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Pterosaur

Source: Wikipedia, the free encyclopedia.

Pterosaurs
Temporal range:
Ma
Pterosauria-collage-01.jpg
A sample of various pterosaurs.
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Ornithodira
Clade: Pterosauromorpha
Order: Pterosauria
Kaup, 1834
Subgroups[1][2]
Pterosaur Fossil Distribution Map.png
Distribution of pterosaur fossil locations. Colored species or genera names correspond to their taxonomic group.[a]

Pterosaurs (

reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to 66 million years ago[7]). 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 ankles to a dramatically lengthened fourth finger.[8]

There were two major types of pterosaurs. Basal pterosaurs (also called 'non-pterodactyloid pterosaurs' or '

pterodactyloids) evolved many sizes, shapes, and lifestyles. Pterodactyloids had narrower wings with free hind limbs, highly reduced tails, and long necks with large heads. On the ground, pterodactyloids walked well on all four limbs with an upright posture, standing plantigrade on the hind feet and folding the wing finger upward to walk on the three-fingered "hand". They could take off from the ground, and fossil trackways show at least some species were able to run and wade or swim.[9] Their jaws had horny beaks, and some groups lacked teeth. Some groups developed elaborate head crests with sexual dimorphism
.

Pterosaurs sported coats of hair-like filaments known as

endothermy
, a good oxygen supply and strong muscles made pterosaurs powerful and capable flyers.

Pterosaurs are often referred to by popular media or the general public as "flying dinosaurs", but dinosaurs are defined as the descendants of the

last common ancestor of the Saurischia and Ornithischia, which excludes the pterosaurs.[14] Pterosaurs are nonetheless more closely related to birds and other dinosaurs than to crocodiles or any other living reptile, though they are not bird ancestors. Pterosaurs are also colloquially referred to as pterodactyls, particularly in fiction and journalism.[15] However, technically, pterodactyl may refer to members of the genus Pterodactylus, and more broadly to members of the suborder Pterodactyloidea of the pterosaurs.[16]

Pterosaurs had a variety of lifestyles. Traditionally seen as fish-eaters, the group is now understood to have also included hunters of land animals, insectivores, fruit eaters and even predators of other pterosaurs. They reproduced by eggs, some fossils of which have been discovered.

Description

The anatomy of pterosaurs was highly modified from their reptilian ancestors by the adaptation to flight. Pterosaur

breastbone for flight muscles and an enlarged brain able to coordinate complex flying behaviour.[17] Pterosaur skeletons often show considerable fusion. In the skull, the sutures between elements disappeared. 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 shoulder blade. Likewise, the sacral vertebrae could form a single synsacrum
while the pelvic bones fused also.

Basal pterosaurs include the clades Dimorphodontidae (Dimorphodon), Campylognathididae (Eudimorphodon, Campyognathoides), and Rhamphorhynchidae (Rhamphorhynchus, Scaphognathus).

Pterodactyloids include the clades Ornithocheiroidea (

).

The two groups overlapped in time, but the earliest pterosaurs in the fossil record are basal pterosaurs, and the latest pterosaurs are pterodactyloids.[18]

The position of the clade Anurognathidae (

nocturnal or crepuscular
habits, mouth bristles, and feet adapted for clinging. Parallel adaptations are seen in birds and bats that prey on insects in flight.

