Pterosaurs With Dentures

Sweeping low over the crowded beach, the big male Tupuxuara presented a magnificent sight. It was not just the white-tipped wings, wider than three lanes of traffic, and it was not just the skull, as long as a javelin and with jaw tips to match. It was, above all else, the extraordinary, the magnificent, the show-stopping crest. It towered above the skull, rising up from its roots on the forehead into a huge sail, flaming red at the front, then shading to maroon at the apex and the rear. As he flew by again, broadside on to the flock of pterosaurs, mostly females, he slowed almost to the point of stalling and executed several slow rolls and turns, each one accentuating the majesty of that crest. Several other males came floating by, far fewer now, as the youngsters and the less well-endowed had been winnowed out by indifference. Several females began to stir, then settled back, waiting. The "king" came in again, this time positioning himself so that the rays of the low sun shone through the crest, making it seem as if it were ablate— a huge flame sailing through the skies. That did it. Three females raised themselves on their stilt-like arms, searched for the bree%e, and, with a kick of their hind legs, were soon aloft on outspread wings. Two more joined them as they trailed off behind the triumphant male. Just as always, big crests were in.'

FIGURE 5.1 The six inch (15 centimeters) long skull of Sinopterus, a toothless azhdarchoid pterosaur from the Lower Cretaceous Jehol Biota of China. (Image courtesy of Wang Xiao-Lin and Zhou Zhonghe.)

Every Body Needs Somebody Ask almost anybody: "What is the most powerful thing in the world?" And they will probably reply: "gravity," or "love," or "money." All these answers and many others, including "Microsoft," are wrong, because the right answer is "evolution." Every single living thing, from the smallest microbe to the biggest blue whale, is a consequence of evolution, even the most complex thing in the known universe— our brains.

Evolution also built pterosaurs. And one of the really clever things about evolution is that it does not start from scratch, with a drawing board, some wobbly pink stuff and lots of sticky-back plastic,2 it begins with a complete, fully functioning organism such as a lizard-like reptile and turns it, bit by bit, into a beautiful flying creature. What is even more surprising is that evolution does not do this by lopping bits off here and adding bits on there, it just slowly modifies the existing bones, teeth, tissues and organs and remodels them in such a way that they can be used for flight, fishing or climbing trees.

The result, for pterosaurs, was a body whose components (insofar as we know and understand them) can be identified in other nonflying vertebrates, but had become uniquely modified for their mode of life in the skies. As we have previously seen, most fossil evidence of the anatomy of these extinct animals consists of hard tissues— their bones and teeth. In life, however, much of the body was composed of soft tissues: major internal organs such as the heart, liver and lungs, and the blood system, nerves, muscles, skin and so on. Usually, as was explained in Chapter 3, very little evidence of such structures survives the rigors of fossilization. Sometimes, however, a serendipitous concatenation of events presents us with the fossilized remains of pterosaur skin, wing membranes, or even throat sacs, to list just some of the soft structures found so far. When added together (Figure 3-9)? the the fossil evidence for soft tissues is rather better for pterosaurs than for many other groups of extinct vertebrates, but it still only reveals a small, if tantalizing, part of the whole picture.

Fortunately, there are two other approaches we can use to fill out our knowledge of pterosaurs' soft anatomy. The pterosaur skeleton, like that of other vertebrates, had a highly intimate relationship with the rest of the body, and individual bones frequently bear the subtle marks of this liaison. Crests, ridges, bumps and scars give away the original position and size of muscles, while holes and grooves, usually tiny, but sometimes large, as in the case of the channel for the spinal cord, reveal the courses of nerves and blood vessels as they snaked their way through, around and over the bones. This same principle applies on a larger scale to the braincase and its vital passenger, the brain, whose external shape and general features can be reconstructed from the internal shape of its bony casing.

