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Pterosaur
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Pterosaurs (from the Greek pte??sa????, pterosauros, meaning "winged lizard", often referred to as pterodactyls, from the Greek pte??d??t????, pterodaktulos, meaning "winged finger" ) were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period (220 to 65.5 million years ago). Pterosaurs are the earliest vertebrates known to have evolved powered flight.
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Pterosaurs (from the Greek pte??sa????, pterosauros, meaning "winged lizard", often referred to as pterodactyls, from the Greek pte??d??t????, pterodaktulos, meaning "winged finger" ) were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period (220 to 65.5 million years ago). Pterosaurs are the earliest vertebrates known to have evolved powered flight. Their wings were formed by a membrane of skin, muscle, and other tissues stretching from the legs to a dramatically lengthened fourth finger. Early species had long, fully-toothed jaws and long tails, while later forms had a highly reduced tail, and some lacked teeth. Pterosaurs spanned a wide range of adult sizes, from the very small Nemicolopterus to the largest known flying creatures of all time, including Quetzalcoatlus and Hatzegopteryx.
Pterosaurs are sometimes referred to in the popular media as dinosaurs, but this is incorrect. The term "dinosaur" is properly restricted to a certain group of terrestrial reptiles with a unique upright stance (superorder Dinosauria), and therefore excludes the pterosaurs, as well as the various groups of extinct aquatic reptiles, such as ichthyosaurs, plesiosaurs, and mosasaurs.
History of discovery
The first pterosaur fossil was described by the Italian naturalist Cosimo Collini in 1784. Collini misinterpreted his specimen as a seagoing creature that used its long front limbs as paddles. A few scientists continued to support the aquatic interpretation even until 1830, when the German zoologist Johann Georg Wagler suggested that Pterodactylus used its wings as flippers. Georges Cuvier first suggested that pterosaurs were flying creatures in 1801, and coined the name "Ptero-dactyle" 1809 for a specimen recovered in Germany; however, due to the standardization of scientific names, the official name for this species became Pterodactylus, though the name "pterodactyl" continued to be popularly applied to all members of this first specimen's order.
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. A famous early UK find was an example of Dimorphodon by Mary Anning, at Lyme Regis in 1828. The name Pterosauria was coined by Johann Jakob Kaup in 1834, though the name Ornithosauria (or "bird lizards", Bonaparte, 1838) was sometimes used in the early literature.
Most pterosaur fossils are poorly preserved. 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 than crushing them. This created three-dimensional fossils for paleontologists to study. The first find in the Araripe Plateau was made in 1974.
Most paleontologists now believe that pterosaurs were adapted for active flight, not just gliding as was earlier believed. Pterosaur fossils have been found on every continent except Antarctica. At least 60 genera of pterosaurs have been found to date, ranging from the size of a small bird to wingspans in excess of 10 meters (33 feet).
Description
The anatomy of pterosaurs was highly modified from their reptilian ancestors for the demands of flight. Pterosaur bones were hollow and air filled, like the bones of birds. They had a keeled breastbone that was developed for the attachment of flight muscles and an enlarged brain that shows specialised features associated with flight. In some later pterosaurs, the backbone over the shoulders fused into a structure known as a notarium, which served to stiffen the torso during flight, and provide a stable support for the scapula (shoulder blade).
Wings
Pterosaur wings were formed by membranes of skin and other tissues. The primary membranes attached to the extremely long fourth finger of each arm and extended along the sides of the body to the legs.
While historically thought of s simple, leathery structures composed of skin, research has since shown that the wing membranes of pterosaurs were actually highly complex and dynamic structures suited to an active style of flight. First, the wings were strengthened by closely spaced fibers called actinofibrils. The wing membranes also contained a thin layer of muscle, fibrous tissue, and a unique, complex circulatory system of looping blood vessels.
As evidenced by hollow cavities in the wing bones of larger species and soft tissue preserved in at least one specimen, some pterosaurs extended their system of respiratory air sacs (see Paleobiology section below) into the wing membrane itself.
