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Extinction, resulting from competition and environmental change, is the inevitable fate of every species and taxon of living organisms. Only when mass extinctions occurred (Table 3) is there any call for explanation or comment. When many groups of organisms disappeared simultaneously within a relatively short space of time, the change recorded in the fossil records appears to have taken place so rapidly that some people are tempted to assume that a catastrophic event must have been responsible. The Rhamphorhynchoidea reached their peak of diversification shortly before their extinction at the end of the Jurassic period, while a few of the Pterodactyloidea persisted almost until the end of the Cretaceous. Toothless pterosaurs which dominated during the Upper Cretaceous appear in the fossil record only as isolated individuals - the toothed forms had already disappeared before this. Pterosaur history therefore included two periods during which they flourished - the Late Jurassic and the middle of the Cretaceous and three extinction events. These were at the end of the Jurassic, in the Early Cretaceous, and towards the end of the Mesozoic Era. There were, however, no mass extinctions at the times when most groups died out.

Bramwell and Whitfield (1974) speculated that the extinction of Pteranodon could have been caused by climate change, and particularly in average wind speed towards the end of the Cretaceous. This would also have applied to Quetzalcoatlus, Titanopteryx, and Azhdarcho. An increase of only 5ms-1 would have been enough to make conditions impossible for the giant pterosaurs. Such a change could have been caused by global cooling, accompanied by the development of clear temperature differences between the equator and the poles. As Wellnhofer (1991, incorporated in Norman and Wellnhofer 2000) argued, longer periods of the year with higher wind speeds, during which the large pterosaurs were unable to fly, would have reduced their numbers to such an extent that they sank below the critical level at which survival was possible.

As the landmasses of Eurasia drifted apart, the marine currents which influence world climatic conditions would have become more like those which obtain today. Consequently,the adaptive process which led to the evolution of the great pterosaurs would have led to their demise. In order to survive, they would have had to become smaller, develop heavier bones and acquire flapping flight once again. According to Unwin (1988), pterosaurs and birds showed differential survival patterns of relative diversity during the Mesozoic. This conforms to a double-wedge pattern typical of the differential survival model of faunal replacements. Birds which began to compete with the pterosaurs during the Lower Cretaceous (Sect. 6.7.1) were better adapted to the changing conditions, and during the Upper Cretaceous would have competed more strongly with them - even over the oceans that were their last stronghold. However, all this is very speculative. Nothing is known of weather patterns in those days and there is no real evidence either for, or against, competition between pterosaurs and birds. Certainly, there is little evidence to suggest that the pterosaurs radiated as birds and bats have. In fact, they appear to be the analogues of the sea birds today (Naish and Martill 2003).

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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