Definition for Pterosaurs

Pterion

Pterygium

Pterosaurs ("winged lizards") were flying reptiles of the clade Pterosauria. They existed from the late Triassic to the Cretaceous periods (from 228 to 65 million years ago). The earlier species had long, fully-toothed jaws and long tails, while later forms had a stump for a tail, no teeth and a jaw more like a beak than the elongated jaw of the earlier species. Pterosaurs are classified within an larger group archosaurs that also includes crocodiles, dinosaurs (including birds). Pterosaurs were first discovered in 1784 by the Italian naturalist Cosimo Collini. He initially believed that pterosaurs were aquatic animals, not flyers. In the 19th century Baron Georges Cuvier proposed that pterosaurs flew.

At least 60 genera of pterosaurs have been found, ranging from the size of a small bird to wingspans in excess of 12m (40 feet). Since the first pterosaur fossil was discovered in the late Jurassic Solnhofen limestone in 1784, twenty-nine kinds of pterosaurs have been found in those deposits alone. Most paleontologists now believe that pterosaurs were adapted for active flight, not just gliding as was earlier believed.

Most pterosaur fossils did not preserve well. Their bones were hollow, and when sediments piled on top of them, the bones were flattened. The best preserved fossils have come from the Araripe Plateau, Brazil. For some reason, when the bones were deposited, the sediments encapsulated the bones, rather then crushing them. This created three dimensional fossils for paleontologists to study. The first find in the Araripe Plateau was discovered in 1974.

Pterosaur wings were thin membranes of skin, strengthened by closely spaced fibers, attached to the extremely long fourth finger of each arm and extending along the sides of the body. There has been considerable argument among paleontologists about whether the wings attached to the hindlimbs as well. Some paleontologists believe material of Sordes, a small rhamphorhynchoid pterosaur indicate that in these animals the webs did in fact connect to the hindlimbs, however it seems that this might not have been the case as the wings are poorly preserved for the most part and are obscured by hairs, thus giving the effect that the wings were broad when they were very likely quite narrow. Modern birds, bats and flying squirrels show considerable variation in the extent of their wing membranes and it is possible that, like these groups, different species may have had different wing designs. Pterosaurs also had webbed feet, although these have been considered to be evidence of swimming, webbed feet are also seen in some gliding animals such as colugos.

There is no fossil evidence of feathers, but pterosaurs were unique among reptiles in that at least some of them were covered with hair, similar but not homologous to mammalian hair. Although in some cases fibers in the wing membrane have been mistaken for hair, some fossils such as those of Sordes pilosus ( the "hairy demon") do show the unmistakable imprints of hair on the head and body, not unlike modern-day bats. The presence of fur (and the demands of flight) imply that pterosaurs were warm-blooded ('endothermic').

There has been considerable debate in the past about whether pterosaurs moved about on the ground as quadrupeds or as bipeds. A large number of pterosaur trackways are now known, with a distinctive four-toed foot and three-toed finger; these are the unmistakable prints of pterosaurs walking on all fours. However, it might be too much to conclude that all pterosaurs were quadrupedal, all the time.

Their bones were hollow and had openings at each end. Unlike typical reptiles, pterosaurs had a keeled breastbone that was developed for the attachment of flight muscles and a brain that was more developed than comparable dinosaurs of similar sizes.

A jumble of pterosaur bones found in the Atacama desert in Chile yielded a proportionally large number of juvenile individuals. This would indicate that, like some modern-day shorebirds, pterosaurs roosted in rookeries, and that the young passed through a nesting stage while their parents cared for them until they were ready to fly on their own.

Pterosaurs are known to have been attacked by spinosaurs: in the 1 July 2004 edition of Nature, paleontologist Eric Buffetaut discusses an Early Cretaceous fossil of three cervical vertebra of a pterosaur with a (broken) tooth of a spinosaur embedded in it. The vertebrae are known not to have been eaten and digested, as the joints still articulate.

The ancestry of pterosaurs is not well understood. Although some argue that pterosaurs are related to dinosaurs, they are so highly modified for flight that it seems they could have evolved from almost any small, generalized Triassic reptile.

It is believed that competition with early bird species may have resulted in the extinction of the pterosaurs. By the end of the Cretaceous, only large species of pterosaurs survived. The smaller versions were extinct, and replaced by birds. After the impact of the famous Chicxulub bolide that ended the Cretaceous period, the larger animals all died, including the pterosaurs. Birds, being smaller creatures, survived.

Examples of pterosaurs include:

- Pterodactylus, with a wingspan of about 50-75 cm (20-30 inches), that lived during the late Jurassic on lake shores.

- Dsungaripterus, with a wingspan of 3 metres (10 feet) wingspan, an unusual bony crest running along its snout, and long, narrow, curved jaws with a pointed tip, that lived during the early Cretaceous period.

- Pteranodon, 1.8 metres (six feet) long with a wingspan of 7.5 m (25 feet), that lived during the late Cretaceous period.

- Quetzalcoatlus, with a wingspan of 12 metres (40 feet) and a weight of only about 50 kg (110 pounds), that lived during the late Cretaceous period.

- Rhamphorhynchus, a Jurassic pterosaur that had a diamond-shaped tail, which may have acted as a rudder in flight.