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Pterosaurs
Fossil range: Late TriassicLate Cretaceous
Ningchengopterus BW
Ningchengopterus, a genus of pterodactyloid pterosaur from the Early Cretaceous-age Yixian Formation of China.
Scientific classification

Class:

Reptilia

Superorder:

Archosauria

Order:

Pterosauria
Kaup, 1834

Suborders:

Pterosaurs, meaning "winged lizard", often referred to as pterodactyls, 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. 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.[1][2][3]

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.

DescriptionEdit

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.[4] 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).

WingsEdit

Azhdarchwingshapewittonnaish2008

Reconstructed wing planform of Quetzalcoatlus compared to the Wandering Albatross and the Andean Condor. (not to scale).

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 wing to to the elbow) were strengthened by closely spaced fibers called actinofibrils.[5] 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 they were made from. 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.[6] The wing membranes also contained a thin layer of muscle, fibrous tissue, and a unique, complex circulatory system of looping blood vessels.[7]

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.[8]

Parts of the pterosaur wingEdit

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.[7] 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.

Pterosaur wing BW2

Anatomy of a pterosaur wing.

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 pteroid-to-shoulder connection traditionally depicted in life restorations.[7] 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.[9] 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.[10]

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,[11] the anurognathid Jeholopterus,[12] and a pterodactyloid from the Santana Formation seem to demonstrate that the wing membrane did attach to the hindlimbs, at least in some species.[13] 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.[14]

Many if not all pterosaurs also had webbed feet.[15]

Skull, teeth and crestsEdit

Pterosauria tooth 01

A pterosaur tooth, possibly Coloborhynchus

Most pterosaur skulls had elongated, beak-like jaws. Some advanced forms were toothless (such as the pteranodonts and azhdarchids, though most sported a full complement of needle-like teeth.[16] 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.[17]

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.[16]

Tapejara heads BW

The crests of three tapejarids. Clockwise from right: Tapejara, Tupandactylus, and "Tapejara" navigans. Not to scale.

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.[7] In the cases of pterosaurs like Pterorhynchus and Pterodactylus, the true extent of these crests has only been uncovered using ultra violet photography.[17][18] 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).[7]

PycnofibersEdit

At least some pterosaurs were covered with hair-like filaments known as pycnofibers, similar to but not homologous (sharing a common structure) with mammalian hair. Pycnofibers were not true hair as seen in mammals, but a unique structure that developed a similar appearance through convergent evolution. Although in some cases actinofibrils (internal structural fibres) in the wing membrane have been mistaken for pycnofibres or true hair, some fossils such as those of Sordes pilosus (which translates as "hairy demon") and Jeholopterus ninchengensis do show the unmistakable imprints of pycnofibres on the head and body, not unlike modern-day bats, another example of convergent evolution.[11] The presence of pycnofibres (and the demands of flight) imply that pterosaurs were endothermic (warm-blooded).

The term "pycnofibre", meaning "dense filament", was first coined in a paper on the soft tissue impressions of Jeholopterus by palaeontologist Alexander W.A. Kellner and colleagues in 2009.[6]

History of discovery Edit

Pterodactylus antiquus 1888 Lydekker R

Diagram of the original P. antiquus specimen by Richard Lydekker, 1888.

Anhanguera-santanae skull

The three-dimensionally preserved skull of Anhanguera santanae, from the Santana Formation, Brazil.

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.[19] 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.[20]Georges Cuvier first suggested that pterosaurs were flying creatures in 1801,[21] 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 genus 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.[1]

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).

PaleobiologyEdit

FlightEdit

Pneumatic Anhanguera santanae

CT scans and photograph illustrating external pneumatic openings and typical pneumatic architecture in the ornithocheirid pterosaur Anhanguera santanae (AMNH 22555).

The mechanics of pterosaur flight are not completely understood or modeled at this time[22][23], 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.[citation needed] 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.

Pterosaur pulmonary air sac system

Models of ventilatory kinematics and the pulmonary air sac system of pterosaurs

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.[24] 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 modern birds and decided that it is impossible for a pterosaur to stay aloft.[25] 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.[26] 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 [2] Furtheremore, Darren Naish concluded that atmospheric differences between the present and the Mesozoic weren't needed for the giant size of pterosaurs: [3]

Air sacs and respirationEdit

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 (beneath the skin) air sac system in at least some pterodactyloids would have further reduced the density of the living animal.[8]

Nervous systemEdit

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.[4]

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.[4]

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.[4]

Ground movementEdit

Haenamichnuswittonnaish2008

The probable azhdarchid trace fossil Haenamichnus uhangriensis.

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.[27] 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.[28][29]

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.[15]

Quetzfeedingwittonnaish2008

Fossil trackways show that pterosaurs like Quetzalcoatlus northropi were quadrupeds.

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.[30] 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.[15]

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.[15]

Natural predatorsEdit

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.[31]

ReproductionEdit

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.[32] The embryo's wing membranes were well developed, suggesting pterosaurs were ready to fly soon after birth.[33] This is corroborated by very young animals found in the Solnhofen limestone beds.[34] It is not known whether pterosaurs practiced parental care, but their comparatively early flight capabilities suggest the young were 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.[34]

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.[35] 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 other archosaurs are incapable of doing.

ReferencesEdit

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  13. ^ Frey et al., (2003) New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion Geological Society London Special Publications
  14. ^ Dyke GJ, Nudds RL, Rayner JM (July 2006). "Limb disparity and wing shape in pterosaurs". J. Evol. Biol. 19 (4): 1339–42. doi:10.1111/j.1420-9101.2006.01096.x. PMID 16780534. 
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  16. ^ Cite error: Invalid <ref> tag; no text was provided for refs named DU06b
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  28. ^ Padian K (2003). "Pterosaur Stance and Gait and the Interpretation of Trackways". Ichnos 10 (2-4): 115–126. doi:10.1080/10420940390255501. 
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External links Edit

See also Edit


Archosauromorphs
Primitive ArchosauromorphsEuparkeriidae • Erythrosuchidae • Proterochampsidae • Proterosuchidae • Choristodera • Prolacertiformes • Rhynchosauria • Trilophosauria

Crurotarsi ArchosaursOrnithosuchidae • Aetosauria • Phytosauria • Rauisuchia • Crocodylomorpha • Crocodilia

Avemetatarsalia and Ornithodira ArchosaursScleromochlus • Pterosauria • Dinosauromorpha • Dinosauria • Ornithischia • Saurischia • Aves

Avian ArchosaursAvialae • Archaeopteryx • Confuciusornis • Ichthyornis • Enantiornithes • Hesperornithes • Neornithes • Paleognathae • Neognathae

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