The Jurassic period, the second of the three Mesozoic periods. It extends from 201.3 to 145 million years ago, immediately after the Triassic (251.9 to 201.3 million years ago) and the Cretaceous (145 to 66 million years ago). In the Morrison Formation of the United States and the Sollenhofen Tuff of Germany, both famous for their exceptionally well-preserved fossils, are geological features that were formed during the Jurassic period.
The Jurassic was a period of major global changes in continental tectonics, oceanographic patterns and biological systems. During this time, the supercontinent Pangaea split apart, eventually forming what is now the Mid-Atlantic and the Gulf of Mexico. The intensification of plate tectonic movements led to significant volcanic activity, mountain building events and the connection of islands to continents. Shallow sea channels covered many continents and marine and marginal marine sediments were deposited, preserving a variety of dinosaur fossils. The rock layers deposited during the Jurassic period yielded gold, coal, oil and other natural resources.
Because of the poor preservation of terrestrial sediments and their fossils, it is not clear whether the mass extinction at the end of the Triassic had the same impact on terrestrial ecosystems as it did in the oceans. However, there were clear changes in vertebrate fauna during the Early Jurassic. In Triassic terrestrial ecosystems, synapsids and therapsids were the dominant modern mammal-ancestors, as were their relatives, commonly known as mammals and reptiles.
They occupied several ecological niches and grew to large sizes. By the early Jurassic, these groups became scarce and a minor component of the fossil assemblage; individuals were very small and their dental and skeletal anatomy suggests that early mammals may have been omnivorous (eating plants and animals) or insectivorous. In contrast, the main dragons (dinosaurs, crocodiles and pterosaurs) were the main terrestrial vertebrates.
It is not clear why this change from a predominantly sauropod fauna to a predominantly sauropod fauna occurred; it may be related to the Triassic-Jurassic extinction, or to adaptations that allowed sauropods to overtake their mammalian and mammalian ancestors. The diversity of terrestrial vertebrates may also have declined during the Late Jurassic, although some marine invertebrates were becoming extinct, but the evidence here is also inconclusive.
Pterosaurs were common throughout the Jurassic. Their light skeleton and wing structure, supported by a finger on each 'hand', made them suitable for flight and gliding.
The dinosaurs are divided into two groups based on a large number of skeletal features: the saurischians (lizard-rumped) and the ornithischians (bird-rumped). The pubic bones of the saurischians extend forward, while those of the ornithischians extend backwards.
Lizard-footed dinosaurs, including lizard-footed dinosaurs and all carnivorous dinosaurs, were the earliest dinosaurs. The lizard-footed dinosaurs (including the bewilderosaurs) appeared in the Early Jurassic and reached their peak of diversity, numbers and size in the Late Jurassic. Lizard-footed dinosaurs were usually long-necked and were probably adapted to feeding on the leaves of tall gymnosperms. Their decline in the Late Jurassic seems to correspond with a decline in such vegetation.
Carnivorous lizard-footed dinosaurs include Allosaurus. Many theropod suborders were globally distributed in the Jurassic. The origin of birds remains controversial, but it is generally accepted that birds are descended from small theropods of the Jurassic period. The earliest undisputed bird fossil found is that of the primitive bird.