Considering Medusa

The storm had been coming for days, and when it hit, its ferocity was overwhelming. The normally placid waters of the lagoon were whipped into a maelstrom, while the trees and shrubs that clothed the archipelago of low islands were blown this way and that. Some, torn from their roots, whirled off toward the horizon. Pterosaurs, who normally rode out storms on the wing, fought vainly for control and found themselves being rudely tossed around the sky. A big, old male seeking escape by climbing to a higher altitude was flipped on his back by a sudden eddy and then blasted by a tremendous gust.

With a dull crack, his wingflnger broke and, simultaneously, a flight membrane tore away from his leg. Crippled beyond hope, torn wings fluttering like forlorn pennants, he spiraled down toward the hundreds of other pterosaurs whose bodies littered the surface of the lagoon far below. Dropping into the sea with a crumpling splash, the pterosaur gradually lost consciousness as water flooded his lungs, and within a few minutes, he was dead. Soon, waterlogged by the surge of the waves, the carcass slipped beneath the surface and slowly began to sink down through the blue waters into the dark, lifeless depths and the pristine mud far below.'

FIGURE 3*1 Fragments of the Upper Cretaceous pterosaur Azzhdarvho rest on the hand of their discoverer, the late Lev Alexandrovich Nesov. This photograph was taken at the fossil locality of Dzharakhuduk in Uzbekistan in the early 1980s. (Photograph courtesy of Lev Nesov.)

The F o s si lF actory The fate of those who looked upon Medusa was to be turned to stone— forever— which is generally considered to be a bad thing, especially if one had other plans for the evening. Pterosaurs, along with millions of other animals and plants, suffered a similar destiny at the hands of geological processes, but in this case, it was a good thing. This goes both for the paleontologist, who now has something to work with, and the original victims, which have achieved an enviable degree of immortality, and, if they get very lucky, a meeting with someone who would, in a sense, like to bring them back to life.

Fossilization, which is primarily about replacing or replicating biological tissues with relatively inert minerals (the basic stuff of stone) that can last practically forever, is the process by which pterosaurs traveled across millions of years from their dinosaur-filled Mesozoic world to our modern mammal-run planet.2 Most of what we know about pterosaurs is founded on fossils that survived this journey. Inevitably, not everything was fossilized. Usually only the hardest, toughest materials, such as teeth and bone, were capable of hanging around long enough in just the right sorts of places, such as the bottom of tranquil lagoons, to stand any real chance of becoming fossilized. This means that in order to be able to extract the most from what remains of pterosaurs, which, because of the relatively delicate nature of their construction, are rare as fossils in any case, it is vitally important to try to understand how they were fossilized in the first place: what survived, what did not, and how the conversion to stone modified what was originally there to what we see now.

In this chapter, then, we explore the transformation of a pterosaur from a living animal to a fossil and its subsequent journey into the hands of a paleontologist, such as Lev Nesov, shown in Figure 3.1, intent on understanding what this animal originally looked like, how it functioned during life and what its role was in the long-extinct communities of the Mesozoic. The road from a living pterosaur to a reconstruction on a computer screen is long (Figure 3.2). It begins with a corpse, a burial, fossilization and a few tens of millions of years underground. It continues with discovery, preparation (the freeing of fossils from their rocky tomb) and identification, and ends beneath the microscope of a paleontologist intent on wringing as much information as possible from his petrified subject. In order to start this process, however, another process must end: life.

When the Grim Reaper Cais As is the case for wild animals today, it was probably rare for a pterosaur to die of old age, and certainly all, or nearly all, of the fossils collected so far probably represent individuals that succumbed to disease or accidents or were killed in some local, or perhaps more widespread, catastrophe. We can be absolutely sure, for example, that several hatchling-size individuals of the Upper Jurassic pterosaur Pterodactylus (one of which is shown in Figure 1.6) were probably only a few days or weeks old when they died and had their brief lives dramatically cut short by some kind of accident— probably related to their inexperience of flight.

