Mark Witton (UK)
Planet Earth was a busy place 225Ma. The super-continent, Pangaea, in the Northern Hemisphere started rifting, creating the beginnings of the North Atlantic Ocean. Dinosaurs began their campaign for global dominance of Mesozoic terrestrial ecosystems and drove many older reptile lineages into extinction. In the seas, enormous marine reptiles began to challenge fish for the role of top predators. Hidden in the shadows of this reptile-dominated world, the first mammals quietly appeared. Even plants were undergoing a revolution, as archaic seed ferns were replaced with sleek new conifers. Yes, the Late Triassic was a busy time indeed. However, away from all this bustle, in the treetops and skies, another branch of reptiles were quietly carving their own place in history, as the first vertebrates to evolve powered flight. These extraordinary animals were pterosaurs – the ‘flying reptiles’.
Most people are not terribly familiar with pterosaurs. Sure, they might have come across Pteranodon or ‘pterodactyls’ in books, films or television documentaries. However, pterosaurs are mostly cast as secondary components of prehistoric landscapes, playing bit parts in productions dominated by their dinosaur contemporaries. At most, pterosaurs have brief cameos in which they carry off scantily clad women or harass explorers on their entry to forgotten, lost worlds. However, these bit parts do little to tell the real pterosaur story. It is one of humble beginnings and eventual domination of Mesozoic skies for 160 million years, of global distribution and tremendous ecological diversity and, in the final days before their extinction 65Ma, the evolution of the largest animals ever to grace the skies.
Of course, all that is left of pterosaurs today is their fossil remains. The first of these fossils (Fig. 1) made known to science was found in the Solnhofen Limestones of Germany in 1784, 38 years before dinosaur studies really kicked off.
Its describer, Cosimo Collini, thought it represented an extinct aquatic reptile. However, this notion was overturned in 1809 when the renowned comparative anatomist, Georges Cuvier, recognised the flight adaptations of the skeleton. Subsequently, he named the animal Ptero-dactyle, which translates as ‘wing-finger’, a name later modified to Pterodactylus (Fig. 2). As more pterosaur discoveries came to light, it was realised that a whole group of these bizarre creatures once existed and, in 1834, the group was christened “pterosauria”, meaning “winged lizards”. Over the last 220 years, approximately 100 different species of pterosaur have been recognised and the continual discovery of new pterosaur fossils, on all seven continents, means this number is set to rise.
Despite centuries of research, the beginning of the pterosaur story remains mysterious. Taxonomically speaking, just what are pterosaurs? They certainly aren’t ‘flying dinosaurs’. This title applies to birds, not pterosaurs. Nor are they lizards, despite their Greek name suggesting the contrary. In fact, establishing exactly where pterosaurs fit into a broader evolutionary picture has proved rather difficult as the earliest most ‘primitive’ pterosaurs already possess highly modified anatomy distinct from all other fossil forms, thereby shadowing their ancestry.
It seems likely that their origins lie in a taxonomic group known as “archosauromorpha” that is a collection of animals represented in modern times by crocodiles and birds. Opinions on where pterosaurs lie in this group differ: some folk include them right at the base with some unusual reptiles known as protorosaurs. Others place them close to crocodiles, whilst several studies present pterosaurs as close relatives of dinosaurs. Answering this rather elementary question is so difficult because the pterosaur fossil record is rather patchy, particularly at the very start of their evolutionary history. It is quite likely that pterosaurs evolved quickly in environments unsuited for fossil preservation (such as dense woodlands or forests) and, consequently, the fossil record misses the opening act of the pterosaur show. Clearly, new discoveries are required to shed light on this murky area of the pterosaur tree.
Pterosaur anatomy: inside and out
Several key sites in China, Brazil, Germany and the USA have yielded complete, high-quality pterosaur fossils that preserve not only skeletal components, but also elements of their soft tissues. These specimens demonstrate that pterosaurs have a skeleton principally adapted for active, powered flight. This method of locomotion is far more energy efficient than walking or running but is very physically demanding, requiring high energy intake and adaptations to meet the stresses it causes. This may explain why, despite the abundance of gliding animals, only insects, birds, bats and pterosaurs have ever evolved self-propelled, powered flight.
