JURASSIC

Paleogeography

The process of rifting continued in Pangea, resulting in the fragmentation of the supercontinent. A new ocean was formed in this process between Laurasia and Gondwana, and continued to spread westwards, toward the Panthalassan Ocean. Rifting in the Tethys Ocean continued at the same time. In the Middle Jurassic, Pangea separated entirely into Laurasia and Gondwana. At the end of the period, Laurasia started rotating clockwise, gradually closing the Tethys Ocean. The process was aided by Gondwana’s counter clockwise rotation. A new mid-ocean ridge formed at the easternmost edge of Gondwana, beginning the opening of the Indian Ocean. The separation of the continents was accompanied by intensive volcanic activity in many of Earth’s regions.

Paleoclimate

The Jurassic climate was somewhat milder than the Triassic one. The breakup of Pangea resulted in longer coastlines and wetter conditions on the mainland. The amount of precipitation increased, but there were still no major ice caps.

Terrestrial life

Fauna – Dinosaurs were at their peak in the Jurassic: the Saurischia (lizard-hipped reptiles) and the Ornithischia (bird-hipped reptiles).

The Saurischia were both carnivorous and herbivorous. The herbivores (sauropods) were mostly quadrupedal, while the carnivores (therapods) were bipedal. The most common herbivores included the Diplodocus (1), the Seismosaurus (2) and the Brachiosaurus (3). The Allosaurus (4) and the Ceratosaurus were common carnivores.

The Ornithischia were exclusively herbivorous and both quadrupedal and bipedal. Some of the most common bipedal dinosaurs in this group were the Iguanodon (5), the Dryosaurus and the Ornithopoda. The Stegosaurus (6) and the Ankylosaurus were the most common representatives of the quadrupedal group.

Flying reptiles from the Pterosauria order also saw a period of intensive development in the Jurassic. Pterosauria were recognisable by leathery wings stretched between one finger on their front limbs and the ankle of their hinds limbs, as well as by hollow bones and teeth in their jaws. The Rhamphorhyncus and the Pterodactylus (7) were the most common pterosaurs. Jurassic pterosaurs were small, the size of present-day larks, unlike their future counterparts. The cynodonts (7) were the most important mammals.

Flora – Jurassic flora was much more abundant than its Triassic predecessor. Pteridophytes and gymnosperms were widely spread. Ferns and horsetails were the most important pteridophytes. Cycads (8), resembling modern-day palms, Ginkgophyta (9), Pteridospermatophyta and Coniferophyta were the most notable gymnosperm classes.

Marine life

Phytoplankton (10) flourished in the Jurassic seas, and foraminifera (11) underwent an intensive growth spurt too. The seas and oceans were crawling with a variety of invertebrates. Sponges, corals, gastropods, bivalves and brachiopods inhabited them in large numbers, and cephalopods were particularly abundant, with ammonites and belemnites as the dominant groups. Various fish from the Chondrichthyes (sharks) and Osteichthyes classes were also present. Plesiosauria, Ichthyosauria (12) and Crocodilia were still the dominant orders among marine reptiles and were at the top of the food chain.

Proto-Velebit sedimentary basin

In Lower Jurassic, and in particular at the end of the period, the Adriatic Microplate detached itself from Gondwana entirely. The Adriatic Microplate consisted of several shallow-marine carbonate platforms, called the Adriatic, the Apulian and the Apennine platforms, which were linked by deep seabeds. The Adriatic-Ionian Basin (present-day Adriatic Sea) was located between the Adriatic and Apulian platforms. The platform domains of the once whole Adriatic Microplate were thus formed in the Jurassic and moved in front of Gondwana toward Eurasia. The Adriatic carbonate platform was approaching Eurasia while simultaneously creating carbonate sediments in shallow-marine conditions.

Extinction

Another mass extinction concluded the Jurassic period. Marine life was affected in particular, with 80% species becoming extinct in shallow seas. The causes of the extinction event have not yet been fully explained, but it is believed to have occurred due to the release of substantial amounts of methane from the sea floor.