Why weren't pterosaurs dinosaurs?

Look at a pterosaur's wing. It's thin skin, stretched from its arm all the way down to its ankle, and held up by one super-long finger. Birds, the flying dinosaurs, grew wings of feathers instead. Two different families found two different ways to fly.

Were the big sea reptiles dinosaurs?

Out in the sea swam big reptiles called ichthyosaurs, plesiosaurs and mosasaurs. Remember how dinosaurs stood tall on legs tucked underneath them? Apart from some birds, dinosaurs lived on land. These sea reptiles came from other parts of the reptile family, so even though people often call them dinosaurs, they weren't.

Was a pterodactyl a dinosaur?

Tiny answers for ages 4–11, then goes 2 steps deeper each time a child asks why.

Tiny says

While dinosaurs walked about below, other reptiles soared through the sky above them. They're called pterosaurs, and pterodactyl is an old name for them. They were related to the dinosaurs, and they lived at the same time. But pterosaurs were a family all their own, so a pterodactyl wasn't a dinosaur.

For ages 4–7

Checked against Pterosaurs: the truth about these 'flying dinosaurs' (Natural History Museum)
Why weren't pterosaurs dinosaurs?

Look at a pterosaur's wing. It's thin skin, stretched from its arm all the way down to its ankle, and held up by one super-long finger. Birds, the flying dinosaurs, grew wings of feathers instead. Two different families found two different ways to fly.

For ages 6–9

Checked against Pterosaurs: the truth about these 'flying dinosaurs' (Natural History Museum)
Were the big sea reptiles dinosaurs?

Out in the sea swam big reptiles called ichthyosaurs, plesiosaurs and mosasaurs. Remember how dinosaurs stood tall on legs tucked underneath them? Apart from some birds, dinosaurs lived on land. These sea reptiles came from other parts of the reptile family, so even though people often call them dinosaurs, they weren't.

For ages 8–11

Checked against What are dinosaurs? (Natural History Museum)