Parts of the pterosaur wing
The pterosaur wing membrane is divided into three basic units. The first, called the propatagium ("first membrane"), was the forward-most part of the wing and attached between the wrist and shoulder, creating the "leading edge" during flight. This membrane may have incorporated the first three fingers of the hand, as evidenced in some specimens. The brachiopatagium ("arm membrane") was the primary component of the wing, stretching from the highly elongated fourth finger of the hand to the hind limbs (though where exactly on the hind limbs it anchored is controversial and may have varied between species, see below). Finally, at least some pterosaur groups had a membrane that stretched between the legs, possibly connecting to or incorporating the tail, called the uropatagium.
A bone unique to pterosaurs, known as the pteroid, connected to the wrist and helped to support a forward membrane (the propatagium) between the wrist and shoulder. Evidence of webbing between the three free fingers of the pterosaur forelimb suggests that this forward membrane may have been more extensive than the simple pteoid to shoulde connection traditionally depicted in life restorations. The position of the pteroid bone itself has been controversial. Some scientists, notably David Unwin, have argued that the pteroid pointed forward, extending the forward membrane. However, this view was strongly refuted in a 2007 paper by Chris Bennett, who showed that the pteroid did not articulate as previously thought and could not have pointed forward, but rather inward toward the body as traditionally though.
There has been considerable argument among paleontologists about whether the main wing membranes (brachiopatagia) attached to the hind limbs, and if so, where. Fossils of the rhamphorhynchoid Sordes, the anurognathid Jeholopterus, and a pterodactyloid from the Santana Formation seem to demonstrate that the wing membrane did attach to the hindlimbs, at least in some species. 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 had different wing designs. Indeed, analysis of pterosaur limb proportions shows that there was considerable variation, possibly reflecting a variety of wing-plans. Many if not all pterosaurs also had webbed feet, and although these have been considered to be evidence of swimming, they may have had an aerodynamic function. Webbed feet are also seen in some gliding animals such as colugos, the "flying lemurs".
Skull, teeth & crests
Most pterosaur skulls had elongated, beak-like jaws. Some advanced forms were toothless (such as the pteranodonts and azhdarchids, though most sported a full compliment of needle-like teeth. In some cases, actual keratinous beak tissue has been preserved, though in toothed forms, the beak is small and restricted to the jaw tips and does not involve the teeth.
Unlike most archosaurs, which have several openings in the skull in front of the eyes, in pterosaurs the antorbital opening and the nasal opening was merged into a single large opening, called the nasoantorbial fenestra. This likely evolved as a weight-saving feature to lighten the skull for flight.
Pterosaurs are well known for their often elaborate crests. The first and perhaps best known of these is the distinctive backward-pointing crest of some Pteranodon species, though a few pterosaurs, such as the tapejarids and Nyctosaurus sported incredibly large crests that often incorporated keratinous or other soft tissue extensions of the bony crest base.
Since the 1990s, new discoveries and more thorough study of old specimens have shown that crests are far more widespread among pterosaurs than previously thought, due mainly to the fact that they were frequently extended by or composed completely of keratin, which does not fossilize as often as bone. In the cases of pterosaurs like Pterorhynchus and Pterodactylus, the true extent of these crests has only been uncovered using ultra violet photography. The discovery of Pterorynchus and Austriadactylus, both crested "rhamphorchynchoids", showed that even primitive pterosaurs had crests (previously, crests were thought to be restricted to the more advanced pterodactyloids).
Hair
Pterosaurs were unique among reptiles in that at least some of them were covered with hair, similar to but not homologous with mammalian hair. Pterosaur "hair" is not true hair as seen in mammals, but a unique structure that developed a similar appearance through convergent evolution. 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, another example of convergent evolution. The presence of hair (and the demands of flight) imply that pterosaurs were endothermic (warm-blooded).
Paleobiology
Flight
The mechanics of pterosaur flight are not completely understood or modeled at this time, but it is almost certain that this group of animals was capable of powered flight in at least as wide a range of conditions as modern birds. Pterosaurs display many extreme morphological changes required for flight - lightweight bones, stiffened torsos, and modification of the forelimbs into large, dedicated flight surfaces. It is unlikely that all the highly flight-specialized skeletal features observed in pterosaur fossils were developed and maintained for hundreds of millions of years if the animals did not fly. Skeletal specializations displayed by the pterosaurs would put them at an enormous disadvantage to terrestrial tetrapods if they were not used for the exploitation of an airborne lifestyle and ecological niches.