Yet another potentially useful way of prying into pterosaurs' innards relies not on the fossils themselves, but on comparisons with living relatives which, naturally, come with a full complement of soft parts. The problem here is that irrespective of which of the modern groups of diapsids— lizards, crocodiles or birds— is eventually shown to be pterosaurs' closest living relative (a matter of some debate, as Chapter 4 revealed), any close relationship can be ruled out, so only speculative inferences regarding soft parts are possible. Still, because pterosaurs were certainly diapsids, we can be fairly confident that what is generally true of all living diapsids, for example, that they breathed using lungs, was also true of pterosaurs.3

By using a wide variety of research techniques, examining as many different fossils as possible and combining every last bit of available evidence, we can piece together an accurate picture of pterosaurs' skeletal structure and a general view of their soft tissue anatomy. Admittedly, large chunks of this picture are still missing— we have no idea, for example, what the liver looked like, or whether it functioned in quite the same way as in other diapsids— but even in these cases, we can fill in the blanks with best guesses, informed by comparison with living relatives such as crocodiles and lizards, and the knowledge that we have already garnered about pterosaurs. This chapter, and the one that follows, takes pterosaurs apart, from the teeth to the tail and to the toes, to see how they were constructed and what this meant for their major bodily functions such as feeding, breathing and, most vital of all, their metabolism. We begin, however, at the beginning— the head.

Head Start The most complex part of a pterosaur, or indeed any vertebrate, was the head— skull, mandibles and all the associated soft bits— quite simply because so many important components and functions were packed into this single structure. The skull contained the brain, of course, the center of consciousness and neural control, but it also housed key sensory organs for four of the five senses: sight, sound, smell and taste. In addition, the primary passageways involved in eating and breathing also passed through the head and, in the case of pterosaurs, the capture of prey was almost exclusively carried out by the jaws and teeth. On top of all this, quite literally, the heads of many pterosaurs bore crests, some of them extremely large and showy, surely a clue to their main purpose.

As Figure 5-2 illustrates, pterosaurs had a highly distinctive skull that, superficially at least, looks quite bird-like. The front half was made up of the snout, often bearing teeth, while the rear half was composed of the cranium, consisting of the orbit (the opening that housed the eye) and the braincase, on the base of which hinged the lower jaw. The snout region was stretched forward, sometimes to a remarkable degree, as in Pteranodon, and tapered to a sharp point. One or two fossils, most notably a specimen of Tapejara, illustrated in Figure 5-3, reveal that in pterosaurs, just as in birds, a horny sheath fitted tightly over the front end of the snout and probably helped to protect it from wear and tear.4

Pterosaur Jaw Muscles

FIGURE 5.2 General anatomy of the pterosaur skull, based on the rhamphorhynchoid Rhamphorhynchus (above), with a length of about 4 inches (10 centimeters), and the pterodactyloid Pteranodon (below), with a length of about 40 inches (1 meter). (Redrawn from Peter Wellnhofer, 1975, and Chris Bennett, 2001.)

FIGURE 5.2 General anatomy of the pterosaur skull, based on the rhamphorhynchoid Rhamphorhynchus (above), with a length of about 4 inches (10 centimeters), and the pterodactyloid Pteranodon (below), with a length of about 40 inches (1 meter). (Redrawn from Peter Wellnhofer, 1975, and Chris Bennett, 2001.)

In rhamphorhynchoids, the main part of the snout was pierced on either side by two openings. Those at the front formed the nostrils, the entrance-way to passages that led to the back of the mouth. The function of the rear pair of openings is less clear: They certainly helped to lighten the skull, but they might also have housed muscles or a salt gland through which pterosaurs, especially the marine forms, were able to dump excess salt.5 As Figure 5-2 shows, pterodactyloids dispensed with the bony bar between the front and rear openings and settled for a single large opening, at the front corner of which lay the nostril.

Teeth erupted along the lower edges of the snout, at least in toothed pterosaurs, but the extent of the tooth rows varied considerably. Except in dsungaripteroids, they usually began at the tip of the snout and continued back to below the orbit, but they could terminate much earlier, and in some pterosaurs, such as Cycnorhamphus, the teeth were restricted to the jaw tip alone. Like other reptiles, pterosaurs constantly shed their teeth and grew

Coloborhynchus Robustus

FIGURE 5.3 Fossil evidence for soft-part structures of the pterosaur head. Above left: horny sheath (rhamphotheca) covering the jaws of Tapejara. Above right: the throat sac in Rhamphorhynchus, Below left: the j aws of the anurognathid Batrachognathus fringed by a beard of bristles. Below right: close up of Batrachognathus' bristles. (Tapejara image courtesy of Dino Frey.)

FIGURE 5.3 Fossil evidence for soft-part structures of the pterosaur head. Above left: horny sheath (rhamphotheca) covering the jaws of Tapejara. Above right: the throat sac in Rhamphorhynchus, Below left: the j aws of the anurognathid Batrachognathus fringed by a beard of bristles. Below right: close up of Batrachognathus' bristles. (Tapejara image courtesy of Dino Frey.)

new ones (often to be seen peeping from the socket of the old tooth) as waves of replacements swept along the tooth row.

