Imagine an animal the size of a small airplane flying above your head. This is what you might have seen 70m years ago when giant pterosaurs roamed the skies. These flying reptiles that lived above the dinosaurs' heads during the Mesozoic era were the largest animals ever to fly. The biggest pterosaurs, giant azhdarchids, had wingspans of between 10 and 11 metres.
But how did these enormous creatures get into the air? The bigger an animal, the harder it becomes for it to fly since more lift is required to counteract its weight so it can take-off. If an animal is too heavy, it won’t be able to produce enough lift to get off the ground. We’d expect an animal the size of a large pterosaur to be too big to fly.
Unlike birds with feathered wings, pterosaurs had a membrane that stretched from the end of an elongated fourth finger to their legs (more similar to a bat than a bird). This membrane had internal structures called actinofibrils to strengthen the wings, and provide structural support. They also had a unique bone in their wrist called the pteroid, which controlled the leading edge of the wing.
Pterosaurs also had a highly specialised respiratory system, similar to that of birds, with air sacs in addition to their lungs. This is a much more effective breathing system, which is important for providing the large amounts of energy needed for flight. Pterosaurs had air sacs in their necks and trunk, and larger creatures also had them in their wings. In many cases, the air sacs invade the bones and hollow them out, making their wing bones extremely thin-walled. This is referred to as skeletal pneumaticity and is another important element contributing to large pterosaurs' ability to fly.
Unfortunately for us, thin-walled hollow bones are easily crushed and don’t preserve as well as thick-walled marrow-filled bones. So well-preserved pterosaur bones are relatively uncommon. But the small number of 3D pterosaur fossils we do have help us to understand just how hollow these bones were.
Bayerische Staatssammlung für Paläontologie und Geologie, Munich/Elizabeth Martin-Silverston, Author provided
Thin-walled hollow bones are important in flight for two reasons. First, they are lighter than bones filled with marrow. Additionally, the bones are more resistant to bending than bones with a smaller diameter and thicker walls and so are better able to deal with the heavy loads created by the pterosaurs' large size.