The Jurassic and Cretaceous periods are known as the time of the dinosaurs. During these two vast eras of time the world was dominated by these giant reptiles. Some of the sauropod herbivores are the largest animals to have ever walked the planet, and the Tyrannosaurus Rex is the largest ever land predator. The periods also saw the development of flowing plants, birds and mammals, all of which are still alive and thriving today.
The start of the Jurassic period was signalled by the Triassic-Jurassic extinction event. With many species dying out, there were large ecological niches that would be filled over time. During the Jurassic the supercontinent Pangea broke up into two separate land masses, Laurasia in the North and Gondwana in the South. The climate at the time was much warmer than that of today, with no land at the North or South poles.
Marine crocodiles, dolphin-like ichthyosaurs and the plesiosaurs dominated the oceans. All reptilian, whales and dolphins would not enter the seas for millions of years to come. The Jurassic also saw a large increase in planktonic species.
During this time many of the 'classic' dinosaurs evolved. Long-necked herbivores such as Diplodocus and Apatosaurus were feeding from the abundant ferns, cyads and conifer trees. Their enormous size would have been a deterrent to predators and would also allow them to browse vegetation at levels that other animals could not reach. With such a long and flexible neck an animal such as Diplodocus would have been able to feed on almost all vegetation growing at the time. Large Tyrannorsaur-like therapods were also living at this time, possibly hunting sauropods. Little is known about the behavior of dinosaurs as so many fossil specimens are incomplete. During the late Jurassic the first birds evolved from smaller bipedal dinosaurs.
The Cretaceous period started around 145 million years ago and lasted until the extinction of the dinosaurs, around 65 million years ago. During the Cretaceous period the continents continued to break up and started to resemble that which we see today. South America, Antarctica and Australia all moved away from Africa forming the Atlantic and Indian Oceans. The planet continued to cool in temperature, although remaining much hotter than current climates. Tropical average temperatures would have averaged at around 37 degrees centigrade, with deep sea temperatures sitting around 20 degrees hotter than they do today.
The Cretaceous is notable for the spread of flowering plants, or angiosperms. Along with the development of flowers was the evolution of bees and other pollen-spreading insects. This flourishing ecosystem is an excellent example of where two organisms can greatly accelerate eachother's development, commonly known as coevolution. Many leafy trees also began to show up during the Cretaceous, and plants started to look much more like they do in modern times.
The dinosaurs would continue to evolve and thrive during the Cretaceous period with many of the most well-known species flourishing during this period. Tyrannosaurus was the top land predator along with smaller bipedal dinosaurs such as Velociraptors. Pterosaurs would face increasing competition from birds and would dwindle significantly in numbers towards the end of the Cretaceous. Mammals were still restricted to smaller nocturnal creatures. At this point in the sea sharks and rays had fully developed and would remain the same even millions of years later.
The Cretaceous period was ended by a huge mass extinction event known as the Cretaceous-Tertiary extinction. It is thought that the extinction was brought on by a huge meteorite impact, the crater of which can be found on the Chicxulub coast in Mexico. The impact would have kicked up a huge dust cloud, blocking much of the sun's light. The impact crater shows that the meteor struck the coastline and so would have caused massive tsunamis, which would have proved fatal to all animal life caught in them. Evidence of tsunamis is prevalent across the USA, with marine sand found a long distance in land. The impact produced a cloud of sulphur dioxide which would have caused reduced sunlight as well as acid rain. This would have killed plants and plankton, severely harming the ecosystems that depended on them.
The only survivors of the event were omnivores, insectivores and carrion-eaters. Ecosystems that were based on consuming detritus would have been able to survive, and mammals with their warm blood and diet of insects would have been able to live through events that killed the larger reptilian dinosaurs. Crocodiles also survived the extinction event, this is thought to be due to their ability to live as scavengers and go for months without food.
After this mass extinction killed off the vast majority of the dominant life-forms a vast number of ecological niches were left open for mammals and birds to fill. The next era is known as the Paleocene Epoch and would see the growth of mammals across the planet.
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