Size

Pterosaurs had a wide range of sizes, though they were generally large. The smallest species had a wingspan no less than 25 centimetres (10 inches).[11] The most sizeable forms represent the largest known animals ever to fly, with wingspans of up to 10–11 metres (33–36 feet).[20]

Standing, such giants could reach the height of a modern giraffe. Traditionally, it was assumed that pterosaurs were extremely light relative to their size. Later, it was understood that this would imply unrealistically low densities of their soft tissues. Some modern estimates therefore extrapolate a weight of up to 250 kilograms (550 pounds) for the largest species.[21]

Skull, teeth, and crests

Conical tooth, possibly from Coloborhynchus

Compared to the other vertebrate flying groups, the birds and bats, pterosaur skulls were typically quite large.

braincase was relatively large for reptiles.[27]

Tupandactylus imperator
(drawn to scale)

In some cases, fossilized 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.[28] Some advanced beaked forms were toothless, such as the Pteranodontidae and Azhdarchidae, and had larger, more extensive, and more bird-like beaks.[23] Some groups had specialised tooth forms. The Istiodactylidae had recurved teeth for eating meat. Ctenochasmatidae used combs of numerous needle-like teeth for filter feeding; Pterodaustro could have over a thousand bristle-like teeth. Dsungaripteridae covered their teeth with jawbone tissue for a crushing function. If teeth were present, they were placed in separate tooth sockets.[25] Replacement teeth were generated behind, not below, the older teeth.[24]

The skull of Thalassodromeus

The public image of pterosaurs is defined by their elaborate head crests.[29] This was influenced by the distinctive backward-pointing crest of the well-known Pteranodon. The main positions of such crests are the front of the snout, as an outgrowth of the premaxillae, or the rear of the skull as an extension of the parietal bones in which case it is called a "supraoccipital crest".[27] Front and rear crests can be present simultaneously and might be fused into a single larger structure, the most expansive of which is shown by the Tapejaridae. Nyctosaurus sported a bizarre antler-like crest. The crests were only a few millimetres thin transversely. The bony crest base would typically be extended by keratinous or other soft tissue.[27]

Since the 1990s, new discoveries and a more thorough study of old specimens have shown that crests are far more widespread among pterosaurs than previously assumed. That they were extended by or composed completely of keratin, which does not fossilize easily, had misled earlier research.[30] For Pterorhynchus and Pterodactylus, the true extent of these crests has only been uncovered using ultraviolet photography.[28][31] While fossil crests used to be restricted to the more advanced Pterodactyloidea, Pterorhynchus and Austriadactylus show that even some early pterosaurs possessed them.[30]

Like the upper jaws, the paired lower jaws of pterosaurs were very elongated.[32] In advanced forms, they tended to be shorter than the upper cranium because the jaw joint was in a more forward position. The front lower jaw bones, the dentaries or ossa dentalia, were at the tip tightly fused into a central symphysis. This made the lower jaws function as a single connected whole, the mandible. The symphysis was often very thin transversely and long, accounting for a considerable part of the jaw length, up to 60%.[26] If a crest was present on the snout, the symphysis could feature a matching mandible crest, jutting out to below.[26] Toothed species also bore teeth in their dentaries. The mandible opened and closed in a simple vertical or "orthal" up-and-down movement.

Vertebral column

A neck vertebra of Arambourgiania

The

vertebral body) was concave and into it fitted a convex extension at the rear of the preceding vertebra, the condyle. Advanced pterosaurs are unique in possessing special processes projecting adjacent to their condyle and cotyle, the exapophyses,[33] and the cotyle also may possess a small prong on its midline called a hypapophysis.[34]

The neck of pterosaurs was relatively long and straight. In pterodactyloids, the neck is typically longer than the torso.[35] This length is not caused by an increase of the number of vertebrae, which is invariably seven. Some researchers include two transitional "cervicodorsals" which brings the number to nine.[35] Instead, the vertebrae themselves became more elongated, up to eight times longer than wide. Nevertheless, the cervicals were wider than high, implying a better vertical than horizontal neck mobility. Pterodactyloids have lost all neck ribs.[34] Pterosaur necks were probably rather thick and well-muscled,[36] especially vertically.[37]

The torso was relatively short and egg-shaped. The vertebrae in the back of pterosaurs originally might have numbered eighteen. With advanced species a growing number of these tended to be incorporated into the

neural spines into a "supraneural plate". Their ribs also would be tightly fused into the notarium.[38] In general, the ribs are double-headed.[39] The sacrum consisted of three to ten sacral vertebrae. They too, could be connected via a supraneural plate that, however, would not contact the notarium.[38]

