Pterosaurs were the first true flying vertebrates, appearing in the Late Triassic. They were not feathered birds, not bats, and not dinosaurs. They were a distinct branch of flying reptiles with wings formed mainly by a greatly elongated fourth finger supporting a wing membrane. The earliest true pterosaurs already appear as capable fliers, which means their branch had likely been developing for some time before the first clear fossils we have found.

Over time, pterosaurs diversified into a wide range of forms. Some stayed relatively small and lightly built, while later species became enormous, with wingspans far beyond any bird alive today. This branch explored many lifestyles, from coastal fish-hunters to inland soarers, and remained successful for a very long time before disappearing at the end of the Cretaceous.

Even though pterosaurs lived alongside dinosaurs and shared a deeper ornithodiran ancestry with them, they were not dinosaurs. They were a sister branch, not part of Dinosauria itself. That distinction matters because people often lump all large prehistoric reptiles into “dinosaurs,” but evolution was more branching and more interesting than that. Pterosaurs were close cousins to dinosaurs, not flying dinosaurs.