Henry Gee

But the unusual thing about vertebrates is the armor is not calcium carbonate.

It's calcium phosphate.

It's called hydroxyapatite.

And it's the same arm.

Our bones are made of hydroxyapatite.

Now, strangely enough, even though vertebrates had the notochord, we had external armor more like invertebrates.

Only later did the backbone become mineralized.

And the armor evolved in various ways.

There are some of these fossil fish where the armor is broken up into scales, a kind of chagrin of scales.

There are these beautiful little fossil jawless fish called coelodonts, which have all sorts of amazingly stylish shapes.

Some of them come from this romantic locality called Moth, which stands for man on the hill.

It's out in the Northwest Territories of Canada.

But jawless fishes are known from all over the place.

In fact, quite a lot have been described in the UK, in Wales and the west of England, where they're Ordovician, Silurian, Devonian rocks.

You know, the clues in the name, the Ordovician and Silurian, were named after the Roman tribes in Wales, and the Devonian is in Devon.

So there were a lot from there, and they were escaping predators.

But also, phosphate is quite rare on Earth, and it's important in the biology of all animals.

So there is an idea that bone was a kind of accessory store of phosphate for very active animals, which vertebrates are much more active than most others.

So that was the origin of bone and fishes.

And then at some point in the Silurian, jaws were invented.