Henry Gee

armor, jaws, teeth, skeletons, shells.

And of course, it's the hard parts that tend to be fossilized.

It's only very rarely that you get fossilization of the soft parts of an animal.

And that really is quite remarkable.

The Cambrian, they keep changing their minds about the beginning of the Cambrian.

There are two ways to decide this.

One is when particular fossils occur.

Traditionally, the beginning of the Cambrian is seen as the occurrence of a kind of burrow called treptichnus.

So when animals started to be able to burrow into the sediment, that caused a big change in the global ecology, rather than skating over the surface.

Once they started mixing up the sediment, that caused a great deal of change to the Earth system.

But the other one is directly dating it using not carbon dating that runs out.

That's useless before about 45,000.

It's recent history.

It's quite recent history.

So there are various radioisotope methods that you can use to date the beginning of the Cambrian.

But they keep changing their minds.

I mean, I looked up this morning and the latest...

agreed date for the beginning of the Cambrian was 538.8 million years ago.

In fact, the reason we're having this today, listeners, is because it's the 538.8 millionth anniversary next Tuesday.

So that's why we're doing it.