Answer:Fossils that are abundant in earlier rock layers are simply not present in later rock layers. A wide range of animals and plants suddenly died out, from tiny marine organisms to large dinosaurs.
Species go extinct all the time. Scientists estimate that at least 99.9 percent of all species of plants and animals that ever lived are now extinct. So the demise of dinosaurs like T. rex and Triceratops some 65 million years ago wouldn't be especially noteworthy—except for the fact that around 50 percent of all plants and animals alive at the same time also died out in what scientists call a mass extinction.
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