ZoomDinosaurs.com Dinosaur and Paleontology Dictionary

A mass extinction is a relatively sudden, global decrease in the diversity of life forms. Mass extinctions have occurred periodically throughout the existence of life on Earth. To be a mass extinction, the following must occur:

• Extinctions occur all over the world.
• A large number of species go extinct.
• Many types of species go extinct.
• The extinctions are clustered in a short amount of geological time (a few million years is very short in terms of geological time).

The five largest mass extinctions in Earth's history occurred during:

• The late Ordovician period (about 438 million years ago) - 100 families extinct - more than half of the bryozoan and brachiopod species extinct.
• The late Devonian (about 360 mya) - 30% of animal families extinct.
• At the end of the Permian period (about 245 mya) - Trilobites go extinct. 50% of all animal families, 95% of all marine species, and many trees die out.
• The late Triassic (208 mya) - 35% of all animal families die out. Most early dinosaur families went extinct, and most synapsids died out (except for the mammals).
• At the Cretaceous-Tertiary (K-T) boundary (about 65 mya) - about half of all life forms died out, including the dinosaurs , pterosaurs, plesiosaurs, mosasaurs, ammonites, many families of fishes, clams, snails, sponges, sea urchins and many others.

Many minor extinctions have occurred through Earth's history.

Enchanted Learning^®
Over 35,000 Web Pages
Sample Pages for Prospective Subscribers, or click below

Click to read our Privacy Policy

E-mail