Tuesday, June 16, 2009

Modern Versus Ancient Extinction pt 2

Rock Volume

Ok let's think a little more about Jack's curve.

Remember we're counting fossils here. The number of fossils you have is going to depend on the amount of rock that you have. The younger the rock, the more of it there is. So there is a lot more Tertiary (the "T" on the graph) rock out there than say Cambrian (the "C" with a line through it) so there are more Tertiary fossils than Cambrian fossils. You can compensate for this but it's always going to be very tricky. So that huge run up of in diversity that you see at the end of Jack's curve might not be "real" just because we have so much more rock for those more recent time periods. So the general shape of the curve is a little suspect. With respect to extinction, I'm not saying that extinction events in the paleontoloogical record are simply a result of not having rock for those time periods, but certainly the volume of rock is going to affect how severe we view the extinction as being.
Next time: taphonomy and counting issues

Monday, June 8, 2009

Modern Versus Ancient Extinction pt 1

I've been meaning to get to this for a while. It hits on something that comes up from time to time in Paleontology. Writing in the New Yorker Elizabeth Colbert says:
The fifth, the end-Cretaceous event, which occurred sixty-five million years ago, exterminated not just the dinosaurs but seventy-five per cent of all species on earth. Once a mass extinction occurs, it takes millions of years for life to recover, and when it does it’s generally with a new cast of characters. In this way, mass extinctions have played a determining role in evolution’s course. It’s now generally agreed among biologists that another mass extinction is under way.
What these biologists are doing is jumping from ancient extinction events to a modern "extinction". This is much trickier than most people think so lets look at it. Let's begin, as Anton Ego would say, with some


This is Jack Sepkoski's Family diversity curve with all five of the major extinctions numbered. Jack generated this curve by going into the literature and tabulating the number of taxonomic families through time. Paleontologists spend a great deal of time pouring over curves like this one so we can understand the broad sweep of life's evolution. We also spend a great deal of time looking at extinction events in great detail. So with that sort of perspective it is certainly fair to ask what can paleontologists or the paleontological record tell us about the current biological crisis.
The answer.... not much.
Why next time.