Extinction is a natural result of evolution. Species go extinct when they are unable to adapt to changes in the environment or compete effectively with other organisms.
Most extinctions (perhaps up to 95 per cent of all extinctions) are background extinctions, occurring throughout time. These extinctions are not caused by major catastrophes or horrendous climactic changes, but by small changes in climate or habitat, depleted resources, competition, and other changes that require adaptation and flexibility.
Here is a graph of invertebrate extinctions over the last 600 million years. The mass extinctions appear as periodic peaks rising above the background extinction levels.
The five largest mass extinctions in Earth's history occurred during:
1. The late Ordovician period (about 438 million years ago) - 100 families extinct - more than half of the bryozoan and brachiopod species extinct.
2. The late Devonian (about 360 million years ago) - 30% of animal families extinct.
3. At the end of the Permian period (about 245 million years ago) - Trilobites go extinct. 50% of all animal families, 95% of all marine species, and many trees die out. This marks the beginning of the age of the dinosaurs.
4. The late Triassic (208 million years ago) - 35% of all animal families die out. Most early dinosaur families went extinct, and most synapsids died out (except for the mammals).
5. At the Cretaceous-Tertiary (K-T) boundary (about 65 million years ago) - 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. This brought an end to the age of the dinosaurs.
The event that caused the K-T extinction was the collision of an asteroid with Earth off the coast of the Yucatan Penninsula coastal town of Puerto Chicxulub. Clouds of smoke and dust shrouded the globe and caused a worldwide drop in temperature.
The crater is about 180 km across. Estimates suggest that the asteroid was about 20 km across and was travelling at anywhere between 20 and 60 kilometers per second (between 44,700 and 134,000 mph).
With the demise of the dinosaurs, mammals moved to the center of the stage:
"You and me, baby, ain't nothin' but mammals ..."
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