Pterosaurs were the first vertebrates known to achieve powered flight. Their wings were supported by an enormously elongated fourth finger, and their anatomy was specialised in ways unlike that of birds or bats. Yet the earliest known pterosaur fossils already show an airborne body plan. The transition from ground-dwelling ancestors to powered flight is therefore missing from the fossil record itself.
The oldest pterosaur fossils are already specialised
Known pterosaurs appear in the fossil record by the Late Triassic, roughly 220 million years ago, although the precise timing is still being refined. Their wings, shoulder girdles and lightweight skeletons show that flight was established by then. The oldest fossils are fragmentary, and many early specimens come from deposits where bones are flattened or incomplete. They reveal what pterosaurs had become more clearly than the exact sequence of earlier changes.
That gap does not mean scientists have no clues. They compare pterosaur anatomy with that of other archosaurs, examine the order in which traits evolved on family trees, and use CT scans to inspect delicate structures hidden inside fossils. Such comparisons can identify close relatives and the shared anatomy that links them.
Lagerpetids illuminate the branch near pterosaurs
Lagerpetids were small, mostly ground-dwelling Triassic archosaurs once known chiefly from hindlimb bones. Better fossils, including skulls and associated skeletons, revealed features shared with pterosaurs across the skull, braincase, limbs and other parts of the body. A 2020 study used these remains and micro-CT scans to identify lagerpetids as the closest known sister group to pterosaurs.
A sister group is a close evolutionary branch, not a direct parent. Lagerpetids were not shown to be the literal ancestors of pterosaurs, and they were not themselves early pterosaurs without wings. Their importance is that they preserve anatomy near the divergence, giving researchers a richer comparison for reconstructing the pterosaur lineage.
Flight probably assembled from several changes
Powered flight requires more than a membrane. It depends on the shoulder and chest, muscle attachment, balance, sensory systems, a wing capable of producing lift, and behaviour that brings the animal into the air. The pterosaur hand and forelimb were transformed into a wing support, while the hindlimbs, pelvis and trunk also show distinctive adaptations. The fossil sequence does not yet tell us which change came first or what the earliest gliding or launching behaviour looked like.
Proposed scenarios include climbing, leaping and launching from the ground, but there is no preserved recording of the transition. Biomechanical models can test whether reconstructed bodies could generate the forces needed for take-off. They rely on estimates of muscle, tissue, wing shape and body mass, so results depend on assumptions. A plausible model is not direct evidence of an observed behaviour.
What remains uncertain
The discovery of lagerpetid relatives has narrowed the evolutionary neighbourhood of pterosaurs, not closed every gap. More complete fossils near the origin could reveal intermediate combinations of traits and better constrain timing. Until then, the careful conclusion is that pterosaur flight arose within a lineage of archosaurs whose close relatives were terrestrial, and that several major anatomical changes must have accumulated before the oldest known pterosaurs.
The story is a reminder that extraordinary adaptations rarely appear in a single leap. Fossils preserve parts of the branching history, while the exact sequence from running to flight remains a research question. See the broader pterosaur catalogue for the diversity that followed.
Frequently asked questions
What were the first vertebrates to fly under their own power?
Pterosaurs were the first known vertebrates to evolve powered flight. Birds and bats evolved powered flight independently later.
Were lagerpetids direct ancestors of pterosaurs?
They are identified as the closest known sister group in an influential 2020 analysis. A sister group is a close relative, not a proven direct ancestor.
When did pterosaurs first appear?
They are known from the Late Triassic, around 220 million years ago, but fragmentary fossils and changing dates leave the earliest timing uncertain.
Do fossils show exactly how pterosaur flight began?
No. Fossils reveal anatomy and evolutionary relationships, while the behavioural steps and order in which flight adaptations arose remain unresolved.