Mesozoic (252 - 66 Ma)

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The Mesozoic (“Middle Era”) began just after the end-Permian mass extinction and is divided into three periods: the Triassic (252-201 Ma), Jurassic (201-145 Ma) and Cretaceous (145-66 Ma). It’s also known as the “Age of Archosaurs”, a group of diapsid amniotes that dominated many Mesozoic ecosystems, the living representatives (birds and crocodilians) which still play an important role in modern ecosystems. The crocodilian-line archosaurs (Pseudosuchia) includes crocodiles, alligators, gavials and their extinct relatives, whereas the bird-line archosaurs (Avemetatarsalia) includes non-avian dinosaurs, pterosaurs and of course, the birds, which evolved during the mid Jurassic as a clade of derived coelurosaurian theropod dinosaurs. Even though the dinosaurs first appeared in the mid Triassic, they didn’t have a major role in ecosystems until the Triassic-Jurassic boundary, when a fourth major mass extinction wiped out dominant crocodilian-line archosaurs such as rauisuchians, phytosaurs and aetosaurs. Since then, dinosaurs have become the most dominant tetrapods. The first mammals also evolved from synapsid ancestors in the Triassic.

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As well as archosaurs, there were many other reptile groups that existed throughout the Mesozoic, where many groups of sauropterygians and other marine sauropsids flourished in Mesozoic seas and lagoons, such as ichthyosaurs, plesiosaurs, nothosaurs, placodonts, mosasaurs, etc. Invertebrates also thrived, where numerous species of ammonites, belemnites, bivalves, brachiopods, gastropods and echinoderms evolved and diversified during this era. Important changes in plant life took place during the early Cretaceous, when flowering plants (angiosperms) evolved from seed-producing plants (gymnosperms), leading to the co-evolution of new insects such as bees; the flower’s brightly-coloured petals attract insects, which cross-pollinate and fertilise the plants so that new seeds are produced.

 

Throughout the Triassic all the world’s land masses were unified into one supercontinent called Pangea. Its centre was so distant from the sea that hot, arid environments were prevalent, with the only habitable environments occurring along the coastal rim. Sea levels were at their lowest since the Precambrian, despite there being no ice caps at the poles. Oxygen levels were also quite low, an impact of the upheaval that concluded the Paleozoic era.

 

In the Jurassic, climatic conditions became more moist and equable, with the temperature gradient towards the poles being more gradual than today's. With the start of the Jurassic, the short-lived Pangea started to break up after becoming thermally unstable and shallow seas spread across continental shelves to form embryonic oceans like the North Atlantic. This break-up also led to the formation of two major landmasses: Laurasia in the north and Gondwana in the South. In middle Jurassic times the North Atlantic ocean started to widen, and east from the newly forming Atlantic was the Tethys ocean - a complex system of sea basins and oceans which lasted from the late Paleozoic right up to the Neogene.

 

In all of the Phanerozoic Eon, the sea levels were at their highest in late Cretaceous times - at over 200 meters higher than modern sea levels. Meanwhile, temperatures peaked as well - the mean global temperatures which were well over 30°C, leading to global anoxia events in the oceans. There were no ice caps at the poles and carbon dioxide levels were higher than todays. As Gondwana started breaking up in the early Cretaceous, the Indian and South Atlantic oceans started opening.

The Mesozoic era came to an end when a 10-15 km wide massive comet or asteroid struck the Earth, leading to the well-known Cretaceous-Paleogene (K-Pg) extinction event that wiped out all non-avian dinosaurs, pterosaurs, mosasaurs, plesiosaurs, ammonites, belemnites and many species of plankton. The impact took place near what is now Yucatan peninsula in southeastern Mexico, forming the 180 km (112 mi) wide Chicxulub crater. The aftermath of this event, in association with intense volcanic activity throughout the late Cretaceous, wiped out 75% or more of all species. This paved the way for the beginning of the Cenozoic Era...