Behind the snout lay the orbit. In pterosaurs, this was relatively large and thus able to accommodate a big eyeball, emphasizing the critical importance of sight for these animals. Within the orbit, a ring of small, overlapping, plate-like bones called scleral ossicles helped to support the eyeball. In some really well-preserved fossils, the position and diameter of this ring pinpoints both the exact size and the original location of the eyeball in its socket.

Behind the orbit lay the main part of the cranium, which, in effect, consisted of two bony boxes, one inside the other. The outer box was constructed from bones that roofed the skull and also extended down the sides of the cranium, forming the cheeks. In early reptiles, these "cheeks" were solid,6 but in pterosaurs, they were pierced by two openings (the upper and lower temporal fenestra), the epitome of the diapsid condition. Deep inside the outer bony box was a second box, the braincase. The roof of the braincase was composed of the same elements as the outer box, but its sides and floor were made up of several distinct bones, often fused together. The lower parts and underside of the braincase were pierced by numerous small openings through which the cranial nerves exited from the brain and ran out into the rest of the head, where they enervated the tongue, eyes, jaw muscles and teeth.7

The space between the outer bony box and the braincase was partly filled by blood vessels, nerves and other soft tissues, but mainly occupied by the muscles that attached to the mandibles and operated the jaws.8 The muscles, shown in Figure 5-4, were not confined to this space, but spread out, via the temporal fenestrae, onto the outer surface of the cheek region, thereby attaining a relatively large size and gaining an extensive attachment area. This enabled pterosaurs to power their many different types of feeding behavior, some of which, such as cracking clams, must have required pretty hefty muscles.

The back of each cheek was buttressed by a single, long, pillar-like bone, the quadrate, just behind the top of which lay the ear drum and on the lower end ofwhich articulated the mandible. In early pterosaurs, this articulation seems to have been relatively simple and only allowed the jaws to swing up and down. In some later pterosaurs, such as the ornithocheiroids, the articulation evolved into a more complex screw-like arrangement so that, as the lower jaw opened, the mandibles on each side were pushed outward, increasing the gap between them. Exactly why this was necessary will be explained in the next section.

Pterosaur Jaw Muscles
FIGURE 5.4 How pterosaurs operated their jaws. Muscles that opened the jaws are shown in blue, those that closed them in pink.

Built from several flat, lath-like bones, the mandibles each consisted of a long hollow bar that met at the jaw tip. This junction was very short and sometimes not even fused in early pterosaurs, but became firmly united in later forms, the fusion spreading farther and farther back along the jaw and forming an elongated blade-like structure called the mandibular symphysis. In toothed pterosaurs, the upper edge of each mandible bore a row of teeth, and in some crested forms, the lower edge supported a decorative bony keel. In several exceptionally well-preserved fossils, one of which is illustrated in Figure 5.3, a large patch of wrinkled skin can be seen curving down below the back part of the lower jaw.9 This is most likely to have been the fossilized remains of a throat sac, which probably looked rather similar to the throat pouches of pelicans and within which lodged the leaf lying between the mandibles of Ludodactylus, the "tree biter" that first appeared in Chapter 3. It also seems to explain why the mandibles of some pterosaurs widened as they were opened: This helped ensure that hard-won prey, such as wet, slippery, wriggling fish, ended up in the sac and not back in the ocean.

Fangs for the Fish Three aspects of the pterosaur head deserve closer inspection. The first of these is the teeth. As Figure 5.5 demonstrates, the dentition ofpterosaurs was remarkably variable: Dimorphodon had a long file of tiny teeth led by several large fangs; Anurognathus sported a row of small, well-spaced, sharp-tipped spikes; while Dsungaripterus was equipped with

FIGURE 5.5 Using only single, simple teeth, but with varied number, size, position and orientation, pterosaurs evolved a remarkable degree of dental diversity. Prey-snagging devices, with a tooth grab at the front and smaller teeth behind, evolved over and over again, appearing independently in many lineages including the rhamphorhynchids, represented here by Dorygnathus, and in the ornithocheirids such as Coloborhynchus. Small, sharp-pointed, spike-like teeth, well suited for gripping and puncturing the hard covering of insects, are typical of anurognathids such as Anuromathus, while large numbers of long, fine filament-like teeth were used by Pterodaustro and other ctenochasmatines to strain and sieve for their prey. By contrast, Dsungaripterus, and other dsungaripterids, had large, bulbous, clam-cracking teeth at the back of the jaw, while Tapejara, its relatives and pteranodontians abandoned teeth altogether and presumably relied on the shape of the jaws to deal with their food.