The most conspicuous flight adaptation of the pterosaur skeleton is their wings. They are structures that share features with those of birds and bats but are anatomically unique. Like birds, the wing was comprised of a single spar formed from bones of the hand and forearm but, in keeping with bats, distal portions of the wing were supported by an enormously elongated finger, albeit only one compared to the three flight fingers of bats (Fig. 3). In flight, this finger was held between 150º to 170º from the supporting wing metacarpal (one of the bones that comprises the palm of the hand) to form an elongate wing. However, when grounded, it could be folded tightly towards the body so as not to impede terrestrial locomotion.
Attached to the elongate forelimb was a wing membrane known as the “cheiropatagium” that extended from the tip of the wing finger to the hindlimb. Exactly where it attached on the hindlimb remains controversial as very few fossils unequivocally preserve the full outline of the wing. The best fossil evidence currently available indicates pterosaurs had broad membranes that attached at their ankles, not at their knees or hips as commonly suggested by palaeontological illustrators. However, some argue that narrow wings would have improved flight efficiency, and favour a model of knee or thigh attachment. Two further membranes complete the pterosaur wing: the uropatagium, located between the legs, and the propatagium, found along the front of the forelimb. The latter was controlled by the pteroid, a unique pterosaur bone that articulated with the wrist and controlled the propatagium as required during flight, manipulating it like flaps on a plane wing.
Exceptional preservation of pterosaur membranes has revealed intricate details of the stratified internal tissues making up these organs. The upper surface was little more than a thin epidermis, with a comparatively thick layer of spongy tissue directly beneath. Underlying this was a network of blood vessels and capillaries and a thin sheet of muscle. A complex of elongate, stiffened rods underlay the muscle tissues and strengthened the cheiropatagium, acting analogously to the fingers of bats or central vane of bird feathers.
These rods were neatly arranged parallel to, but beneath, the wing finger in the distal part of the wing. They were shorter and more fibrous towards the body, suggesting the membrane was comparatively elastic in this region. This would have allowed flex and movement of the limbs during flapping and terrestrial locomotion, while keeping the wingtip stiff and rigid to form an efficient flight surface.
All pterosaurs share this wing anatomy, but some broad distinctions can be made between pterosaurs based on other aspects of their skeletons. Classically, pterosaurs have been divided into two groups: a series of predominately long tailed, long bodied and short-handed forms referred to as “basal pterosaurs” or “rhamphorhynchoids”; and the short-tailed, short bodied but long-handed pterodactyloidea. As their name suggests, basal pterosaurs represent the earlier stages of pterosaur evolution and have a slightly different flight apparatus to the more advanced pterodactyloids. Along with generally shorter wings, the uropatagium is broader and supported by elongate fifth digits on the feet. The long, often stiff tail overlies this membrane and bore a vertically orientated sail or rudder (Fig. 4).
By contrast, Pterodactyloids have narrow uropatagia and correspondingly small fifth toes (Fig. 5).
Although all pterosaurs are thought to have been competent fliers, the reduction of the tail and uropatagia in pterodactyloids may indicate these forms had more sophisticated flight styles than their forebears, being less stable in the air but gaining increased agility and speed because of it.
Other fossilised pterosaur soft tissues reveal surprising details about their anatomy. Given their reptilian origins, the presence of thick, bristly fur across the head (excluding the jaws), neck, torso and proximal regions of the limbs is rather unusual. This fur was particularly thick along the back of the neck and torso and, apparently, independently evolved from both mammalian hair and bird feathers. The presence of a furry, insulating integument suggests that pterosaurs shared an elevated body temperature with almost all other flying animals.
However, even more eyebrow raising are the often elaborate, soft-tissue headcrests that adorn the skulls of many pterosaurs. Some of these headcrests are truly enormous being up to 75% of the skull area (Fig. 6).
Other pterosaurs had bony headcrests that apparently lacked soft-tissue adornment, but were also probably strikingly coloured. The function of these crests is still unclear, but it is unlikely that they were used as rudders during flight or crestless pterosaurs would be unable steer. Rather, their reduced nature in immature pterosaurs and apparent role in sexual dimorphism suggest they may have had a key role in display between individuals. Intriguingly, new finds of Pterodactylus reveal that it also had a headcrest (Fig. 2), albeit one that left no evidence on the skull anatomy. It seems quite likely that if a pterosaur known for over 220 years from numerous specimens can keep a secret like this hidden for so long, other well-known pterosaurs may also have some soft-tissue surprises in store.
