The anatomy of pterosaurs was highly modified from their reptilian ancestors by the adaption to 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 shoulder blade.
Reconstructed wing planform of Quetzalcoatlus northropi
(A) compared to the wandering albatross Diomedea exulans
(B) and the Andean condor Vultur gryphus
(C). These are not to scale; the wingspan of Q. northropi
, the largest known flying animal, was more than three times as long as that of the wandering albatross.
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 ankles.
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. 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 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 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 (see Paleobiology section below) into the wing membrane.
Parts of the wing
, as depicted here, evidences the possibility that pterosaurs had a cruro
patagium – a membrane connecting the legs that, unlike the chiropteran uro
patagium, leaves the tail free.
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; 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). It is generally agreed though that non-pterodactyloid pterosaurs had a broader uro/cruropatagium, with pterodactyloids only having membranes running along the legs.
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. 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. 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 inward toward the body as traditionally thought. Peters (2009) proposed that the pteroid articulated with the ‘saddle' of the radiale (proximal syncarpal) and both the pteroid and preaxial carpal were migrated centralia. This view of the articulation of the pteroid has since been supported by specimens of Changchengopterus pani and Darwinopterus linglongtaensis, both of which show the pteroid in articulation with the proximal syncarpal.
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. In derived pterodactyloids like pteranodontians and azhdarchoids, metacarpals I-III are small and do not connect to the carpus, instead hanging in contact with the fourth metacarpal; in nyctosaurids the forelimb digits besides the wingfinger have been lost altogether.
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.
Skull, teeth and crests
Most pterosaur skulls had elongated jaws with a full complement of needle-like teeth. 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. Some advanced beaked forms were toothless, such as the pteranodonts and azhdarchids, and had larger, more extensive, and more bird-like beaks.
Unlike most archosaurs, the nasal and antorbital openings of pterodactyloid pterosaurs merged into a single large opening, called the nasoantorbital fenestra. This feature likely evolved to lighten the skull for flight.
Some species of pterosaurs featured 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 extremely 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 case of pterosaurs like Pterorhynchus and Pterodactylus, the true extent of these crests has only been uncovered using ultraviolet photography. 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).
At least some pterosaurs had hair-like filaments known as pycnofibers on the head and body, similar to, but not homologous (sharing a common origin) with, mammalian hair. A fuzzy integument was first reported from a specimen of Scaphognathus crassirostris in 1831 by Goldfuss, and recent pterosaur finds and the technology for histological and ultraviolet examination of pterosaur specimens have provided incontrovertible proof: pterosaurs had pycnofiber coats. Pycnofibers were not true hair as seen in mammals, but a unique structure that developed a similar appearance. Although, in some cases, actinofibrils (internal structural fibers) in the wing membrane have been mistaken for pycnofibers 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 pycnofibers on the head and body, not unlike modern-day bats, another example of convergent evolution. The head-coats do not cover the pterosaur's large jaws in many of the specimens found so far.
The presence of pycnofibers (and the demands of flight) imply that pterosaurs were endothermic (warm-blooded). The absence of pycnofibers on pterosaur wings suggests that the coat did not have an aerodynamic function, lending support to the idea that pycnofibers evolved to aid pterosaur thermoregulation, as is common in warm-blooded animals, insulation being necessary to conserve the heat created by an endothermic metabolism.[c]
Pterosaur "hair" was so obviously distinct from mammalian fur and other animal integuments, it required a new, separate name. The term "pycnofiber", 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. Research into the genetic code of American alligator embryos could suggest that pycnofibres, crocodile scutes and avian feathers are developmentally homologous, based on the construction of their beta-keratin.
Some researchers (Czerkas and Ji, 2002) have speculated that pycnofibers were an antecedent of proto-feathers, but the available impressions of pterosaur integuments are not like the "quills" found on many of the bird-like maniraptoran specimens in the fossil record. Pterosaur pycnofibers were structured similarly to theropod proto-feathers. Pycnofibers were flexible, short filaments, "only 5–7mm in some specimens" and rather simple, "apparently lacking any internal detail aside from a central canal". Pterosaur "pelts" found "preserved in concentrated, dense mats of fibers, similar to those found surrounding fossilized mammals" suggest coats with a thickness comparable to many Mesozoic mammals,[d] at least on the parts of the pterosaur covered in pycnofibers. The coat thickness, and surface area covered, definitely varied by pterosaur species.
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 homogenous structure generally considered to cover them. The analysis found that at least some of these were very similar in structure to 4 different feather structures commonly known from birds and dinosaurs but almost never known from pterosaurs prior to the study. It is unknown if these "feathers" share a common ancestry with bird feathers or independently evolved from them, but the study's senior author, Mike Benton, has stated that despite heavy analysis, there was virtually no recorded difference between the structures and bird or dinosaur feathers, and they thus likely share a common ancestry with them.