a battery of clam-crushing dentures. Yet all these different dentitions are composed of a single, rather simple tooth design. Unlike our mammalian teeth with their complex cusps and roots, pterosaur teeth had one large root that anchored them firmly in the jaw and a crown that usually consisted of a single elongated cone, built of dentine and capped with enamel. The striking variation evident in pterosaur dentition was achieved quite simply, just by varying the length and degree of curvature of individual teeth, or by varying their number, size, position and orientation within the jaws.

The simple construction of pterosaur teeth had its drawbacks, though. With a few rare exceptions,10 the teeth had no cutting edges that could be used to dismember prey or snip off bite-sized chunks, nor did they have cusps and basins, which might have helped to grind or pulp their food. This means that, essentially, pterosaurs were the "fast-food" feeders of the Meso-zoic, preferring items that required little or no preparation or processing and could be swallowed as quickly as possible.

Predictably, therefore, the most common type of dentition found in pterosaurs is a prey grab. Typically, this consisted of several pairs of large, slightly curved, sharp-pointed fangs, whose job was to get hold of the prey. This was followed by a row of smaller teeth, whose task was to maintain a tight grip on the victim before it was swallowed. Judging from their construction, and a few fossils in which the contents of the stomach are still preserved,11 most prey grabs were used to catch fish, although pterosaurs were probably not averse to consuming other delicacies, such as squid. Presumably, most prey was caught on the wing, although some pterodactyloids might have waded in lakes or ponds and hunted in a manner similar to that of herons and egrets today.

Prey grabs evolved on many separate occasions, and although they appear rather similar, each has its own unique features. Dimorphodontids had a long row of tiny, lancet-like teeth behind the prey grab, while the teeth of Eudimorphodon and its relatives had several points that may have helped to grip prey items more tightly. Both scaphognathines and rhamphorhynchines had well-developed tooth grabs. In Rhamphorhynchus, this was taken to an extreme. In addition to several pairs of murderous-looking fangs, the tips of the mandibles were fused into a narrow, blade-like prow that projected forward from the front of the lower jaw and skimmed through the water surface during flight. As soon as an object was contacted, the tooth-grab-armed jaws snapped shut on what the pterosaur hoped was its prey, but what might on occasion have turned out to be a log.12

Tooth grabs evolved in at least two different pterodactyloid groups, reaching their most spectacular development in ornithocheirids. In some species, the main fangs attained 8 centimeters (more than 3 inches) in length, and their highly worn tips suggest that they saw a great deal of use.

Sustenance was to be had not only on the ground or from the waters, it could also be found in the air. The small, sharp-pointed, peg-like teeth of anurognathids seem well-adapted for gripping and puncturing the hard outer covering of insects, which swarmed through the Mesozoic air in the billions. Just like insectivorous birds today, anurognathids had very broad mouths that could gape extremely wide in order to maximize their chances of snapping up a dragonfly that was doing its best to avoid this fate.

The similarity with avian insectivores goes even further. Not only do some of them, such as nightjars, have a very wide gape, but also, like their pterosaurian counterparts, they have short bristles rimming the edges of the mouth13— as did anurognathids. A well-preserved example of Batrachogna-thus, illustrated in Figure 5.3, shows that the jaws of these pterosaurs were also fringed with short bristles, similar to those of the nightjar, except that in this case, the bristles seem to have been modified from the furry covering of the skin, discussed in the next chapter, rather than from feathers.

Ctenochasmatids opted for a radically different style of feeding— filtration. To do this, they dramatically increased the number of teeth, which reached more than a thousand in the most specialized forms such as Pterodaustro, and the teeth themselves became increasingly long and thin. Presumably, ctenochasmatids plunged their open jaws into the prey-filled waters of lakes and pools and, after closing the jaws to form a trap, lifted the prey into the air and allowed the water to drain away. Then, using a long and flexible tongue, they transferred the results of their labors— crustaceans, tiny mollusks and insect larvae— to the back of the throat ready for swallowing.

