A graph of invertebrate extinctions over the last 600 million years. The mass extinctions appear as periodic peaks rising above the background extinction levels. This data is from the work of D. M. Raup and J. J. Sepkoski.

• 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 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.

THE EXTINCTION OF THE DINOSAURS

Most dinosaur species perished in background extinctions that occurred throughout their Mesozoic Era. The other dinosaur species died out during the several mass extinctions that occurred in the Mesozoic: at the end of the Triassic (213 million years ago), during and at the end of the Jurassic (at 190, 160, 144 mya), and during and at the end of the Cretaceous (at 120, 82, and 65 mya).