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 ammonites vanished. Shark diversity shriveled. Most vegetation withered. In all, more than half of the world's species were obliterated. Why Dinosaurs Disappeared The cause of the mass extinction that marks the end of the Cretaceous and the beginning of the Paleogene is a scientific mystery. The extinction wiped out the dinosaurs while most mammals, turtles, crocodiles, salamanders, and frogs survived. Birds escaped. So did snails, bivalves, sea stars (starfish), and sea urchins. Even hardy plants able to weather climate extremes fared okay. Scientists tend to huddle around one of two hypotheses that may explain the Cretaceous extinction: an extra...
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