Steve Brusatte

They'd been modified into these display structures, these advertising billboards sticking off of the arms.

Then some of those dinosaurs got small enough and those advertising billboards big enough that when they moved them around, they could generate a little bit of lift and a little bit of thrust just by the laws of physics.

And a threshold had been crossed accidentally with no plan, with no purpose.

Evolution doesn't work that way.

But now these wings could provide some aerodynamic forces and then natural selection could take over and fashion those wings into ever better airfoils.

And so there were wings.

Lots of different types and shapes of wings started to turn up.

You see some raptor dinosaurs with wings on their arms and on their legs, four wings.

You see other raptor dinosaurs that still had long tails with a wing on their tail.

You even see some small dinosaurs, there's one called Yiqi from China.

It is a tiny little thing, would have been really cute.

You could have held this thing in the palm of your hand.

And this thing had a wing, but that wing was made of skin, kind of like a bat's wing.

So very clearly there was this zone on the family tree of dinosaurs where you had all these different species of small dinosaurs that had feathers and had some kind of wing, a zone of experimentation.

But all we have left of that is the one lineage.

So it's basically a link between the two arms and the two shoulder girdles.

It helps stabilize the arms as the arms were being used for grabbing and slashing at prey.

By sometime late in the Cretaceous, their teeth were definitively gone.

They only had beaks.

What's really interesting is that birds today, in the lab, developmental biologists can tweak the genes of chickens and quails and so on and make them grow little teeth that look like the teeth of tiny raptor dinosaurs.