The tails of pterosaurs were always rather slender. This means that the

chevrons.[40] Such tails acted as rudders, sometimes ending at the rear in a vertical diamond-shaped or oval vane.[41] In pterodactyloids, the tails were much reduced and never stiffened,[41] with some species counting as few as ten vertebrae.[38]

Shoulder girdle

The

shoulder blade, was a straight bar. It was connected to a lower bone, the coracoid that is relatively long in pterosaurs. In advanced species, their combined whole, the scapulocoracoid, was almost vertically oriented. The shoulder blade in that case fitted into a recess in the side of the notarium, while the coracoid likewise connected to the breastbone. This way, both sides together made for a rigid closed loop, able to withstand considerable forces.[39] A peculiarity was that the breastbone connections of the coracoids often were asymmetrical, with one coracoid attached in front of the other. In advanced species the shoulder joint had moved from the shoulder blade to the coracoid.[43] The joint was saddle-shaped and allowed considerable movement to the wing.[39] It faced sideways and somewhat upwards.[41]

The breastbone, formed by fused paired sterna, was wide. It had only a shallow keel. Via sternal ribs, it was at its sides attached to the dorsal ribs.[40] At its rear, a row of belly ribs or gastralia was present, covering the entire belly.[41] To the front, a long point, the cristospina, jutted obliquely upwards. The rear edge of the breastbone was the deepest point of the thorax.[43] Clavicles or interclavicles were completely absent.[41]

Wings

Pterosaur wings were formed by bones and 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. Where they ended has been very controversial but since the 1990s a dozen specimens with preserved soft tissue have been found that seem to show they attached to the ankles. The exact curvature of the trailing edge, however, is still equivocal.[44]

While historically thought of as simple leathery structures composed of skin, research has since shown that the wing membranes of pterosaurs were highly complex dynamic structures suited to an active style of flight.

actinofibrils.[46] The actinofibrils themselves consisted of three distinct layers in the wing, forming a crisscross pattern when superimposed on one another. The 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.[47] The wing membranes also contained a thin layer of muscle, fibrous tissue, and a unique, complex circulatory system of looping blood vessels.[30] The combination of actinofibrils and muscle layers may have allowed the animal to adjust the wing slackness and camber.[45]

As shown by 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 into the wing membrane.[48]

Parts of the wing

Sordes, as depicted here, evidences the possibility that pterosaurs had a cruropatagium – a membrane connecting the legs that, unlike the chiropteran
uropatagium, leaves the tail free

The pterosaur wing membrane is divided into three basic units.[49] The first, called the propatagium ("fore membrane"), was the forward-most part of the wing and attached between the wrist and shoulder, creating the "leading edge" during flight. The brachiopatagium ("arm membrane") was the primary component of the wing, stretching from the highly elongated fourth finger of the hand to the hindlimbs. Finally, at least some pterosaur groups had a membrane that stretched between the legs, possibly connecting to or incorporating the tail, called the uropatagium;[49] the extent of this membrane is not certain, as studies on Sordes seem to suggest that it simply connected the legs but did not involve the tail (rendering it a cruropatagium). A common interpretation is that non-pterodactyloid pterosaurs had a broader uro/cruropatagium stretched between their long fifth toes, with pterodactyloids, lacking such toes, only having membranes running along the legs.[50]

There has been considerable argument among paleontologists about whether the main wing membranes (brachiopatagia) attached to the hindlimbs, and if so, where. Fossils of the rhamphorhynchoid

Santana Formation seem to demonstrate that the wing membrane did attach to the hindlimbs, at least in some species.[53] 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.[54]

The bony elements of the arm formed a mechanism to support and extend the wing. Near the body, the

pronation
.

