The Pterosaurs

The name ‘pterosaur’ means ‘wing lizard,’ and these flying reptiles lived from the late Triassic to the end of the Cretaceous. They are the earliest vertebrates known to have evolved powered flight. This isn’t to say that there was nothing in the air before them, only that the skies were dominated by flying invertebrates such as insects. These primitive insects bloomed to huge sizes in the Carboniferous and early Triassic, but the appearance of pterosaurs changed all that: suddenly flying insects weren’t at the top of the totem pole, and their large sizes made them easy prey to the new predators. The smallest insects had a distinct advantage over their large brethren, and insect sizes dwindled due to natural selection. Ever since then it has paid well for insects to be smaller. 

Pterosaurs, it must be said again and again, were not dinosaurs. Though they were more closely related to dinosaurs and modern birds than crocodiles or reptiles, they were nonetheless not dinosaurs. These flying reptiles belonged to a category all their own. They shared a common ancestor with dinosaurs but ‘branched off’ from the archosaurs before dinosaurs struck out on their own track. Another important aside is that though these flying reptiles are sometimes called ‘pterodactyls,’ this is incorrect: pterodactyls refer to members of the genus Pterodactylus and, more broadly, to members of the suborder Pterodactyloidea, which is a subset of pterosauria. In other words, all pterodactyls are pterosaurs, but not all pterosaurs are pterodactyls. 

Sharovipteryx, ancestor of the pterosaurs?

Pterosaur origins aren’t fully understood. Pterosaurs emerge in the fossil record fully adapted to a flying lifestyle. Scholars speculate that the earliest known ancestor was Sharovipteryx, a gliding reptile from the Middle Triassic that preceded pterosaur emergence and diversification in the Late Triassic. How pterosaurs evolved flight is also unknown; as is the case with the origin of flight for birds, scholars tend towards one of two theories. The ‘Ground Up’ theory holds that they were terrestrial creatures that somehow developed flight whereas the ‘Tree Down’ theory postulates that they were arboreal creatures that weaseled their way into powered flight. This latter theory is the most likely. Perhaps pterosaur ancestors were tree climbers that developed a membrane between limbs that was first used to ‘parachute’ tree-to-tree; over time, the gradual lengthening of the fourth fingers enabled them to glide. How they got from gliding to powered flight is unknown, as the mechanics necessary to go from gliding to flying are entirely different. 

Hatzegopteryx

Pterosaur wings were formed by a membrane of skin, muscle, and other fleshy tissues that stretched from the ankles to dramatically-lengthened fourth fingers on each hand. Early species had long, fully toothed jaws and long tails whereas later forms had small tails (and some lacked teeth altogether). Pterosaurs spanned a wide range of sizes: some had a wingspan of less than ten inches while others won the coveted titles to the largest flying creatures of earth history. Quetzalcoatlus (of late Cretaceous North America) and Hatzegopteryx (of late Cretaceous Romania) had wingspans that reached up to thirty-three to thirty-six feet; when standing, these monstrous pterosaurs could reach the height of a modern giraffe. Fossilized skin patches reveal small, round, non-overlapping scales on the soles of pterosaur feet, ankles, and ends of the toes. These covered pads cushioned the impact of walking. Interestingly, the reptilian scales didn’t spread to the rest of the body. Some (if not all!) pterosaurs sported furry coats made up of hair-like filaments known as pycnofibers, which covered their bodies and parts of their wings. These pycnofibers (or ‘dense filaments’) insulated their warm-bloodied bodies. Pycnofibers were unique structures similar to, but not sharing common origin with, mammalian hair; they’re an example of convergent evolution. These short, flexible filaments were about five to seven millimetres long and simple in structure with a hollow central canal. Their ‘furry’ coats may have been comparable in density to those of Mesozoic mammals. Because most artistic portrayals of pterosaurs give them a leathery, reptilian look, it’s no surprise that the existence of a ‘furry’ coat was resisted; but since the 1990s, however, more pterosaur discoveries and ultraviolet examination of pterosaur specimens old and new have proven that pterosaurs did have insulating pycnofiber coats. The undeniable presence of such coats implies pterosaurs were warm-blooded; the pycnofibers would’ve aided in thermoregulation to prevent excessive heat loss (a staple form of bio-engineering for warm-blooded organisms). 