Heading in an altogether different direction, Dsungaripterus and its relatives opted for a life of clam-crushing. The first problem was to apprehend the object of their desire— clams, other shellfish, perhaps even crabs— either by probing for it in sand and mud or, in the case of mussels and oysters, by levering it from its rocky holdfast. Either way, their long, pointed, winkle-picking jaw tips would have served well for these tasks. The second problem was to remove the edible soft parts from their protective wrapping. To do this, dsungaripterids had specially enlarged teeth at the back of the tooth row where the jaw-closing muscles could exert the most effective crunch. In the king of the clam-crushers, Dsungaripterus, the hindmost teeth in both the upper and lower jaws were large and chunky, forming a pair of anvils that were firmly embedded in deep sockets and between which oysters were doomed.

Not all pterosaurs relied on teeth as their dinner winners. At least two distinct lineages, pteranodontians and azhdarchoids, made do without any teeth at all. How they fed and exactly what they fed on are not really clear, but in both groups, most species had long, narrow, sharply tipped jaws that were probably used rather like tweezers to pluck up small prey, either while they were flying over water or walking around on land.14 By contrast, Tapejara and its relatives, such as Sinopterus, had relatively short, rather powerful-looking beaks that, in some respects, look similar to those ofparrots. Perhaps these pterosaurs had opted for a more herbivorous diet that consisted, at least partly, of seeds and fruit.15

Gray Matter Matters Tightly enclosed in its bony box, the pterosaur brain left a clear impression of its external shape on the internal surface of the braincase. Casts, or rather "endocasts," as they are referred to, produced from the infilling of the braincase by minerals after the soft tissues had decayed away, can replicate the general shape and external details of the brain with remarkable fidelity. The problem is how to get at the endocast. There are a few fossils, mainly from the Solnhofen Limestone, where fortuitous breaks expose some details, and in one case, in an uncrushed Dorygnatbus skull from Yorkshire, England, a section of the skull roof was removed to reveal part of the endocast of the brain.16 But these and other fossils in which internal details of the braincase are visible present only an incomplete picture. Fortunately, a new technique, CAT scanning, in which images from a series of X-rays are reconstructed into a digital (virtual) endocast, means that we can now get the data we need without damaging the fossil at all. Larry Witmer of Ohio State University and his colleagues have applied this technique to two pterosaurs,1' focusing on the skulls shown in Figure 5.6, one of which belongs to the rhamphorhynchoid Rhamphorhynchus, the other to the pterodactyloid Anhanguera.

The CAT scanning study rapidly confirmed the main conclusion that had been slowly and laboriously arrived at by several older studies18—pterosaurs had remarkably bird-like brains. The lobes at the front, concerned with the sense of smell, were very small, suggesting that odors and scents were

Bird Brain Size
FIGURE 5.6 Schematic drawings of the brain (all drawn to the same size) in the rhamphorhynchoid Rhamphorhynchus (above left), the pterodactyloid, Anhanguera (above right), a bird (below left) and a crocodile (below right). (Redrawn with permission from Witmer et al., 2003).

relatively unimportant for pterosaurs. By contrast, the cerebral hemispheres, centers for consciousness and cognitive activities that formed the main part of the forebrain, were relatively large and, like birds, but unlike reptiles, even had furrowed surfaces, suggestive of some internal complexity.

Another strikingly bird-like feature of the pterosaur brain was the position and size of the optic lobes, which are part of the mid-brain and connected to each eyeball by an optic nerve." In reptiles, the optic lobes lie on the main axis of the brain and are rather small, whereas in pterosaurs, as in birds, they were situated in a low position, almost beneath the cerebral hemispheres, and were relatively large. This indicates that pterosaurs relied heavily on eyesight and processed considerable amounts of visual information, which is to be expected, because the eyes must have played a key role in critical behaviors such as flight and the hunting and catching of prey.

Where the CAT scanning work really broke into new territory was in showing details of the hind-brain, a region concerned with reflex activities, such as balance and posture. It was already known from older studies that the main component of this region, the cerebellum, exhibited a remarkably bird-like condition in that, although it lacked the intense folding seen in birds, it lapped forward over the midbrain to contact the forebrain. What Witmer and colleagues established, however, was that in pterosaurs the semicircular canals, which form part of the inner ear and act as the main organs of balance,20 were extraordinarily large. They are large in birds and bats, too, as one might expect in flying animals where a high degree of sensitivity to any changes in orientation is absolutely vital, but they were even larger in pterosaurs.