A bone unique to pterosaurs,[57] known as the pteroid, connected to the wrist and helped to support the 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.[30] The position of the pteroid bone itself has been controversial. Some scientists, notably Matthew Wilkinson, have argued that the pteroid pointed forward, extending the forward membrane and allowing it to function as an adjustable flap.[58] This view was contradicted 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 was directed inward toward the body as traditionally interpreted.[59] Specimens of Changchengopterus pani and Darwinopterus linglongtaensis show the pteroid in articulation with the proximal syncarpal, suggesting that the pteroid articulated with the 'saddle' of the radiale (proximal syncarpal) and that both the pteroid and preaxial carpal were migrated centralia.[60][61]

The pterosaur wrist consists of two inner (proximal, at the side of the long bones of the arm) and four outer (distal, at the side of the hand) 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, according to Wilkinson.[62]

In derived pterodactyloids like

induced drag. The wingfinger is also bent somewhat downwards.[64]

When standing, pterosaurs probably rested on their metacarpals, with the outer wing folded to behind. In this position, the "anterior" sides of the metacarpals were rotated to the rear. This would point the smaller fingers obliquely to behind. According to Bennett, this would imply that the wingfinger, able to describe the largest arc of any wing element, up to 175°, was not folded by flexion but by an extreme extension. The wing was automatically folded when the elbow was bowed.[37][65]

A laser-simulated fluorescence scan on Pterodactylus also identified a membranous "fairing" (area conjunctioning the wing with the body at the neck), as opposed to the feathered or fur-composed "fairing" seen in birds and bats respectively.[66]

Pelvis

The

hip joint was not perforated and allowed considerable mobility to the leg.[63] It was directed obliquely upwards, preventing a perfectly vertical position of the leg.[64]

The front of the pubic bones articulated with a unique structure, the paired prepubic bones. Together these formed a cusp covering the rear belly, between the pelvis and the belly ribs. The vertical mobility of this element suggests a function in breathing, compensating the relative rigidity of the chest cavity.[63]

Hindlimbs

The hindlimbs of pterosaurs were strongly built, yet relative to their wingspans smaller than those of birds. They were long in comparison to the torso length.

metatarsus was always splayed to some degree.[69] The foot was plantigrade, meaning that during the walking cycle the sole of the metatarsus was pressed onto the soil.[68]

There was a clear difference between early pterosaurs and advanced species regarding the form of the fifth digit. Originally, the fifth

abduction of the thighbone, meaning that the legs would be spread. This would also turn the feet into a vertical position.[68] They then could act as rudders to control yaw. Some specimens show membranes between the toes,[70] allowing them to function as flight control surfaces. The uropatagium or cruropatagium would control pitch. When walking the toes could flex upwards to lift the membrane from the ground. In Pterodactyloidea, the fifth metatarsal was much reduced and the fifth toe, if present, little more than a stub.[71] This suggests that their membranes were split, increasing flight maneuverability.[50]

The first to fourth toes were long. They had two, three, four and five phalanges respectively.[67] Often the third toe was longest; sometimes the fourth. Flat joints indicate a limited mobility. These toes were clawed but the claws were smaller than the hand claws.[69]

Soft tissues

The rare conditions that allowed for the fossilisation of pterosaur remains, sometimes also preserved soft tissues. Modern

petrifications, natural casts and transformations of the original material. They may include horn crests, beaks or claw sheaths as well as the various flight membranes. Exceptionally, muscles were preserved.[73] Skin patches show small round non-overlapping scales on the soles of the feet, the ankles and the ends of the metatarsals.[74] They covered pads cushioning the impact of walking. Scales are unknown from other parts of the body.[75]

Pycnofibers

Most or all pterosaurs had

Scaphognathus crassirostris in 1831 by Georg August Goldfuss,[77] but had been widely doubted. Since the 1990s, pterosaur finds and histological and ultraviolet examination of pterosaur specimens have provided incontrovertible proof: pterosaurs had pycnofiber coats. Sordes pilosus (which translates as "hairy demon") and Jeholopterus ninchengensis
show pycnofibers on the head and body.