a tyrannosaur charges a flock of Quetzalcoatlus

Pterosaurs utilized a respiratory system that consisted of air sacs that hollowed out their bones. These hollow, air-filled bones resemble those of birds, and they provided higher muscle attachment for their skeletal weight. Pterosaur bones were often razor-thin, and these factors combined with warm-bloodedness, a good oxygen supply, and strong muscles enabled pterosaurs to be powerful flyers. Pterosaurs had a large and keeled breastbone for flight muscles, and their enlarged brain helped coordinate complex flying behavior. Many of their bones fused together, and in some later pterosaurs, the backbone over the shoulders fused into a structure called a notarium, which stiffened the torso during flight and provided stable support for the shoulder blade. The evolving shape of their skulls, too, helped them became better flyers: as pterosaur evolution crept on, their skulls followed a general trend of lengthening, sometimes even to the point of surpassing the neck and torso in length. This was caused by a stretching and fusion of the front snout bone (the premaxilla) with the upper jaw bone (the maxilla). It’s interesting that the classical ‘archosaur’ nasal and antorbital fenestrae (skull openings) merged into a single large opening in the pterodactyloids, creating a ‘nasoantorbital’ fenestrae. This likely evolved to lighten the skull for flight. 

a pterosaur in flight

Pterosaurs are traditionally viewed as fish-eaters, but many would’ve hunted land animals, insects, fruits, and even other pterosaurs. In toothed pterosaurs, the beaks are small and restricted to the jaw tips. Some advanced beaked forms were toothless and had larger, more extensive bird-like beaks. Earlier pterosaurs had a variety of teeth in their jaws, but in later pterosaurs, the teeth became mostly conical. The front teeth were often larger, enabling the pterosaur to grab its prey. Some pterosaurs had specialized teeth that give us a window into their eating habits: the Istiodactylidae had recurved teeth for eating meat, the Ctenochasmatidae used combs of needle-like teeth for filter feeding (the pterosaur Pterodaustro had over a thousand bristle-like teeth), and the Dsungaripteridae covered their teeth with jawbone tissue so that they could crush their food. They reproduced by means of eggs, some of which have been discovered.

a pterosaur nesting ground

Many if not all pterosaurs had elaborate head crests. The main position of these crests were at the front of the snout as an outgrowth of the front snout bone, or at the rear of the skull as an extension of the parietal bones. Front and rear crests could be present simultaneously, and in some species they fused into a single large structure. This is seen vividly in the Tapejaridae (Tapejara has become infamous for its crest). Another pterosaur, Nyctosaurus, had an eye-catching antler-like crest. These crests were usually only a few millimeters thin, and the bony crest base was likely extended by keratinous or other soft tissues. Recent studies with ultraviolet photography have revealed many pterosaurs had paper-thin crests that failed to fossilize. Below are some examples of pterosaur head crests:

The pterosaurs went extinct at the end of the Cretaceous Period at the end of the Mesozoic in the Cretaceous-Tertiary Extinction Event – along with non-avian dinosaurs and most avian dinosaurs as well. It’s been postulated that in the wake of the extinction event, pterosaurs were at a distinct disadvantage against their smaller bird competition. By the end of the Cretaceous, most pterosaurs were larger-than-life, and in the post-apocalyptic landscape where survivors barely scraped by, they were unable to cope. Smaller pterosaur species existed at the same time, but the available niches may have been monopolized by birds. However, studies show that ecological overlap between early birds and late pterosaurs were minimal; they tended to occupy different spheres. At the same time, the ‘brave new world’ of the beginning of the Cenozoic demolished some niches and opened up others, forcing these parallel spheres-of-operation into competition. This idea is bolstered by the fact that some niches dominated by pterosaurs in the late Cretaceous were dominated by birds in the next stage of history.