The study of pterosaur biomechanics and modeling of flight is a field still in development. Direct comparisons with the most successful vertebrate flyers of today, the birds, leaves gaps in our ability to reproduce their flight mechanics and models. However, pterosaurs certainly were successful flyers, based on their skeletal evidence and the distribution of their fossils in size, shape, geography, and evolutionary longevity.
Every group of animals that has developed the ability of true flight has done it different ways. Some insects (those with wing muscles attached directly to the wings) fly differently from other insects (whose wing muscles attach indirectly to the wings), which fly differently from birds, which fly differently from bats, which fly differently from pterosaurs. The flight dynamics of all the preceding groups, with the probable exception of pterosaurs, have been extensively studied and modeled and copied. And because all the flight mechanisms are different, the models are different, and while each may be valid in their specific case, they are not inter-applicable. This is clearly the case of the current state of the field in pterosaur flight.
Pterosaurs flew using their forelimbs, which are modified by hypertrophy of the fourth finger into a long spar supporting a membrane of tissue which was the flight surface. The wings were probably flapped in a manner grossly similar to that seen in birds (a group which displays many different flapping strategies among and within different species and different situations). One of the chief arguments against active pterosaur flight has been their relatively shallow sternum keel, which is the anchor point for the pectoralis muscles, the main flapping muscle. However, pterosaurs display other skeletal features that may have made this less problematic than a direct comparison to birds may indicate. The pterosaur group is notable for a unique bone, called the pteroid, in the forearm, which may have supported a flight structure not reproduced in other flying animals. Recent wind tunnel tests on model pterosaur wings with the pteroid bone in an extended antero-ventral orientation supporting a large, highly cambered propatagium show that such a configuration enables the wing to develop up to 30% more lift, even at very high angles of attack. This anatomical feature, based on the pteroid bone - the bone unique to the pterosaur clade - may have enabled pterosaurs to be active, powered flyers in spite of the lack of other features associated with strong fliers. While the orientation of the pteroid is disputed, it should be noted that it, or some other combination of features must have efficiently enabled flight for the group, supporting even the evolution of giant forms, like the famous Quetzalcoatlus, to a size unmatched by modern birds.
Katsufumi Sato, a Japanese scientist, did calculations using current birds and decided that it is impossible for a pterosaur to stay aloft. In the book Posture, Locomotion, and Paleoecology of Pterosaurs it is theorized that they were able to fly due to the oxygen-rich, dense atmosphere of the Late Cretaceous period. However, one must note both Katsufumi and the authors of Posture, Locomotion, and Paleoecology of Pterosaurs based their research on the now outdated theories of pterosaurs being seabird-like, and the size limit doesn't apply to terrestrial pterosaurs like azhdarchids and tapejarids Furtheremore, Darren Naish concluded that atmospheric differences between the present and the Mesozoic weren't needed for the giant size of pterosaurs:
Air sacs and respiration
A 2009 study showed that pterosaurs had a lung-air sac system and a precisely controlled skeletal breathing pump, which supports a flow-through pulmonary ventilation model in pterosaurs, analogous to that of birds.
Nervous system
A study of pterosaur brain cavities using X-rays revealed that the animals (Rhamphorhynchus muensteri and Anhanguera santanae) had massive flocculi. The flocculus is a brain region that integrates signals from joints, muscles, skin and balance organs.
The pterosaurs' flocculi occupied 7.5% of the animals' total brain mass, more than in any other vertebrate. Birds have unusually large flocculi compared with other animals, but these only occupy between 1 and 2% of total brain mass.
The flocculus sends out neural signals that produce small, automatic movements in the eye muscles. These keep the image on an animal's retina steady. Pterosaurs may have had such a large flocculus because of their large wing size, which would mean that there was a great deal more sensory information to process.
Ground movement
Pterosaur's hip sockets are oriented facing slightly upwards, and the head of the femur (thigh bone) is only moderately inward facing, suggesting that pterosaurs had a semi-erect stance. It would have been possible to lift the thigh into a horizontal position during flight as gliding lizards do.
There was considerable debate whether pterosaurs ambulated as quadrupeds or as bipeds. In the 1980s, paleontologist Kevin Padian suggested that smaller pterosaurs with longer hindlimbs such as Dimorphodon might have walked or even run bipedally, in addition to flying, like road runners. However, a large number of pterosaur trackways were later found with a distinctive four-toed hind foot and three-toed front foot; these are the unmistakable prints of pterosaurs walking on all fours.