Pterosaurs are now accepted as confident, competent fliers, but their terrestrial capabilities have traditionally been more suspect. Until the 1980s and 1990s, pterosaurs were envisaged as sprawling or belly-dragging beasts when grounded, only capable of taking off when they clumsily hurled themselves off a nearby cliff. A totally opposing view was offered in the late 1980s with the suggestions that pterosaurs were bird-like bipeds. However, none of these hypotheses took the best evidence for grounded pterosaur locomotion into account – pterosaur trackways. The first pterosaur trace fossils were identified in Utah during the 1950s and, following years of controversy over the ‘pterosaur-or-crocodile’ identity of the track-maker, they were demonstrated to belong to a pterodactyloid pterosaur in the 1990s.
Analysis of this trackway showed its maker was not dragging itself along the ground, walking with sprawled limbs nor strutting about on two legs. Instead, this pterosaur was moving on four limbs held almost vertically beneath its body, and, furthermore, it was running. Pterodactyloid trackways are now known from Mesozoic strata all over the world and, based on the evidence they’ve provided, the terrestrial competence of pterosaurs is no longer questioned. In fact, it is thought that this ability to run was crucial for some pterosaurs to fly. That is, when pterodactyloids decoupled their hindlimbs from the uropatagium, the potential was created for run-up assisted take-offs. Although this may not have been utilised by smaller forms, larger pterosaurs almost certainly needed a running start to become airborne. Therefore, in association with elongation of the pteroid bone (allowing for a deeper, greater lift-generating propatagium), running enabled pterodactyloids to grow far larger than their basal pterosaur ancestors.
When giants ruled the skies
Pterosaur body size gradually increased throughout their 160 million year evolutionary history. Pterosaurs of the Late Triassic were generally rather small, no greater than 1m or so across the wings. They quickly surpassed this in the Jurassic and went on to grow 3m wingspans at the end of this period, a size comparable to the largest modern birds. Such sizes were pretty average for Cretaceous pterosaurs and forms with 6m to 8m wingspans were common throughout this time. However, it was not until the very end of the Cretaceous that pterosaurs really made it big. In the dying light of the Mesozoic, monsters such as Quetzalcoatlus attained wingspans in excess of 10m (Fig. 8), making them the largest flying animals ever. Unfortunately, it is difficult to assess exactly how big these forms were as they are only represented by very fragmentary material, but wingspans up to 14m have been suggested.
Of course, even the most gigantic animals start out small. No insights were offered into pterosaur reproduction for well over 200 years until, in 2004, not one, not two, but three pterosaur eggs all turned up with uncanny timing. These not only conclusively demonstrated that pterosaurs laid eggs but also had soft, thin eggshells in the manner of turtles and crocodiles. This suggests that pterosaurs buried their eggs underground to prevent them drying out and, unlike birds, would not have incubated them. Among modern animals, soft-shelled eggs typically take far longer to hatch than hard-shelled: up to three months in some cases.
However, hatchlings from soft-shelled eggs are often far more precocial than those born from parentally incubated hard-shelled eggs. The well-ossified bone structure of embryonic pterosaurs suggests this was true of them as well. Hatchling pterosaurs, or ‘flaplings’, may have been quite capable of looking after themselves from the moment they emerged from the egg. Along with fully developed bones in one embryo, preserved wing membrane suggests flaplings possessed full flight apparatus at hatching, creating the possibility that they may have been flying around at a very, very young age. Such an achievement is unknown among modern flying animals that all grow close to adult size before taking wing. It is quite remarkable to think that, in the biggest pterosaurs, their flaplings would grow from the size of a sparrow to that of a light aircraft and, while this transformation was taking place, the pterosaur could fly around as competently as any adult.
The pterosaur ecological spectrum
One of the most controversial areas of research into pterosaurs, perhaps because it is also one of the most under-researched, is pterosaur ecology. Traditionally, pterosaurs have been thought to be Mesozoic analogues of seabirds that plundered fish from the water in flight. This is almost certainly true for some pterosaurs such as the rhamphorhynchines, ornithocheirids and pteranodontids. These pterosaurs have long jaws, often brimming with procumbent, fish-seizing teeth and long gull or albatross-like wings (Fig. 5).