Taken at face value, this might suggest that, in some respects, the flight ability of pterosaurs was at least as good as, if not even better than, that of birds and bats, but there may be another reason for the large size. The semicircular canals enclose another important part of the brain, a lobe called the flocculus, which receives impulses from several parts of the body, including the neck muscles, the eyes and the skin. Birds have proportionately the largest flocculi of all living animals, but in pterosaurs they were even larger, prompting the question: Why? The answer, detailed in Chapter 8, is rather unexpected and possibly related to pterosaurs' wings.

Apart from their size, the semicircular canals had another surprise, illustrated in Figure 5.7. They revealed, for the first time, the likely position in which pterosaurs held their heads during flight and, perhaps even more importantly, on the ground. The theory behind this is quite simple. Studies of mammals and birds have shown that in the posture typically adopted by the head, the semicircular canals are aligned so that the lateral canal is more or less parallel to the ground.21 Consequently, ifwe take a virtual pterosaur brain endocast and rotate it so that the lateral canal is also in a horizontal position, we can discover the typical head posture for that particular species. The result for Rhamphorhynchus, which probably holds true for other rham-phorhynchoids as well, was unsurprising: The head, together with the neck and body, appears to have lain in an almost straight line, both in flight and when it was moving on the ground.

In pterodactyloids, however, things were quite different. Here, as the results for Anhanguera show, the lateral canal is inclined to the long axis of the head, so rotating the brain to bring the canal back into normal alignment results in a head-down posture. From this, we can deduce that in flight, the body and neck were probably near horizontal, while the head slanted

Body Weight (Kilograms)

FIGURE 5.7 Ground truth? Pterosaur head orientations interpreted in terms of posture when on the ground. Above left: The horizontal alignment of the lateral semicircular canal, indicated by the red line, is consistent with a crouching posture and forward-directed head in long-tailed pterosaurs, represented by Rhamphorhymhus. Above right: In pterodactyloids such as Anhanguera, the reorientation of the canal can be interpreted in terms of an upright position and downward-pointing head. Below: graph showing the relative mass of pterosaur brains (circles) compared with their body size, with polygons showing the same relationship for reptiles and birds including Archaeopteryx (triangle). (Redrawn from David Unwin, 2003, Larry Witmer et al., 2003, and Jim Hopson, 1980.)

downward, which might have been important during hunting.22 Down on the ground, though, the combination of this head posture with a horizontal neck and body would have resulted in a rather peculiar position with the eyes facing downward, rather than forward. This problem is easily solved by canting the body and neck steeply upward, bringing the head up into a forward-facing posture. What's more, this position, although quite different from that of rhamphorhynchoids, was easily supported by the relatively long arms of pterodactyloids. It also matches details of the numerous tracks that they left behind—a neat and attractive set of interlocking ideas that will show up again in Chapter 9.

Before we take our leave of pterosaur's brains, we might ask one final question: Do their brains tell us anything about their intelligence? The quickest way to answer this question is to calculate the ratio of brain mass to body mass and compare it with living tetrapods. This comparison gives a very rough guide to degrees of intelligence, in that the highest values, reflecting the largest and most complex brains, are found in men and other apes, while reptiles and amphibians, not known for their intellectual capacities, have the lowest values.

The downside is that calculating this ratio for fossils is difficult, especially for pterosaurs, where most skulls are crushed flat, so the few available data for this group, shown on the graph in Figure 5.7, should be treated with caution. Still, the location of pterosaurs right between the clusters for reptiles and birds is suggestive: Pterosaurs seem to have been rather more intelligent than your average living reptile and, as one might expect for a flier, they were able to exert more precise control over their movements. They may also have been a little more sophisticated in terms of their social behavior. On the other hand, although their brains were in many respects quite bird-like, primarily reflecting adaptation to an aerial mode of life rather than any close relationship with birds, pterosaurs might not have been quite as intelligent or endowed with the same degree of behavioral complexity as modern birds.23

Does My Head Look Big With This? Visible from a great distance— surely a clue to their function— and quite breathtaking both in their size and variety, the most splendid feature of many pterosaurs was their head crest. Such crests have been known since the mid-1800s, following the discovery of the bony-finned snouts of ornithocheirids in the Cambridge Greensand and the extraordinary weather-vane crest that decorated the head of Pteranodon. It is only in the last decade, however, with the finding of numerous new kinds of pterosaurs and of fossilized soft parts that extended crests to new heights, that it has become possible to comprehend the true diversity and function of these extraordinary structures.

