Pterosaurs aren't considered true dinosaurs. Why not?
Every so often I read about certain prehistoric reptiles not being true dinosaurs. A trip to the encyclopedia yielded the statement that at the same time there were dinosaurs there were also pterosaurs, crocodiles, etc., who were not dinosaurs. But it never actually defined what a dinosaur was. So what exactly differentiates a dinosaur from other lizards?
I understand your confusion. When I was a kid my idea of a dinosaur was that it was big, ugly, and dead. Not the most scientific definition, but no book or museum exhibit ever offered a better one. Now I know why: until the 1970s paleontologists believed, in their heart of hearts, that there weren't any true dinosaurs. "Dinosaur" was an informal term used to describe two distinct groups of animals, the Saurischia and the Ornithischia. The two groups were related, but they were equally related to the crocodiles and the pterosaurs (flying reptiles), all of whom were thought to have descended from a common ancestor. There was no real justification for saying tyrannosaurus (a saurischian) and stegosaurus (an ornithischian) were dinosaurs but a pterodactyl wasn't. Nonetheless the term dinosaur had been around for a long time and the public had gotten used to it. So a definition of sorts evolved: dinosaurs were (a) land bound but (b) nonflying (c) reptiles who (d) lived between 230 and 65 million years ago and (e) had upright legs like mammals rather than splayed-out legs like lizards. But the definition was arbitrary, and scientists knew it. So they didn't go out of their way to explain it to anybody else.
Just as well. Though the old definition still turns up in books, it's pretty much out the window. For one thing, many experts now agree modern birds descended from saurischians--in short, birds are dinosaurs. This kills (b) and (d) above. A few heretics say dinosaurs weren't reptiles either, which shoots (c). Definitionwise, you may conclude, we're back to big, ugly, and dead.
Not to worry. A new, and this time scientifically grounded, definition of dinosaurs has emerged. Having reexamined the fossils in light of a relatively new approach to classification called cladistics, paleontologists have decided the Saurischia and Ornithischia were more closely related than previously thought and together constitute the Dinosauria, a true order--that is, they and only they were descended from some yet undiscovered Big Mama Dinosaur, their common ancestor.
Unfortunately, that's not going to help you explain to Junior why some critters are dinosaurs and others aren't. In the inevitable way of science, the experts haven't worked out all the details. For example, the majority view at the moment is that pterosaurs weren't dinosaurs because they split off from the protodinosaurian lineage before the Big Mama Dinosaur appeared on the scene. However, Robert Bakker, whose brilliant but controversial work has done much to shake up orthodox paleontology, argues to the contrary--that pterosaurs descended from Big Mama and so really are dinosaurs. The fossil record is too spotty for the question to be settled now, but lots of folks are out digging, and new specimens and species are discovered all the time. The question probably won't be settled in time for you to explain it to your kids, but maybe they'll be able to explain it to theirs.
In the meantime, let 'em chew on this. Pterosaurs flew. Birds fly. Birds probably are descended from dinosaurs. Pterosaurs maybe are dinosaurs. However . . . birds didn't descend from pterosaurs. What's more, bats didn't descend from either one. In other words, the ability to fly, that most magical of nature's gifts, evolved among the vertebrates not just once, not twice, but three times. Don't know about you, but I say: huh.