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Pterosaur

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Pterosaur
Conservation status: Fossil
Missing image
Pteronodon2.jpg



A Pteronodon from a 1914 study — image

with permission from The Pterosaur
Database, October 2004

Scientific classification
Kingdom:Animalia
Phylum:Chordata
(unranked)Reptilia
Class:Archosauria
Order:Pterosauria
Suborders

Rhamphorhynchoidea
Pterodactyloidea

Pterosaurs (TEH-row-sore, "winged lizards") were flying reptiles of the clade Pterosauria. They existed from the late Triassic to the end of the Cretaceous period (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 as archosaurs, along with crocodiles and dinosaurs (and their descendants, the birds).

Contents

Fossil evidence

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 12 meters (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.

Missing image
Pterosaur_tooth.jpg
Fossil Pterosaur tooth from the Cretaceous of Morocco.

Flight

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. A bone called the pteroid connects to the wrist and helped to support a membrane (the propatagium) between the wrist and shoulder. The pteroid might have been able to swing forward to extend this membrane.

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; material of a Brazilian pterosaur also shows the wing extending to the ankle. However, modern bats and flying squirrels show considerable variation in the extent of their wing membranes and it is possible that, like these groups, different species of pterosaur may have had different wing designs. Many if not all pterosaurs also had webbed feet, and although these have been considered to be evidence of swimming, webbed feet are also seen in some gliding animals such as colugos (the "flying lemurs").

Pterosaur bones were hollow and air filled, like the bones of birds. 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.

Hair

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').

Ground movement

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 hind foot and three-toed front foot; 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.

It has been suggested that smaller pterosaurs with longer hindlimbs such as Dimorphodon might have walked or even run bipedally, in addition to flying, not unlike modern road runners. Other small pterosaurs such as Rhamphorynchus may have scurried around on all fours. The larger, bulkier pterosaurs with porportionately smaller hindlimbs and massive forebodies are (almost?) universally thought to have moved about on all fours while on the ground, like modern bats.

Behaviour

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. Excavations in Argentina on a Pterodaustro bone-bed uncovered two eggs, containing embryonic bones. The eggs evidently had a leathery shell, like that of modern reptiles.

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 vertebrae of a pterosaur with the broken tooth of a spinosaur embedded in it. The vertebrae are known not to have been eaten and digested, as the joints still articulated.

Evolution and extinction

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.

Classification

Classification of pterosaurs is difficult, because there are many gaps in the fossil record. Traditionally, they are organized into two suborders:

  • Rhamphorhynchoidea (Plieninger, 1901): The early, or basal pterosaurs. They flew by flapping their wings, and had long tails and short wing metacarpals. They were small, and their fingers were still adapted to climbing. They appeared in the late Triassic period, and lasted until the late Jurassic.
  • Pterodactyloidea (Plieninger, 1901): The more advanced pterosaurs. They flew by soaring, with short tails and long wing metacarpals. They appeared in the middle Jurassic period, and lasted until the Cretaceous-Tertiary extinction event wiped them out at the end of the Cretaceous.

Rhamphorhynchoidea paraphyletic group, so with the increasing use of cladistics it has fallen out of favor.

Well-known genera

Examples of pterosaur genera include:

  • Dsungaripterus had a wingspan of 3 metres (10 feet), an unusual bony crest running along its snout, and long, narrow, curved jaws with a pointed tip. It lived during the early Cretaceous period.
  • Pteranodon was 1.8 metres (six feet) long, with a wingspan of 7.5 m (25 feet), and lived during the late Cretaceous period.
  • Pterodaustro was a Cretaceous pterosaur from South America with over 500 tall, narrow teeth, which presumably were used in filter-feeding, much like modern flamingos. Like flamingos, this pterosaur's diet may have resulted in a pink hue.
  • Rhamphorhynchus was a Jurassic pterosaur with a diamond-shaped tail, which may have acted as a rudder in flight.

Further reading

  • Wellnhofer P (1991): Illustrated Encyclopedia of Pterosaurs, Crescent Books
  • Bramwell, C. and G. R. Whitfield (1974). Biomechanics of Pteranodon. Philosophical Transactions of the Royal Society B 267: 503-81.

External links

da:Flyvegler de:Flugsaurier ja:翼竜 nl:Pterosauria pt:Pteurosauria nl:Pterosaurus

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