However, this model does not apply to all pterosaurs. Anurognathids, for instance, were a group of small, broad-mouthed pterosaurs that possess numerous features labelling them as expert insect-chasers, zipping after their prey like ancient swifts (Fig. 9). Dsungaripterid pterosaurs, with their robust jaws and short, blunt teeth, would have crunched up shellfish taken from ponds and streams. This contrasts markedly with the slender jawed ctenochasmatids, which dabbled in water, like modern day spoonbills, in search of small aquatic invertebrates and fish. These pterosaurs had mouths brimming with hundreds of laterally splayed teeth, extending the catchment of their jaws as they searched for food. The most derived ctenochasmatid is the bizarre Pterodaustro, a form with hundreds of baleen-like teeth on its lower jaw but only stunted, peg-like teeth on the upper jaw.
This animal lived in huge flocks around Argentinean lakes during the Cretaceous, filtering foodstuffs from the water with bristle-like dentition. A wholly different lifestyle was taken by the istiodactylids that may have occupied a vulture-like niche. These forms were well adapted for soaring over land in search of rotting dinosaur carcasses, which they would tear apart with their robust, intermeshing dentition. Tapejarids, a group of often flamboyant pterosaurs with short faces, may have spent their time foraging for small animals and fruits on the ground and, in some species, perhaps in trees (Figs. 6 and 10).
A similar lifestyle is suggested for the azhdarchids, the group that includes the biggest pterosaurs of all (Fig. 11). When the biggest of these animals stalked over Cretaceous plains, they would have been capable of catching and devouring fox-sized dinosaurs along with many other types of small animals. Azhdarchids are perhaps furthest removed from the conventional ‘seabird’ image of pterosaurs, being predominately found in terrestrial settings and well adapted for thermal-soaring like modern storks and vultures. With new pterosaur discoveries revealing ever more bizarre, specialised forms, it appears that the conventional image of pterosaurs as shorebird analogues sells the ecological success of these animals very short indeed.
Of course, all this diversity begs the question, “If pterosaurs were so ecologically successful, where have they all gone?” Why do we not go to the park to feed flocks of tapejarids or watch pteranodontids come in to roost as the sun sets? Unfortunately, the pterosaur fossil record is not complete enough to provide a totally clear answer to this mystery. Most of our knowledge of pterosaur diversity stems from a few fossil lagerstätten, sites of exceptional preservation, which lend themselves well to preserving fragile pterosaur skeletons.
Many pterosaur groups are known exclusively from such sites and, with none of these sites known from the late Cretaceous, establishing the health of pterosaur populations at this time is difficult. Still, the evidence currently available indicates pterosaur diversity was low at the end of the Mesozoic, with highly specialised giant forms representing almost every pterosaur fossil from this time. In defiance of their splendour, giant, specialised animals are some of the most vulnerable to extinction as they reproduce slowly and generally require productive environments to exist. Smaller generalists tend to fare much better against extinction, as they are more adaptable and replenish their populations quickly.
It is also possible that birds, new-kids on the block that almost certainly muscled in on some niches once exclusive to pterosaurs, replaced the generalist pterosaur species throughout the Cretaceous, though there is no evidence to confirm this speculation. However it happened, when the curtain call of the Cretaceous came, with all its meteorite impacts, volcanism and sea level changes, pterosaurs had no small generalists to pull them through and were vulnerable to the environmental stresses of this time. As the last pterosaurs perished, rule of the skies was finally relinquished to the birds, leaving only dusty fragments as remnants of a once great aerial dynasty.
Buffetaut, E. and Mazin, J. M. (eds.) 2003. Evolution and Palaeobiology of Pterosaurs, Geological Society Special Publication, 217, 347 pp.
Unwin, D. M. 2005. The Pterosaurs from Deep Time. Pi Press, New York, 347 pp.
Wellnhofer, P. 1991. The Illustrated Encyclopaedia of Pterosaurs. Salamander Books Ltd., London. 192 pp.
About the author
Mark Witton works at the Palaeobiology Research Group at the University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 5DF. He can be contacted at: Mark.Witton@port.ac.uk.