Dr. Steven Novella

So what they found were ambulacraria.

Now, why would that matter?

Because they are... They're walking.

They are deuterostomes.

Deuterostomes, which means mouth second...

So basically, animal split into mouth first and anus first, or mouth second and anus second.

I've never heard that before.

Jay starts listening.

So it has to do with what opens first, the anus or the mouth, in the cord, in the axis of an animal.

Pro tip, don't Google that.

So the deuterostomes, so we are deuterostomes, right?

We are deuterostomes, and we are chordates, and we are vertebrates.

Deuterostomes split into ambulacrarian fossils and ambulacraria and chordates.

So if we're seeing the other branch, that means the split already happened, and therefore chordates must also exist at the same time.

Otherwise, we would be seeing creatures from before the split into the two groups.

Does that make sense?

Yeah.

Sure.

All right.

So we can infer that there must have been chordates at the same time, because we know they evolved into the common ancestor with the ambulacraria, the deuterostomes.