How Was The "Five Extinction" Theory Made Up?

[will_ebert]

I am wondering about the five extinctions. How did scientists come up with the number five? How do they know there hasn't been five-teen extinctions, or fifty, or five hundered? Does it have something to do with the layers of sediment in the Earth's crust?

Websites with info on the five extinctions would be greatly appreciated, but i care more about the above queston.

 

[wet1]

Welcome to sciforums, will_ebert.

The first thing that came up was that there seemed to be some kind of time rhythm or synchronicity to the events that caused a massive loss of living species on the globe. During these times large percentages of all living species literally became extinct almost over night. The only thing that could cause such worldwide would be a worldwide disaster of some nature. Around 1980 came the testable theory that maybe it could have been a large asteroid impact. Iridium is associated with the platinum group. Platinum is a noble element meaning that it does not combine with other elements easily. The element iridium was found at the KT boundary. Iridium is a very rare element on earth. Further what is found on earth is different that the iridium that is found in space. This thin layer of iridium is found worldwide indicating that it must have been airborne to spread.

It would have been nice if they had a smoking gun. In other words, if there was a crater left by such a deep impact. Over time most impacts slowly fade on the surface due to weathering. Because of oil exploration, such a site came up. The place called Chicxulub. (Mayan devil's tail) was found to have been severally fractured deep within the earth. The porportions were huge. From 150 to 300 km in diameter. Some geologists thought that it might be the largest impact site on earth. Time estimations match with the 65 million year date of the KT extinction.

 

[James R]

The short answer is: analysis of the fossil record. Prior to each extinction, there are fossils of many lifeforms which do not appear after the extinction time. In each major extinction, up to 90% of all species appear to disappear from the fossil record.

The causes of these extinctions are debateable.