About Mass Extinctions


brontosaurus

Any conversation about dinosaurs inevitably reaches a point at which someone asks, "So what caused them to go extinct, anyway?"

The subject of mass extinction fascinates us. The idea of something eliminating 60% of the species of animal life on Earth is overwhelming (and not a little scary). And that extinction — the one that got the dinosaurs — wasn't even the biggest of them. That distinction goes to the Permian extinction which happened before dinosaurs arose. That extinction probably wiped out 96% of species on the planet!

There have actually been quite a few mass extinctions recorded in our Earth's stratigraphic encyclopedia, though the Cretaceous (dinosaurs) and Permian extinctions were the biggest. So what causes mass extinctions? Is there one reason for all of them; is each one different? What can cause such a planet-wide disaster?

Tyrannosaurus rex

Over the years since human interest in dinosaurs awoke, dozens of ideas have been proposed to attempt to explain how species such as Tyrannosaurus rex could have been completely wiped from the face of the Earth.

Most of these proposals are, frankly, pretty silly. A favorite 30 years or so ago was that dinosaurs went extinct because they were "out competed" by mammals. There are a number of problems with this suggestion. The first of these is that mammals appeared on earth at pretty much the same time dinosaurs did. They coexisted for more than 150 million years, and the mammals never managed to develop into anything bigger than a large rat, while dinosaurs occupied every terrestrial ecological niche available for an animal bigger than a chicken. Then, too, there is the problem that, while the dinosaurs are the stars of our Mesozoic landscape, they didn't have the world to themselves. The world was full of creatures of all types — all of which suffered greatly during that mass extinction. Any explanation for what happened would have to explain a lot more than just why dinosaurs disappeared.

There are a couple of ideas that seem to explain the most about mass extinctions: continental drift and impact of a large rock from outer space. Both of these causes are supported by significant evidence.

If you consider these causes carefully, you will see that both of them produce the same effect — probably the true cause of mass extinctions. They significantly decrease the habitat space available for organisms, either by severely reducing the resources available, or by eliminating the physical space that populations need to survive. Niles Eldredge has written an excellent book called The Miner's Canary which explores the idea that the primary reason for mass extinctions is habitat loss.


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Updated 25 September 2004