Pterosaurs (, from the Greek πτερόσαυρος, ''pterosauros,'' meaning "winged lizard") 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. Many sported furry coats made up of hair-like filaments known as pycnofibres, which covered their bodies and parts of their wings. 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 reptiles with a unique upright stance (superorder Dinosauria, which includes birds), and therefore excludes the pterosaurs, as well as the various groups of extinct marine reptiles, such as ichthyosaurs, plesiosaurs, and mosasaurs.

Pterosaurs are also incorrectly referred to as "pterodactyls", particularly by journalists. This usage is discouraged. "Pterodactyl" refers specifically to members of the genus ''Pterodactylus'', and more broadly to members of the suborder Pterodactyloidea.

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 as 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 outer wings (from the tip to the elbow) were strengthened by closely spaced fibers called actinofibrils. The actinofibrils themselves consisted of three distinct layers in the wing, forming a crisscross pattern when superimposed on one another. The actual function of the actinofibrils is unknown, as is the exact material from which they were made. Depending on their exact composition (keratin, muscle, elastic structures, etc.), they may have been stiffening or strengthening agents in the outer part of the wing. 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. 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 thought.

Three lines of evidence, morphological, developmental and histological, indicate that the pteroid is a true bone, rather than ossified cartilage. The origin of the pteroid is unclear: it may be a modified carpal, the first metacarpal, or a neomorph (new bone).

The pterosaur wrist consists of two inner (proximal) and four outer (distal) carpals (wrist bones), excluding the pteroid bone, which may itself be a modified distal carpal. The proximal carpals are fused together into a "syncarpal" in mature specimens, while three of the distal carpals fuse to form a distal syncarpal. The remaining distal carpal, referred to here as the medial carpal, but which has also been termed the distal lateral, or pre-axial carpal, articulates on a vertically elongate biconvex facet on the anterior surface of the distal syncarpal. The medial carpal bears a deep concave fovea that opens anteriorly, ventrally and somewhat medially, within which the pteroid articulates.

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.

Unlike most archosaurs, which have several openings in the skull in front of the eyes, in pterodactyloid 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. The discovery of ''Pterorynchus'' and ''Austriadactylus'', both crested "rhamphorhynchoids", showed that even primitive pterosaurs had crests (previously, crests were thought to be restricted to the more advanced pterodactyloids).

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 the specimen recovered in Germany. However, due to the standardization of scientific names, the official name for this genus became ''Pterodactylus'', though the name "pterodactyl" continued to be popularly and incorrectly applied to all members of Pterosauria. Paleontologists now avoid using "pterodactyl" and prefer the term "pterosaur". They relegate the term "pterodactyl" specifically for members of the genus ''Pterodactylus'' or more broadly for members of the suborder Pterodactyloidea.

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. 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 metres (33 ft).

Paleobiology

Flight

The mechanics of pterosaur flight are not completely understood or modeled at this time.

Katsufumi Sato, a Japanese scientist, did calculations using modern 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 Furthermore, Darren Naish concluded that atmospheric differences between the present and the Mesozoic weren't needed for the giant size of pterosaurs.

However, Mark Witton and Mike Habib, of the University of Portsmouth and Johns Hopkins University, respectively, argue that pterosaurs used a vaulting mechanism to obtain flight. Once in air, pterosaurs could reach speeds up to and travel thousands of kilometres.

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. The presence of a subcutaneous air sac system in at least some pterodactyloids would have further reduced the density of the living animal.

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. Furthermore, 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.

Reproduction and life history

Very little is known about pterosaur reproduction, and pterosaur eggs are very rare. The first known pterosaur egg was 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. This was supported by the description of an additional pterosaur egg belonging to the genus ''Darwinopterus'', described in 2011, which also had a leathery shell and, also like modern reptiles but unlike birds, was fairly small compared to the size of the mother. A study of pterosaur eggshell structure and chemistry published in 2007 indicated that it is likely pterosaurs buried their eggs, like modern crocodiles 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.

