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 the first animals with backbones to achieve powered flight, appearing in the Late Triassic. Birds came later, evolving from small feathered theropod dinosaurs, and bats arrived much later still from mammal ancestors. Each group solved the same problem with different parts. Pterosaurs stretched a membrane from one enormous finger. Birds carry feathers on arm and fused hand bones. Bats spread a membrane across several long fingers, so their wing works more like a webbed hand.

This is a textbook case of convergent evolution, where unrelated lineages arrive at similar solutions because they face similar physical demands. It also clears up a common mistake: birds did not descend from pterosaurs. Birds are living dinosaurs, while pterosaurs left no descendants when they died out at the end of the Cretaceous. Even the popular word pterodactyl causes confusion, because Pterodactylus is one specific, fairly small genus found in Germany, not a label for every flying reptile.

The first pterosaur known to science came from the fine-grained limestone of Bavaria, the same rock that later yielded Archaeopteryx. When the Italian naturalist Cosimo Collini described the specimen in the 1780s, he could not place it among any living animal and suggested it might have lived in the sea, using its long arms as paddles. It was the French anatomist Georges Cuvier who recognised it as a flying reptile and gave it the name that became Pterodactylus, meaning wing finger.

That early confusion shows why pterosaurs are hard to study. Their bones were thin-walled and filled with air spaces, which saved weight in flight but made them fragile, so most skeletons were crushed or scattered long before they could fossilise. Much of what is known comes from a few exceptional sites where fine mud or limestone buried animals quickly, such as the Bavarian limestones, the Santana Formation of Brazil and lake deposits in north-east China. Those rare places preserve wing membranes, head crests and even soft tissue, so a single good site can reshape the whole picture.

Pterosaurs were not all fish-snatching gliders, and their jaws show a wide range of diets. Pterodaustro, from South America, had a lower jaw lined with hundreds of long, thin, bristle-like teeth, which most researchers think it used to strain small creatures from water, rather like a flamingo. Other pterosaurs had stout teeth suited to crushing shells, or long jaws with interlocking fangs for seizing slippery fish. The giant azhdarchids, the group that includes Quetzalcoatlus, had long stiff necks and toothless beaks, and a widely discussed idea is that they stalked small animals on the ground, much as storks do today.

Many pterosaurs also carried head crests, some made of bone and others extended by soft tissue that only exceptional fossils preserve. In some Brazilian species the crest towered above the skull like a sail. Because the crests seem too awkward for steering or cooling to be their main job, many researchers think they were for display. A fossil of Darwinopterus from China, described in 2011, strengthened this idea. It was preserved with an egg beside its body, marking it as female, and it had no crest, while other specimens of the species do. That suggests crests may have been mainly male signals, rather like a peacock's tail.

Pterosaurs are not the only famous prehistoric reptiles wrongly called dinosaurs. The great marine reptiles of the same era belong on separate branches too. Ichthyosaurs had streamlined, dolphin-like bodies, and fossils of mothers preserved with unborn young inside them show that they gave birth to live young at sea rather than laying eggs on land. Plesiosaurs swam with four large flippers, and many had very long necks and small heads. Mosasaurs, giant marine predators of the Late Cretaceous, were not dinosaurs at all but lizards, belonging to the same group as today's lizards and snakes.

Some of the first of these animals were found by Mary Anning at Lyme Regis on the Dorset coast. Her family's discovery of an ichthyosaur skeleton in the early 1810s and her find of a nearly complete plesiosaur in 1823 came before the word dinosaur had even been coined. The simplest test is the one used for pterosaurs: dinosaurs held their legs straight beneath the body, while these reptiles had limbs turned into paddles. Another frequent mix-up is Dimetrodon, the sail-backed animal included in many toy dinosaur sets. It lived tens of millions of years before the first dinosaurs and is more closely related to mammals.

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.