A wide range of plants and animals, both on land and in the sea, went extinct 65 million years ago. The ferocious Tyrannosaurus rex was one of the last species of nonavian dinosaurs. The ancient mollusks known as ammonites disappeared quite suddenly 65 million years ago.
Around 65 million years ago, something unusual happened on our planet-and we can see it in the fossil record. 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.
Large dinosaurs like T. Rex are the most famous creatures that went extinct some 65 million years ago. But a wide range of other species also perished around the same time.
During their long reign, dinosaurs were the top predators on land. In the oceans, meanwhile, three groups of large reptiles more closely related to lizards and snakes dominated the food chain, eating fish and other animals. All three groups, plesiosaurs, ichthyosaurs and mosasaurs, went extinct around 65 million years ago.
Around 66 million years ago, a wide range of large dinosaurs roamed the continents, while winged reptiles called pterosaurs flew overhead. By 65 million years ago, the pterosaurs, most of these dinosaurs and many other land animals had vanished. The age of the dinosaurs, which had lasted 170 million years, was over.
What happened to cause such widespread devastation 65 million years ago? Scientists agree that species go extinct primarily as a result of changes in their environment.
In the late 1970s a group of researchers led by physicist Luis Alvarez and geologist Walter Alvarez made an important discovery. While analyzing rock samples from 65 million years ago, the father-son team measured exceptionally high concentrations of a rare element, iridium.
Some researchers think that intense volcanism could have contributed to the dinosaurs' decline well before a comet or asteroid impact sealed their fate.
Our view of the mass extinction 65 million years ago is somewhat hazy, thanks to the unavoidable limitations of the fossil record. Yet there's no doubt that all nonavian dinosaurs went extinct. No fossils of nonavian dinosaurs have been found that are younger than 65 million years old.
Paleontologists are actively investigating what caused all nonavian dinosaurs to die out 65 million years ago. Most agree that the impact of a large asteroid or comet played a major role, while scientists continue to debate how volcanic activity or climate change caused by retreating sea levels might have contributed.