Unlike most vertebrates, which walk on their toes with ankles held off the ground (digitigrade), fossil footprints show that pterosaurs stood with the entire foot in contact with the ground (plantigrade), in a manner similar to humans and bears. Footprints from azhdarchids show that at least some pterosaurs walked with an erect, rather than sprawling, posture.
Though traditionally depicted as ungainly and awkward when on the ground, the anatomy of at least some pterosaurs (particularly pterodactyloids) suggests that they were competent walkers and runners. The forelimb bones of azhdarchids and ornithocheirids were unusually long compared to other pterosaurs, and in azhdarchids, the bones of the arm and hand (metacarpals) were particularly elongated, and azhdarchid front limbs as a whole were proportioned similarly to fast-running ungulate mammals. Their hind limbs, on the other hand, were not built for speed, but they were long compared with most pterosaurs, and allowed for a long stride length. While azhdarchid pterosaurs probably could not run, they would have been relatively fast and energy efficient.
The relative size of the hands and feet in pterosaurs (by comparison with modern animals such as birds) may indicate what type of lifestyle pterosaurs led on the ground. Azhdarchid pterosaurs had relatively small feet compared to their body size and leg length, with foot length only about 25%-30% the length of the lower leg. This suggests that azhdarchids were better adapted to walking on dry, relatively solid ground. Pteranodon had slightly larger feet (47% the length of the tibia), while filter-feeding pterosaurs like the ctenochasmatoids had very large feet (69% of tibial length in Pterodactylus, 84% in Pterodaustro), adapted to walking in soft muddy soil, similar to modern wading birds.
Predation
Pterosaurs are known to have been eaten by spinosaurids. 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 exposed to digestion, as the joints still articulated.
Reproduction
Very little is known about pterosaur reproduction. A single pterosaur egg has been found in the quarries of Liaoning, the same place that yielded the famous 'feathered' dinosaurs. The egg was squashed flat with no signs of cracking, so evidently the eggs had leathery shells, as in modern lizards. The embryo's wing membranes were well developed, suggesting pterosaurs were ready to fly soon after birth. This is corroborated by very young animals found in the Solnhofen limestone beds. It is not known whether pterosaurs practiced parental care, but their comparatively early flight capabilities suggest the young only dependent on their parents for a short period of times while the wings grew long enough to fly. It's possible they even used stored yolk products for nourishment during this time, as in modern reptiles, rather than depend on parents for food.
A study of pterosaur eggshell structure and chemistry published in 2007 indicated that it is likely pterosaurs buried their eggs, like modern crocodile and turtles. Egg-burying would have been beneficial to the early evolution of pterosaurs, as it allows for more weight-reducing adaptations, but this method of reproduction also would have put limits on the variety of environments pterosaurs could live in, and may have disadvantaged them when they began to face ecological competition from birds. The alternative would be for the mother to retain the egg within the body until just prior to hatching, as some lizards do, but which archosaurs are incapable of doing.
Evolution and extinction
Origins
Because pterosaur anatomy has been so heavily modified for flight, and immediate "missing link" predecessors have not so far been described, the ancestry of pterosaurs is not well understood. Several hypotheses have been advanced, with the most common in recent years being links to ornithodirans like Scleromochlus, an ancestry among the archosauriforms like Euparkeria (a more traditional view), or related to prolacertiformes like Sharovipteryx.
They were thought to have evolved flight from some manner other than the 'tree-down' route possibly taken by birds, because pterosaurs demonstrated no adaptations useful for tree living.
Most scenarios have pterosaurs evolving from long-legged, ground-running ancestors like Scleromochlus or Sharovipteryx, both of which had webs of skin from long hind legs to their bodies or tails. This suggested a 'ground-up' evolution of flight or even a route that evolved by gliding from cliff-tops.
However, new (2008) findings suggest that the earliest pterosaurs were small, tree dwelling, insectivorous organisms.