Until quite recently, it seemed that bony crests, at least, were confined to the pterodactyloids, but their discovery in four different rhamphorhynchoids demonstrates that they were present in all groups of pterosaurs, as Figure 5.8 illustrates. Perhaps the most important and stunning find in this respect was the long, low, blade- like crest adorning the head of Austriadactylus.24 This was the first evidence of a crest in a rhamphorhynchoid and, significantly, is from the Triassic, proving that pterosaur head crests were a constant feature of the group's 150-million-year history.

The uneven, incomplete edge of the crest in Austriadactylus hints at the possibility that, in life, it was further extended by soft tissues, an idea that has been confirmed by the exceptionally well-preserved remains of another new rhamphorhynchoid, Pterorhynchus. This pterosaur, a scaphognathine from the Lower Cretaceous of China, has a low bony ridge on the snout, from which rose a large, keel-like crest that appears to have been constructed from a stiffened and probably rather leathery sheet of skin.25 Although overlooked by a string of researchers beginning with Richard Owen, Dimorphodon, one of the most primitive of pterosaurs, also had a small fin-like crest on the tip of the mandibles. By contrast, other long-tailed forms, most notably Rham-phorhynchus and its close relatives, do not seem to have sported any bony head crests at all, although the possibility that they bore flaps or sails of stiffened skin, as yet unrepresented or unrecognized in fossils, cannot be ruled out.26

Head crests reached their greatest diversity and exuberance in the ptero-dactyloids, yet are constructed in two quite different ways, hinting perhaps at separate origins. One type of crest, confined to the ornithocheiroids, had a completely smooth outer edge, suggesting that originally it was covered with a close-fitting, thin layer of skin that added little or nothing to its overall size and shape. All the same, ornithocheiroid head crests were remarkably variable, ranging from pug-like prows on the tips of the jaws and half-moon crests on the top of the snout, to the blade-thin fins that spring from the crown of the head in Ludodactylus and Pteranodon, and not forgetting the extraordinary forked crest of Nyctosaurus. Not all pterosaurs were so gaudily adorned, however. Many crested ornithocheiroids had relatives, in some cases members of their own species, in which the crest was relatively small or even completely absent.

FIGURE 5.8 Cranial crests show a remarkable degree of diversity in pterosaurs, ranging from the keel-like jaw tip decorations of Omithocheirus to the extraordinarily tall "sail" borne by Tupuxuara. In each case the crest is picked out with a pink fill above.

Nyctosaurus

FIGURE 5.8 Cranial crests show a remarkable degree of diversity in pterosaurs, ranging from the keel-like jaw tip decorations of Omithocheirus to the extraordinarily tall "sail" borne by Tupuxuara. In each case the crest is picked out with a pink fill above.

All other pterodactyloid clans, except ornithocheiroids, had a second type of crest, its base composed of bone, but, as shown by several exceptionally preserved skulls of Tapejara from the Crato Limestone, its upper part formed from a leathery sheet of skin that was supported internally by a stiff, fiber-like network.2' These same fossils also show that the narrow, blade-like sail was given additional support through a thickening of the leading edge. Contrasting strongly with the situation in ornithocheiroids, this second crest type is almost always found in the same location, extending from the crown of the head forward along the snout, sometimes, as in Tupuxuara, almost reaching its tip. Except for one or two lonchodectids, bony crests on the lower jaw are unknown, although, occasionally, as in the dsungaripterids and at least one species of Tapejara, a second bony crest extended upward from the back of the skull. In many cases, the main bony crest has an unfinished outer edge, marking the junction with the base of its leathery continuation, but only in Tapejara is the true extent of this development known. Astonishingly, in this pterosaur, the crest reached a height equivalent to five times the height of the skull— if the situation in other pterodactyloids was even remotely similar, then the Mesozoic skies must have been a sight to behold.

Do Ya Think I'm Sexy? Pterosaurs clearly invested substantial resources in growing, maintaining, and coping with their head crests (imagine flying in gusty winds with a weather-vane rooted to your head), which must have been of considerable importance to them. But for what? It's tempting to believe they had some kind of mechanical function, and there are plenty of ideas on offer. Several paleontologists have suggested that perhaps the larger, flap-like crests were employed as a rudder that enabled pterosaurs to steer themselves through the skies. Or, a clever variation, if the softer part of the crest had been able to develop a camber and behave like a sail it might have allowed pterosaurs to tack into the air flow— ingenious, but impossible for Pteranodon, or any other bony-crested ornithocheiroid.28 Other functional explanations for the crests include the following: a device to guide the jaws through the water while fishing, although this must have been restricted to pterosaurs with crested jaw tips;29 a counterbalance to the jaws, in Pteranodon and similar forms, helping to reduce muscle mass that would otherwise have been needed to stabilize the head;30 or perhaps even a means of dissipating excess heat.31

The problem is that all these mechanical explanations, and their variants, suffer from at least two fatal drawbacks. First, with the possible exception of the "radiator theory," they explain only one or two particular kinds of crest.

