First and foremost, pterosaurs are ruling reptiles (Archosauria). The skulls of ruling reptiles have an opening between the eye socket and the nostril (antorbital fenestra), and an opening in the back of the lower jaw (mandibular fenestra). Included in the ruling reptiles are dinosaurs, the Crocodilia (crocodiles, alligators, caimans and garials) and flying reptiles. All dinosaurs are ruling reptiles, but not all ruling reptiles are dinosaurs.
But are pterosaurs dinosaurs? There is a Middle Triassic archosaur from Argentina called Marasuchus (formerly Lagosuchus — it is still captioned as such in the MACN). Its skull and rear limbs have features similar to those found in both dinosaurs and pterosaurs. Since it lived before the rise of these two groups, Marasuchus may belong to the line that eventually split into dinosaurs and pterosaurs. Some scientists argue that Marasuchus is a dinosaur, and therefore the pterosaurs should be considered dinosaurs too. Not all scientists agree with this. For a more detailed discussion of dinosaur ancestry and relationships, see the various dinosaur encyclopedias in the Bibliography.
