Many mass extinctions have occured in the history of Earth. Two of those in particular were the Mesozoic and Permian extinctions.
At the end of the Mesozoic Era, Australia was located a little bit off of the coast of Antarctica, much shorter of a distance than it is now. The great extinction of the mesozoic was the most famous of all extinctions, in which the dinosaurs died. It is becoming more and more accepted that the reason for this extinction is that a huge meteor from outer space collided with the earth. The result was a great cloud of debris that covered the earth, blocking out the sun and preventing photosynthesis for plants which die at the bottom of the food chain. This disrupted the entire chain, and the majority of animals died out.
At the end of the Paleozoic Era, Australia was still connected to Antarctica, and was part of the supercontinent Pangea. Scientists believe that the great extinction of the Paleozoic era was caused by a toxic build-up of carbon dioxide gas. Paleontologists believe that as rocks were forming, warmer conditions near the poles prevented cold, oxygen-rich waters from sinking to the deep oceans. The ocean floors became stagnant and carbon dioxide and hydrogen sulfide built up. The resulting "sucking" of carbon dioxide from the atmosphere would have reduced the greenhouse effect and plunged the world's climate into an ice age. If you would have been sitting on the beach, you would have seen an Alka-Seltzerish effect occuring in the ocean.
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