All the other variations are either unaccounted for or, worse still, would have had exactly the opposite effect. A second problem is that these ideas completely fail to explain a peculiar thing about crests: why they are present in one pterosaur, but not in another. And it doesn't matter if these two fossils are thought to have belonged to the same species, two different species, or even two different genera, because what is the benefit to one pterosaur of investing in a crest when another almost identical individual seems to have managed perfectly well without this expensive adornment?

The alternative is much more attractive: sex. Or, to be more explicit, crests served as display devices to attract members of the opposite gender or discourage members of the same gender. So, what's the evidence? Analyze lots of fossils of Pteranodon longiceps, as Chris Bennett did,32 and one finds that some individuals have a relatively large, well-developed crest, while in others it is much smaller and far less conspicuous. Now, carefully examine the other end of the same animals, and one discovers that big-crested individuals have a relatively small, narrow pelvis, while small-crested forms have a relatively large and wide pelvis, presumably to permit the passage of an egg.

The message seems clear: These were males and females. Interestingly, on average, males of Pteranodon longiceps seem to have been somewhat larger than females, a pattern known as size dimorphism and quite common in reptiles,33 though rare in birds and bats. Apart from reinforcing Bennett's ideas regarding Pteranodon, this pattern can also be linked to another rather curious aspect of pterosaurs, discussed in detail in Chapter '— the remarkable extent to which adult size varies.

Pteranodon longiceps is by no means the only pterosaur where the so-called "dimorphic" pattern of crest size is to be found. It also occurs in species of Pterodactylus, Germanodactylus, Lonchodectes, Anhanguera, Coloborhynchus and Nyctosaurus and several more cases where closely related species, or even genera, appear almost identical, apart from the presence or absence of crests. Ctenocbasma elegans and Ctenocbasma porocristata provide one instance, Bras-ileodactylus and Anhanguera another. All these examples are most easily and most convincingly interpreted as cases of sexual dimorphism— in which one gender, often (but not necessarily always) the male, bears a display device that is smaller or absent in the other. First documented in detail by Charles Darwin more than 100 years ago in The Descent of Man and Selection in Relation to Sex, examples of sexual dimorphism in the modern world are legion with spectacular examples, including peacocks' tails, deer antlers, chameleons' horns and the crests of newts and salamanders.34

Another advantage of the idea that crests served for display is that it neatly explains why their size, shape and position varied so much. Quite simply, it did not matter where the advertisement was located— on the tip of the jaws or the back of the head— as long as it was clearly visible to everyone else. This leads to another important point. Among living animals, a secondary effect of display devices is that they often act as a means of distinguishing among different species. Consequently, closely related forms living in the same area, ducks for example, tend to have distinctly different display devices, in their particular case, patterns of feather coloration. If the same held true for pterosaurs, then crest size, shape and position would be expected to vary among species found at the same locality. Which indeed they do as, for example, in the Santana Formation pterosaurs: Omithocheirus mesembrinus has a crest right on the tip of the snout; in Coloborbynchus robustus, it is set a little farther back and has a different shape; Tapejara wellnhoferi has a very tall, sail-like crest on top of the head; while the crest of Tupuxuara leonardii extends almost the whole length of the skull.

That crests were used for display seems fairly certain, but how they were used and under what circumstances is not yet clear. That said, it is most likely that their main function was to draw attention to the owner, to make it appear larger and to impress upon the opposite sex his (or her) superior fitness as a mate. This means that sight, which, as already detailed, was well-developed in pterosaurs, is likely to have played a key role in the affair and that the crested sex displayed in some way, either on the ground or perhaps in the air. It might even be supposed that males and females gathered together for these displays, as some species of birds, mammals and insects do today when they take part in leks.35

Wild speculation, surely? Well, not necessarily. Some recently discovered pterosaur track sites, detailed in Chapter 8, seem to show lots of individuals milling around together. Is this a record of some long-ago parade in which bizarrely crested pterosaurs strutted their stuff or craned their necks to catch a glimpse of the new kid in the air?

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