Pterosaurs are not classified as birds, and they are also not dinosaurs.
They're an extinct group of flying reptiles that are more closely related to dinosaurs
and birds than they are to any other reptile. Pterosaurs first appear about 225 million
years ago, and become extinct about the same time that avian dinosaurs did, about 65 million
years ago. There are two broad categories of pterosaur
body plan. Early pterosaurs tended to have long tails,
relatively short heads, and relatively short limbs.
The later pterosaurs tended to have very short tails, longer limbs when standing on the ground,
and very large heads and necks. In recent years, there have been several trackways
that have been found, and that we could refer to pterosaurs.
The main reason for that is because pterosaurs have a particular morphology of their hands
and their feet, and you could see them replicated in those trackways. And those trackways showed
us that they moved on the ground using all four limbs.
Pterosaur research is an ongoing process, and we have a lot of new discoveries and a
lot of new aspects that we might learn in the future.