The Jurassic Period is a geologic period and system that extends from 201 to 145 million years ago, from the end of the Triassic to the beginning of the Cretaceous. The start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Toarcian extinction in the Early Jurassic, and the Late Tithonian event at the end; however, neither event ranks among the ‘Big Five’ mass extinctions.
By the beginning of the Jurassic, the super-continent Pangaea had begun rifting into two landmasses, Laurasia to the north and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, changing many of the arid deserts of the Triassic to lush rainforests. On land, the fauna transitioned from the Triassic fauna dominated by archosaurs, to one dominated by dinosaurs. The first birds also appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, and the evolution of therian mammals, including primitive placentals. Crocodylians made the transition from a terrestrial to an aquatic mode of life. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates.
The Toarcian extinction was a wave of extinctions that marked the end of the Pliensbachian stage and the start of the Toarcian stage of the Early Jurassic period around 183 million years ago. The Toarcian extinction was most strongly manifested in aquatic lifeforms, notably in mollusk groups like ammonites.(1)Paul B. Wignall and Anthony Hallam (1997) Mass Extinctions and Their Aftermath, Pages 164-5 Its reach was global in extent, as evidenced by research in Portuguese and Japanese waters, and the Andean Mountains which were the floor of the Tethys Sea at the time.(2)Kenneth G. MacLeod and Christian Koeberl (2002) Catastrophic Events and Mass Extinctions: Impacts and Beyond, Pages 525-9 Evidence points to anoxic bottom waters as the probable cause of these marine extinctions, linked in turn to the massive volcanism of the Karoo-Ferrar eruptions.(3)Joan Marti and Gerald Ernst (2005) Volcanoes and the Environment, Page 219
The Late Tithonian extinction event was another minor extinction period lasting a few thousand years at the end of the Jurassic Period. Again it is unclear what caused this extinction, however evidence for both meteor impacts and massive volcanic activity is missing from the geological record. Again the data suggests the activity of an intelligent organism may have been at work. Again there does not appear to have been anything in the geologic record that indicates an intelligent animal had evolved, so the possibility exists that this was another short lived extraterrestrial colony.
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|1.||⇑||Paul B. Wignall and Anthony Hallam (1997) Mass Extinctions and Their Aftermath, Pages 164-5|
|2.||⇑||Kenneth G. MacLeod and Christian Koeberl (2002) Catastrophic Events and Mass Extinctions: Impacts and Beyond, Pages 525-9|
|3.||⇑||Joan Marti and Gerald Ernst (2005) Volcanoes and the Environment, Page 219|