Mass extinction is known by the boundary between one geologic time period and the next. The Cretaceous mass extinction, better known as the K-T mass extinction, happened at the boundary between the Cretaceous Period and the Tertiary Period. The K stands for Kreide, which is the German word for chalk. It
describes the chalky texture of the clay found in sedimentary rocks from that time. This clay layer is also known as the K-T boundary.
The Cretaceous mass extinction marked the end of the Age of the Dinosaurs. But the dinosaurs were not the only victims. Around 75% of all species were destroyed. All land animals over 55 pounds (25 kilograms) became extinct.
Some groups of animals escaped this mass extinction. There are many theories to explain why. Crocodiles, turtles, lizards, mammals, and birds were affected. However, they survived with most of their species intact. Many plants either died out or suffered heavy losses. However, the roots of many plants managed to survive and eventually grew again.
Mammals were no larger than a cat at this time. Most mam-
In the early 1800s, the fossils of only three dinosaur species were known in Britain. The dinosaurs were considered to be huge lizards and were classified as reptiles. Sir Richard Owen was a prominent scientist and expert in anatomy (a science that studies the physical structures of living creatures) of the time. In 1841 he studied newly discovered dinosaur fossils and concluded that while the dinosaurs were related to each other, they were not related to lizards. In 1842 Sir Owen coined the name Dinosauria, from two Greek words meaning "terrible lizard," for these new members of the reptile world.
mals lived underground, as if waiting for their turn to dominate the planet. They waited about 140 million years! After the dinosaurs became extinct, the Age of Mammals began. It still took nearly 60 million more years for the first humanlike ancestor to walk the Earth.
The extinction of the dinosaurs has been studied extensively. For the past 25 years, most scientists have thought that a very large asteroid hit the planet. Other scientists think that a massive volcanic eruption in India caused this extinction.
Are extraterrestrial asteroids responsible for the mass extinction of the dinosaurs? Could asteroids have caused any other mass extinction? Come share in the discovery that changed the course of paleontology.
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