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Word Count: 1,745
The history of life has been punctuated by brief episodes of mass extinction The five major ones ended the Ordivician Devonian Permian Triassic and Cretaceous eras see fig ure below A general pattern was found when the extinctions were studied 1 Extinction strikes in both the land and the sea 2 On the land while animals suffer repeatedly plants tend to be highly resistant to mass extinctions 3 Preferential disappearance of tropical forms of life during mass extinctions 4 Tendency of certain groups of animals to experience them repeatedly for example trilobites and ammonoids 5 Alleged equal spacing or periodicity in geological time occurring about every 26 million years These similarities between distinct extinction occurrences aid paleontologists in determining the agents the agents that perpetuated the disappearances of species in each extinction event Such agents are currently divided into two types 1 Catastrophic Agents- such as meteorite impacts and comet showers 2 Earth Agents- such as volcanism glaciation variations in sea level global climatic changes and changes in ocean levels of oxygen or salinity Although these agents can explain mass extinction the causes of mass extinction events remain relatively unknown The CretaceousTertiary boundary also known as the KT boundary is not the extinction that resulted in the greatest amount of extinct species that title belongs to the PermianTriassic boundary Yet the KT boundary is a rather popular one to study because of its victims the dinosaurs Over the years there have been several theories regarding the demise of the dinosaurs One proposal is that a worldwide rise in temperature occurred at the closing of the Cretaceous period which caused the testes of dinosaurs to stop functioning and led to their extinction by sterilization of males Another proposal has to do with angiosperms flowering plants which first evolved toward the end of the dinosaur reign Many of these plants have psychoactive agents but are avoided by
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