Were Pterosaurs Dinosaurs?

Pterosaurs and dinosaurs share a common ancestor and both belong to a wider group called archosaurs, which also includes crocodiles and birds. But the two lineages split before dinosaurs existed as a group.

The definition of a dinosaur is based on specific skeletal features, particularly in the hip and the ankle, that allow the legs to be held directly under the body. Pterosaurs do not have that arrangement. They are close cousins rather than members of the family.

The wing structure is completely different from a bird's or a bat's. A pterosaur wing was a membrane of skin and muscle stretched mainly from an enormously elongated fourth finger to the body and hind limb.

That membrane was not simple skin. Fossils show it contained stiffening fibres and blood vessels, which would have let the animal control its shape in flight. The other three fingers stayed free and clawed, and were used for climbing and walking.

Pterosaurs covered an enormous size range across their long history:

• •Some early forms were roughly the size of a sparrow
• •Pteranodon had a wingspan of six to seven metres
• •Quetzalcoatlus reached an estimated ten to eleven metres across
• •The largest stood roughly as tall as a giraffe when on the ground

Pterosaurs walked on all fours, folding the wing finger up and putting weight on the other three fingers plus the hind feet. Fossil trackways confirm this, and it explains why the largest species could move around on land at all.

Many pterosaurs were also covered in hair-like structures called pycnofibres, which likely worked as insulation. That points to an active, warm bodied animal rather than a cold sluggish reptile. Fossil eggs and embryos have been found, showing they laid eggs with soft or thin shells.

Launching a ten metre wingspan animal from the ground is a serious engineering problem, and birds solve it with powerful hind legs. Pterosaurs appear to have done something different.

The leading model is a quadrupedal launch, in which the animal vaulted off its powerful forelimbs rather than its legs, using the same muscles that power flight. Because the forelimb muscles of a flying animal are far stronger than its leg muscles, this would allow much larger sizes than a bird style launch permits, and it may be the reason pterosaurs reached dimensions no bird has ever matched.

Pterosaurs were not dinosaurs but a separate group of flying reptiles, with wings built from an elongated finger, insulating body fibres and a size range from sparrow to small aircraft, living alongside dinosaurs for well over a hundred million years.