Which group of flying reptiles first appeared in the Triassic?

The group of flying reptiles that first appeared in the Triassic is pterosaurs.

Pterosaurs were members of the extinct clade Pterosauria and lived from the Late Triassic, roughly 228 million years ago, until the end of the Cretaceous. They were the earliest vertebrates known to evolve powered flight. Their wings were membranes of skin, muscle, and other tissues supported mainly by an enormously elongated fourth finger.

Pterosaurs were archosaurs, placing them on the bird-line branch of reptile evolution, but they were not dinosaurs. Dinosaurs and pterosaurs shared an ancient archosaur ancestry, while birds evolved much later from theropod dinosaurs. This is why calling pterosaurs “flying dinosaurs” is a popular but technically inaccurate description.

The earliest pterosaurs were generally small, long-tailed animals, unlike later giants such as Pteranodon and Quetzalcoatlus. They also differed from bats, whose wings are modified mammalian forelimbs, and from birds, whose wings are feathered rather than stretched across a single elongated finger. Plesiosaurs were marine reptiles, not flying animals.