Pterosaurs ("winged lizards") hold a special place in the history of life on earth: they were the first creatures, other than insects, to successfully populate the skies. The evolution of pterosaurs roughly paralleled that of their terrestrial cousins, the dinosaurs, as the small, "basal" species of the late Triassic period gradually gave way to bigger, more advanced forms in the Jurassic and Cretaceous. (See a gallery of pterosaur pictures and a complete, A to Z list of pterosaurs).)
Before we proceed, though, it's important to address one important misconception. Paleontologists have found indisputable proof that modern birds are descended not from pterosaurs, but from small, feathered, land-bound dinosaurs (in fact, if you could somehow compare the DNA of a pigeon, a Tyrannosaurus Rex and a Pteranodon, the first two would be more closely related to each other than either would be to the third). This is an example of what biologists call convergent evolution: nature has a way of finding the same solutions (wings, hollow bones, etc.) to the same problem (how to fly).
The First Pterosaurs
As is the case with dinosaurs, paleontologists don't yet have enough evidence to identify the single ancient, non-dinosaur reptile from which all pterosaurs evolved (the lack of a "missing link"--say, a terrestrial archosaur with half-developed flaps of skin--may be heartening to creationists, but you have to remember that fossilization is a matter of chance. Most prehistoric species aren't represented in the fossil record, simply because they died in conditions that didn't allow for their preservation.)
The first pterosaurs for which we have fossil evidence flourished during the middle to late Triassic period, about 230 to 200 million years ago. These flying reptiles were characterized by their small size and long tails, as well as obscure anatomical features (like the bone structures in their wings) that distinguished them from the more advanced pterosaurs that followed. These "rhamphorhynchoid" pterosaurs, as they're called, include Eudimorphodon (one of the earliest pterosaurs known), Dorygnathus and Rhamphorhynchus, and they persisted into the early to middle Jurassic period.
One problem with identifying the rhamphorhynchoid pterosaurs of the late Triassic and early Jurassic periods is that most specimens have been unearthed in modern-day England and Germany. This isn't because early pterosaurs liked to summer in western Europe; rather, as explained above, we can only find fossils in those areas that lent themselves to fossil formation. There may well have been vast populations of Asian or North American pterosaurs, which may (or may not) have been anatomically distinct from the ones with which we're familiar.
By the late Jurassic period, rhamphorhynchoid pterosaurs had been pretty much replaced by pterodactyloid pterosaurs--larger-winged, shorter-tailed flying reptiles exemplified by the well-known Pterodactylus and Pteranodon. (The earliest identified member of this group, Kryptodrakon, lived about 163 million years ago.) With their larger, more maneuverable wings of skin, these pterosaurs were able to glide farther, faster, and higher up in the sky, swooping down like eagles to pluck fish off the surface of oceans, lakes and rivers.
During the Cretaceous period, pterodactyloids took after dinosaurs in one important respect: an increasing trend toward gigantism. In the middle Cretaceous, the skies of South America were ruled by huge, colorful pterosaurs like Tapejara and Tupuxuara, which had wingspans of 16 or 17 feet; still, these big fliers looked like sparrows next to the true giants of the late Cretaceous, Quetzalcoatlus and Zhejiangopterus, the wingspans of which exceeded 30 feet (far larger than the largest eagles alive today).
Here's where we come to another all-important "but." The enormous size of these "azhdarchids" (as giant pterosaurs are known) has led some paleontologists to speculate that they never actually flew. For example, a recent analysis of the giraffe-sized Quetzalcoatlus shows that it had some anatomical features (such as small feet and a stiff neck) ideal for stalking small dinosaurs on land. Since evolution tends to repeat the same patterns, this would answer the embarrassing question of why modern birds have never evolved to azhdarchid-like sizes.
In any event, by the end of the Cretaceous period, the pterosaurs--both large and small--went extinct along with their cousins, the terrestrial dinosaurs and marine reptiles. It's possible that the ascendancy of true feathered birds spelled doom for slower, less versatile pterosaurs, or that in the aftermath of the K/T Extinction the prehistoric fish that these flying reptiles fed on were drastically reduced in number.
Next Page: Pterosaur Behavior and Physiology, and a List of Pterosaur Genera