Wing membranes preserved in pterosaur embryos are well developed, suggesting pterosaurs were ready to fly soon after birth. Fossils of pterosaurs only a few days to a week old (called flaplings) have been found, representing several pterosaur families, including pterodactylids, rhamphorhinchids, ctenochasmatids and azhdarchids. All preserve bones which show a relatively high degree of hardening (''ossification'') for their age, and wing proportions similar to adults. In fact, many pterosaur flaplings have been considered adults and placed in separate species in the past. Additionally, flaplings are normally found in the same sediments as adults and juveniles of the same species, such as the ''Pterodactylus'' and ''Rhamphorhynchus'' flaplings found in the Solnhofen limestone of Germany, and ''Pterodaustro'' flaplings from Brazil. All are found in deep aquatic environment far from shore.

It is not known whether pterosaurs practiced any form of parental care, but their ability to fly as soon as they emerged from the egg and the numerous flaplings found in environments far from nests and alongside adults has led most researchers, including Christopher Bennett and David Unwin, to conclude that the young were only dependent on their parents for a very short period of time, while the wings grew long enough to fly, and left the nest to fend for themselves within days of hatching. Alternatively, they may have used stored yolk products for nourishment during their first few days of life, as in modern reptiles, rather than depend on parents for food.

Growth rates of pterosaurs once they hatched varied across different groups. In more primitive, long-tailed pterosaurs ("rhamphorhynchoids") such as ''Rhamphorhynchus'', the average growth rate during the first year of life was 130% to 173%, slightly faster than the growth rate of alligators. Growth in these species slowed after sexual maturity, and it would have taken more than three years for ''Rhamphorhynchus'' to attain maximum size. In contrast, the more advanced, large pterodactyloid pterosaurs such as ''Pteranodon'' grew to adult size within the first year of life. Additionally, pterodactyloids had ''determinate growth'', meaning that the animals reached a fixed maximum adult size and stopped growing.

Daily activity patterns

Comparisons between the scleral rings of pterosaurs and modern birds and reptiles have been used to infer daily activity patterns of pterosaurs. The pterosaur genera ''Pterodactylus'', ''Scaphognathus'', and ''Tupuxuara'' have been inferred to be diurnal, ''Ctenochasma'', ''Pterodaustro'', and ''Rhamphorhynchus'' have been inferred to be nocturnal, and ''Tapejara'' has been inferred to be cathemeral, being active throughout the day for short intervals. As a result, the possibly fish-eating ''Ctenochasma'' and ''Rhamphorhynchus'' may have had similar activity patterns to modern nocturnal seabirds, and the filter-feeding ''Pterodaustro'' may have had similar activity patterns to modern anseriform birds that feed at night. The differences between activity patterns of the Solnhofen pterosaurs ''Ctenochasma'', ''Rhamphorhynchus'', ''Scaphognathus'', and ''Pterodactylus'' may also indicate niche partitioning between these genera.

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, including links to ornithodirans like ''Scleromochlus'', an ancestry among the basal archosauriforms like ''Euparkeria'', or among the prolacertiformes (which include gliding forms like ''Sharovipteryx'').

Two researchers, Chris Bennett (1996) and David Peters (2000), have found pterosaurs to be prolacertiformes or closely related to them. Bennett only recovered pterosaurs as close relatives of the prolacertiformes after removing characteristics of the hind limb from his analysis, in an attempt to test the idea that these characters are the result of convergent evolution between pterosaurs and dinosaurs. However, subsequent analysis by Dave Hone and Michael Benton (2007) could not reproduce this result. Hone and Benton found pterosaurs to be closely related to dinosaurs even without hind limb characters. They also criticized previous studies by David Peters, raising "serious questions" about the methods he used to recover pterosaurs among the prolacertiformes. Hone and Benton concluded that although more primitive pterosauromorphs are needed to clarify their relationships, pterosaurs are best considered archosaurs, and specifically ornithodirans, given current evidence. In Hone and Benton's analysis, pterosaurs are either the sister group of ''Scleromochlus'' or fall between it and ''Lagosuchus'' on the ornithodiran family tree. Sterling Nesbitt (2011) found strong support for a clade composed of ''Scleromochlus'' and pterosaurs.

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 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.

Pterodactyloidea (Plieninger, 1901): The more derived ("advanced") pterosaurs, 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.

Listing of families and superfamilies within Pterosauria, after Unwin 2006 unless otherwise noted.