The presence of pycnofibers strongly indicates that pterosaurs were endothermic (warm-blooded). They aided thermoregulation, as is common in warm-blooded animals who need insulation to prevent excessive heat-loss.[76] Pycnofibers were flexible, short filaments, about five to seven millimetres long and rather simple in structure with a hollow central canal.[76] Pterosaur pelts might have been comparable in density to many Mesozoic mammals.[b][76]

Relation with feathers

Pterosaur filaments could share a common origin with feathers, as speculated in 2002 by Czerkas and Ji.

maniraptoran specimens too fundamental.[76]

A 2018 study of the remains of two small Jurassic-age pterosaurs from Inner Mongolia, China, found that pterosaurs had a wide array of pycnofiber shapes and structures, as opposed to the homogeneous structures that had generally been assumed to cover them. Some of these had frayed ends, very similar in structure to four different feather types known from birds or other dinosaurs but almost never known from pterosaurs prior to the study, suggesting homology.[78][79] A response to this study was published in 2020, where it was suggested that the structures seen on the anurognathids were actually a result of the decomposition of aktinofibrils: a type of fibre used to strengthen and stiffen the wing.[80] However, in a response to this, the authors of the 2018 paper point to the fact that the presence of the structures extend past the patagium, and the presence of both aktinofibrils and filaments on Jeholopterus ningchengensis[81] and Sordes pilosus.[82] The various forms of filament structure present on the anurognathids in the 2018 study would also require a form of decomposition that would cause the different 'filament' forms seen. They therefore conclude that the most parsimonious interpretation of the structures is that they are filamentous proto-feathers.[83] But Liliana D’Alba points out that the description of the preserved integumentary structures on the two anurogmathid specimens is still based upon gross morphology. She also points out that Pterorhynchus was described to have feathers to support the claim that feathers had a common origin with Ornithodirans but was argued against by several authors. The only method to assure if it was homologous to feathers is to use a scanning electron microscope.[84]

In 2022, a new fossil of Tupandactylus cf. imperator[85] was found to have melanosomes in forms that signal an earlier than anticipated development of the patterns found in extant feathers than previously thought. In these fossils, it appears as though the feather melanosomes took on a more complex form than the melanosome organization in scales that near relatives of Tupandactylus had. This discovery is one of many that leads us away from many previous theories of feathers evolving directly from scales in reptiles, given the significant distinction of melanosome organization and content between the two. This indicates a distinct form of melanosomes within feather structures at the time, different from other contemporary feathers that did not carry this formation. The feather fossils obtained from this specimen also suggested the presence of Stage IIIa feathers, a new discovery which may also suggest that more complex feather structures were present at this time. Previously, no Stage III feather forms had been discovered in this time. This study contains multiple indications about the development of feather forms. These include a more precise estimate for the development of avian feather forms, as well as a more ancient ancestor that contained the origins of feather-specific melanosome signaling found in extant birds.

History of discovery

First finds

Pterodactylus antiquus specimen by Egid Verhelst II
, 1784

Pterosaur

In 1800, Johann Hermann first suggested that it represented a flying creature in a letter to Georges Cuvier. Cuvier agreed in 1801, understanding it was an extinct flying reptile.[93] In 1809, he coined the name Ptéro-Dactyle, "wing-finger".[94] This was in 1815 Latinised to Pterodactylus.[95] At first most species were assigned to this genus and ultimately "pterodactyl" was popularly and incorrectly applied to all members of Pterosauria.[15] Today, paleontologists limit the term to the genus Pterodactylus or members of the Pterodactyloidea.[16]

In 1812 and 1817,

marsupials.[98] As the bat model correctly depicted pterosaurs as furred and warm-blooded, it better approached the true physiology of pterosaurs than Cuvier's "reptile model". In 1834, Johann Jakob Kaup coined the term Pterosauria.[99]

