eli5: Why are pterosaurs not considered dinosaurs?
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This is mostly an accident of how dinosaurs wer first discovered and classified.
Among the earliest dinosaurs discover were f Megalosaurus and Iguanodon. People got a lot wrong when they tried to figure out how these may have looked like in life, but they did notice similarities besides just the size and being dead and extinct for a long time.
So dinosaur was sort of coined as term for creatures like them.
Later more scientifically took these two species and found that they were examples of two different branches of huge group of related animals. They created a definition where the entire family of was dinosaurs. The two branches that these animals were on and all the animals living in them.
Scientifically they just took two animals from either branch and declared that every descendant of their most recent common ancestor was a dinosaur.
This definition cover all the big extinct reptiles walking on land that had all these common features. It also includes modern and extinct birds.
If you want to use the scientific definition and exclude birds you use terms like “non-avian” dinosaur.
A big problem is that alongside the dinosaurs lived other big now extinct reptiles that flew though the air or swam in the ocean. pterosaurs, pleiosaurs and others like that.
Those don’t fall in that particular definition.
Pterosaurs are still closer related to dinosaurs than any living organism (except birds), so we could have easily defined dinosaurs as including them without having to worry about including other living reptiles.
With other creatures that are big and extinct but less closely related this sort of definition would start including all sorts of living reptiles like crocodiles.
Some creatures often marketed as dinosaurs like dimetrodon the one with the sail on its back, they are actually more closely related to us humans than dinosaurs.
So due to the definition of dinosaur being based on how closely related creatures are and the first few dinosaurs we found and recognized and scientifically described, pterosaurs are out and birds are in.
There is no reason why dinosaurs have to be defined the way the were. We have names for groups that include dinosaurs and pterosaurs. Avemetatarsalia is a name that includes dinosaurs and pterosaurs and a bunch of other extinct reptiles. It just isn’t commonly used by non-scientist, but a group that covered the same definition could easily have become the popular name all people knew and the further subdivision be something only scientist care about.
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