Phylogeny and classification
Classification of pterosaurs has historically been difficult, because there were many gaps in the fossil record. Many new discoveries are now filling in these gaps and giving us a better picture of the evolution of pterosaurs. Traditionally, they are organized into two suborders:
- Rhamphorhynchoidea (Plieninger, 1901): A group of early, basal ("primitive") pterosaurs, many of which had long tails and short metacarpal bones in the wing. They were small, and their fingers were still adapted to climbing . They appeared in the Late Triassic period, and lasted until the late Jurassic. Rhamphorhynchoidea is a paraphyletic group (since the pterodactyloids evolved directly from them and not from a common ancestor), so with the increasing use of cladistics it has fallen out of favor in most technical literature.
Listing of families and superfamilies within Pterosauria, after Unwin 2006.
* ORDER PTEROSAURIA (extinct)
The precise relationships between pterosaurs is still unsettled. However, several newer studies are beginning to make things clearer. Cladogram simplified after Unwin.
Extinction
It is often thought that competition with early bird species may have resulted in the extinction of many of the pterosaurs. By the end of the Cretaceous, only large species of pterosaurs are known. The smaller species seem to have become extinct, their niche filled by birds,; however, pterosaur decline (if actually present) seems unrelated to bird diversity . At the end of the Cretaceous period, the great extinction which wiped out all non-avian dinosaurs plus most avian dinosaurs as well, and many other animals, seemed to also take the pterosaurs. Alternatively, most pterosaurs may have been specialised for an ocean-going lifestyle. Consequently, when the K-T mass-extinction severely affected marine life that most pterosaurs fed on, they went extinct. However, forms like azhdarchids and istiodactylids weren't marine in habits.
Well-known genera
Examples of pterosaur genera include:
- Dsungaripterus had a wingspan of 3 metres (10 ft), 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 ft), and lived during the late Cretaceous period.
- Pterodactylus had a wingspan of 50 to 75 centimeters (20 to 30 inches), and lived during the late Jurassic on lake shores.
- Pterodaustro was a Cretaceous pterosaur from South America with a wingspan around 1.33 metres and with over 500 tall, narrow teeth, which were presumably used in filter-feeding, much like modern flamingos. Also like flamingos, this pterosaur's diet may have resulted in the animal having a pink hue. It was South America's first pterosaur find.
- Quetzalcoatlus had a wingspan of 10-11 metres (33-36 feet), and was among the largest flying animals ever. It lived during the late Cretaceous period.
- Rhamphorhynchus was a Jurassic pterosaur with a vane at the end of its tail, which may have acted to stabilise the tail in flight.
Pterosaurs in popular culture
Pterosaurs are a staple of popular culture. While the generic term "pterodactyl" is often used to describe these creatures, the animal depicted is frequently a Pteranodon or some other specific species of pterosaur, or a fictionalized hybrid of several species. Many children's toys and cartoons feature "pterodactyls" with Pteranodon-like crests and long, Rhamphorhynchus-like tails and teeth, a combination that never existed in nature. However, at least one type of pterosaur did have at least the Pteranodon-like crest and teeth--for example, the Ludodactylus, a name that means "toy finger" for its resemblance to old, inaccurate children's toys. Notable examples of older fictional works featuring pterosaurs include Arthur Conan Doyle's book The Lost World and the 1933 film King Kong. Rodan from the Godzilla series is a mutant pterosaur.
Living pterosaur hoax
It was reported in an article in The Illustrated London News (February 9, 1856, page 166) that, in 1856, workmen laboring in a tunnel for a railway line, between Saint-Dizier and Nancy, in France, were cutting through Jurassic limestone when a large creature stumbled out from inside it. It fluttered its wings, made a croaking noise and dropped dead. According to the workers, the creature had a wingspan, four legs joined by a membrane, black leathery skin, talons for feet and a toothed mouth. A local student of paleontology identified the animal as a pterodactyl. The report had the animal turn to dust, as soon as it had died.
This incredible hoax was stimulated in part by contemporary Franco-Prussian palaeontological rivalry. The Solnhofen limestone from Bavaria (in which Archaeopteryx would later be discovered) was producing many prized fossils, each of which was proudly announced by German paleontologists. The tunnel in question was through limestone of similar age to the Solnhofen Limestone, so it presented an opportunity for a shocking story.
External links
- , by Raymond Thaddeus C. Ancog.
- , by Paul Pursglove.
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- , by Alexander W. A. Kellner. (technical)
See also
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