Expanding research

In 1828,

Harry Govier Seeley, at the time the main English expert on the subject, who also wrote the first pterosaur book, Ornithosauria,[103] and in 1901 the first popular book,[90] Dragons of the Air. Seeley thought that pterosaurs were warm-blooded and dynamic creatures, closely related to birds.[104] Earlier, the evolutionist St. George Jackson Mivart had suggested pterosaurs were the direct ancestors of birds.[105] Owen opposed the views of both men, seeing pterosaurs as cold-blooded "true" reptiles.[106]

In the US,

Niobrara Chalk, then the largest known pterosaur,[106] the first toothless one and the first from America.[107] These layers too rendered thousands of fossils,[107] also including relatively complete skeletons that were three-dimensionally preserved instead of being strongly compressed as with the Solnhofen specimens. This led to a much better understanding of many anatomical details,[107]
such as the hollow nature of the bones.

Meanwhile, finds from the Solnhofen had continued, accounting for the majority of complete high quality specimens discovered.

paleoneurologist Tilly Edinger determined that the brains of pterosaurs more resembled those of birds than modern cold-blooded reptiles.[113]

In contrast, English and American paleontologists by the middle of the twentieth century largely lost interest in pterosaurs. They saw them as failed evolutionary experiments, cold-blooded and scaly, that hardly could fly, the larger species only able to glide, being forced to climb trees or throw themselves from cliffs to achieve a take-off. In 1914, for the first time pterosaur aerodynamics were quantitatively analysed, by

David Meredith Seares Watson, but they interpreted Pteranodon as a pure glider.[114] Little research was done on the group during the 1940s and 1950s.[90]

Pterosaur renaissance

The situation for dinosaurs was comparable. From the 1960s onwards, a

Robert Bakker named a renaissance of pterosaurs.[115] Kevin Padian especially propagated the new views, publishing a series of studies depicting pterosaurs as warm-blooded, active and running animals.[116][117][118] This coincided with a revival of the German school through the work of Peter Wellnhofer, who in 1970s laid the foundations of modern pterosaur science.[86] In 1978, he published the first pterosaur textbook,[119] the Handbuch der Paläoherptologie, Teil 19: Pterosauria,[120] and in 1991 the second ever popular science pterosaur book,[119] the Encyclopedia of Pterosaurs.[121]

This development accelerated through the exploitation of two new Lagerstätten.

Santana Formation in Brazil began to produce chalk nodules that, though often limited in size and the completeness of the fossils they contained, perfectly preserved three-dimensional pterosaur skeletal parts.[119] German and Dutch institutes bought such nodules from fossil poachers and prepared them in Europe, allowing their scientists to describe many new species and revealing a whole new fauna. Soon, Brazilian researchers, among them Alexander Kellner
, intercepted the trade and named even more species.

Even more productive was the Early Cretaceous Chinese

CAT-scans.[124] Insights from other fields of biology were applied to the data obtained.[124]
All this resulted in a substantial progress in pterosaur research, rendering older accounts in popular science books completely outdated.

In 2017 a fossil from a 170-million-year-old pterosaur, later named as the species

Jurassic period, and it has been described as the world’s best-preserved skeleton of a pterosaur.[125]

Evolution and extinction

Origins

Because pterosaur

tanystropheids. A placement among basal archosauriforms like Euparkeria was also suggested.[23] Some basal archosauromorphs seem at first glance to be good candidates for close pterosaur relatives due to their long-limbed anatomy; one example is Sharovipteryx, a "protorosaur" with skin membranes on its hindlimbs likely used for gliding.[127] A 1999 study by Michael Benton found that pterosaurs were avemetatarsalians closely related to Scleromochlus, and named the group Ornithodira to encompass pterosaurs and dinosaurs.[128]