tyrannosaurs watch a comet streak
Dinosaur Image Gallery

Two tyrannosaurs watch a comet streak through the sky. See more dinosaur images.

Brian Franczak

Dinosaurs became extinct at the end of the Cretaceous Period. How and why is a puzzle that paleontologists are trying to solve by studying fossils and rock formations. These fossils and rock formations do not give all the facts, so scientists must take the information and make educated guesses. Sometimes different scientists see the same material but come to different conclusions, so there are many different theories.

Dinosaur Image Gallery

Some paleontologists think the extinction was caused by a catastrophe such as a meteorite or comet hitting the earth or a gigantic volcano erupting. Others believe that a more gradual process was responsible. Some theories are that competition between dinosaurs and mammals was the cause, or possibly climate changes. Scientists also disagree about the amount of time it took for the extinction to take place. Some think it happened in several days. Others say it took from hundreds of generations to over half a million years. Any extinction theory must account for the extinctions that occurred in the sea, including some types of clams and coiled mollusks.

The extinction event did not kill all animal and plant life. Many kinds of animals survived, including fishes, frogs, turtles, crocodilians, birds, and mammals. Scientists must take the fossil record and find reasons for all extinctions.