Pterosaurs are actually reptiles and were the first vertebrates to evolve flight capabilities. Pterosaurs varied greatly in size, ranging from sparrow-sized to having a wingspan of 40 feet.
These flying reptiles were originally thought to have been related to bats but are now thought to be more closely related to crocodiles. Pterosaurs were good flyers, having evolved a wing from the greatly elongated fourth finger of their hand.
Hall of Vertebrate Origins