ORDER PTEROSAURIA (extinct)

* Suborder Rhamphorhynchoidea *

** Family Anurognathidae

** Family Campylognathoididae

** Family Dimorphodontidae

** Family Rhamphorhynchidae

* Suborder Pterodactyloidea

** Superfamily Ornithocheiroidea

*** Family Istiodactylidae

*** Family Nyctosauridae

*** Family Ornithocheiridae

*** Family Pteranodontidae

** Superfamily Ctenochasmatoidea

*** Family Ctenochasmatidae

*** Family Gallodactylidae

*** Family Pterodactylidae

** Superfamily Dsungaripteroidea

*** Family Dsungaripteridae

*** Family Germanodactylidae

** Superfamily Azhdarchoidea

*** Family Azhdarchidae

** Family Chaoyangopteridae

*** Family Lonchodectidae

*** Family Tapejaridae

The precise relationships between pterosaurs is still unsettled. However, several newer studies are beginning to make things clearer. Cladogram simplified after Unwin.

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Extinction

It was once 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 Cretaceous–Tertiary extinction event which wiped out all non-avian dinosaurs and 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 Cretaceous–Tertiary extinction event severely affected marine life that most pterosaurs fed on, they went extinct. However, forms like azhdarchids and istiodactylids were not marine in habits.

Well-known genera

Examples of pterosaur genera include:

''Pteranodon'' was 1.8 metres (six ft) long, with a wingspan of 7.5 m (25 ft), and lived during the late Cretaceous period.

''Pterodactylus'' had a wingspan of 50–75 centimetres (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 ft), and was among the largest flying animals ever. It lived during the late Cretaceous period.

''Hatzegopteryx'' was the largest flying animal known to science. Although no complete fossil has been discovered, what little fossils paleontologists ''have'' found give the creature an estimated wingspan of roughly 12 metres (40 ft.)

''Rhamphorhynchus'' was a Jurassic pterosaur with a vane at the end of its tail, which may have acted to stabilise the tail in flight.

In popular culture

Pterosaurs have been a staple of popular culture for as long as their cousins the dinosaurs, though they are usually not featured as prominently in films, literature or other art. Additionally, while the depiction of dinosaurs in popular media has changed radically in response to advances in paleontology, a mainly outdated picture of pterosaurs has persisted since the mid 20th century.

The number and diversity of pterosaurs in the popular consciousness is also not as high as it has been historically for dinosaurs. While the generic term "pterodactyl" is often used to describe these creatures, the animals depicted frequently represent either ''Pteranodon'' or ''Rhamphorhynchus'', or a fictionalized hybrid of the two. 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. Also, some depictions of pterosaurs incorrectly identify them as "birds", when in real life they were flying reptiles, and birds are actually descended from theropod dinosaurs.

Pterosaurs were first used in fiction in Arthur Conan Doyle's 1912 novel ''The Lost World'', and subsequent 1925 film adaptation. They have been used in a number of films and television programs since, including the 1933 film ''King Kong'', and 1966 ''One Million Years B.C.''. In the latter, animator Ray Harryhausen had to add inaccurate bat-like wing fingers to his stop motion models in order to keep the membranes from falling apart, though this particular error was common in art even before the film was made. Pterosaurs were mainly absent from notable film appearances until 2001, with ''Jurassic Park III''. However, paleontologist Dave Hone has noted that even after the 40 intervening years, the pterosaurs in this film had not been significantly updated to reflect modern research. Among the errors he noted as persisting from the 1960s to the 2000s were teeth even in toothless species (the ''Jurassic Park III'' pterosaurs were intended to be ''Pteranodon'', which translates as "toothless wing"), nesting behavior that was known to be inaccurate by 2001, and leathery wings, rather than the taut membranes of muscle fiber which was actually present and required for pterosaur flight.

A fictionalized mutation of a pterosaur was introduced in the 1956 Japanese film ''Rodan''. The film was released by Toho, the same studio responsible for ''Godzilla''. The character later appeared in a number of Godzilla films between 1964 and 2004.

See also

List of pterosaurs

Mary Anning

Wyvern

Flying and gliding animals

References

External links

Pterosaur.net, multi-authored website about all aspects of pterosaur science

The Pterosaur Database, by Paul Pursglove.

Mark Witton's Pterosaur Art

Comments on the phylogeny of the pterodactyloidea, by Alexander W. A. Kellner. (technical)

Category:Cretaceous extinctions

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