shallow sea. Their remains are frequent in marine sediments; exceptionally the outlines and even impressions of the skin of these giant reptiles have been preserved. On land, dinosaurs included the meat-eating Allosaurus, the giant herbivorous sauropods such as Diplodocus, Stegosaurus, and an early iguanodontid called Camptosaurus. Fossils of flying reptiles (pterosaurs) can be found in Jurassic rocks. Perhaps one of the most famous denizens of the Jurassic is the “missing-link” Archaeopteryx, the toothed bird from the Lithographic Limestone of Bavaria in Germany. Mammals which had first appeared during the Triassic were often diminutive, probably weighing between 20 and 30 g (0.7 and 1.06 oz), comparable to the smallest mammals living today. However, some Jurassic mammals were larger, measuring between 0.26 and 0.5 m (10 and 20 in).

The Jurassic macroflora was dominated by the gymnosperms; angiosperms (flowering plants) are first unequivocally recorded in rocks of the succeeding Cretaceous. Among the fossil flora of the Jurassic are many groups still with us today, such as the horsetails, ferns, cycads and conifers. The leaves of the ginkgo or maidenhair tree, a distinctive variety of gymnosperm which first appeared during the Late Palaeozoic and is represented by one extant species, can be abundant in Jurassic rocks. Petrified remains of various Jurassic plants are well known. In Great Britain an unusual form of fossilization has occurred where the trunks of conifers were buried in organic-rich marine muds. Over millions of years the trunks have turned into the intensely black semi-precious stone called jet. These trunks can be identified as belonging to the genus Araucaria which includes the extant Monkey Puzzle Tree and the Norfolk Island Pine.

The colossal productivity of the oceans, plus the plant and animal remains carried into the sea by rivers, led to huge accumulations of organic matter on the sea bed which have been converted to oil (see Petroleum) and gas (see Fuel Gases). Many of the major hydrocarbon fields of Europe’s north-west continental shelf, including the North Sea, contain oil and gas formed from Jurassic organisms or trapped in rocks of Jurassic age.