Mass extinction at the end of the Cretaceous Period (66 million years ago)
Best known for killing off the dinosaurs, the end-Cretaceous mass extinction also caused many other casualties. Ammonoids (marine mollusks), pterosaurs (gliding reptiles), mosasaurs (swimming reptiles), and a host of other plants and animals died out completely or suffered heavy losses. However, some that did survive the extinction—including mammals, birds, crocodiles, turtles, and redwood trees—were barely scathed.
During the Cretaceous Period, shallow seas often covered all or parts of Kansas. Well-preserved and world-renowned fossils of marine animals that did not survive the mass extinction have been found in the chalk beds of western Kansas. They include 20-foot-long mosasaurs and pterosaurs with 20-foot wingspans.
Several theories have been proposed to explain what caused the end-Cretaceous mass extinction, including a much-publicized giant asteroid strike near the Yucatan peninsula that was so massive its impact likely led to world-wide climate change. Scientists continue to research and debate the issue. Regardless of the cause, one thing is certain—the event marked a drastic change in the planet’s biological makeup and set the stage for the rise of mammals.