Why did dinosaurs become extinct?

Sixty-five million years ago, the last of the non-avian dinosaurs went extinct. So too did the giant mosasaurs and plesiosaurs in the seas and the pterosaurs in the skies. Plankton, the base of the ocean food chain, took a hard hit. Many families of brachiopods and sea sponges disappeared. The remaining hard-shelled amonites vanished. Shark diversiry shriveled. Most vegetation withered. In all, more than half of the world's species were obliterated.     
                            
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