FIGURE 3-2 Pterosaurology begins with a living pterosaur and proceeds through death, decay, burial and fossilization, discovery, preparation and study to arrive back at a restoration of a living pterosaur.

Generally speaking it is not clear exactly how most pterosaurs met their doom, but there are one or two cases where the cause of death can be pinpointed. Returning to Pterodactylus, among the more than 100 specimens already found, there is one pigeon-sized adult individual with a wing-finger bone that is clearly snapped in two: a major accident that, irrespective of its true cause, perhaps a storm, was undoubtedly fatal.3 The discovery of the victim, preserved in the Upper Jurassic Solnhofen Limestones, a thick sequence of limey muds that accumulated at the bottom of lagoons about 148 million years ago in the region of what is now Bavaria4 and is still the world's single most important source of pterosaur fossils, certainly supports this idea, because these sediments are considered, by some, to have been generated as a result of powerful storms. if this is true, it might be that many of the more than 1,000 individual pterosaurs thought to have been recovered from this deposit,5 including the hatchling Pterodactylus just mentioned, perished in similar deadly events.

Perhaps the most spectacular example of a pterosaur for which cause of death seems clear is the so-called "tree-biter," a large pterodactyloid from Lower Cretaceous rocks of the Araripe Plateau in Brazil. Described by Dino Frey and colleagues from the Staatliches Museum fur Naturkunde in Karlsruhe, Germany, and named Ludodactylus,6 one of the several surprising features of this pterosaur is a large spike-like leaf, similar to those of the yucca plant, wedged between its mandibles (see Figure 3.3). Attractive as the idea is, this accident probably did not happen as a result of Ludodactylus flying into, or even attacking, a tree, but most probably occurred while this pterosaur was fishing. Ludodactylus had large teeth at the front of the jaws that formed a grab-like structure that it used to snatch up its prey as it flew low over the water surface. it seems that on this occasion this particular pterosaur may have mistaken the leaf for a fish and, after it snapped it up, the point speared through its throat sac' and became stuck between the mandible and the tongue. Hindered by this encumbrance and unable to close its jaws or feed properly, the pterosaur must have slowly starved to death. As Dino Frey and colleagues point out, the same accidents occasionally befall pelicans today: Victims may change, but death, even in its strangest of forms, is always waiting.

Not all pterosaurs seem to have died alone, though. There are several fossil localities in Argentina, Mongolia and China, for example, where large numbers of pterosaur bones, and sometimes whole skeletons, have been found preserved together in just a single or several closely spaced rock layers. Although this cannot yet be demonstrated with any certainty, it is possible that these accumulations reflect the results of natural disasters such as droughts, volcanic eruptions, hurricanes or perhaps even long-term events such as major changes in weather patterns. Such catastrophes may have been the direct cause of mass mortalities among pterosaurs. Or, more subtly, they may have led to temporary breakdowns in the food web, wreaking havoc on animals such as pterosaurs that were near the top of the pile, in much the same fashion that aberrant weather conditions such as El Nino devastate bird populations today.6

FIGURE 3.3 "The tree-biter" Ludodactylus, from the Lower Cretaceous Crato Limestone Formation of Brazil, and its deadly cargo, a yucca leaf lodged between its mandibles. Unable to close its beak fully, or dislodge the leaf by rubbing it against the ground (resulting in its frayed end), the pterosaur either died of starvation, or an illness or accident, brought on by its half starved state. The main part of the skull of this pterosaur was almost 19 inches (a half meter) long. (Photograph courtesy of Dino Frey.)

FIGURE 3.3 "The tree-biter" Ludodactylus, from the Lower Cretaceous Crato Limestone Formation of Brazil, and its deadly cargo, a yucca leaf lodged between its mandibles. Unable to close its beak fully, or dislodge the leaf by rubbing it against the ground (resulting in its frayed end), the pterosaur either died of starvation, or an illness or accident, brought on by its half starved state. The main part of the skull of this pterosaur was almost 19 inches (a half meter) long. (Photograph courtesy of Dino Frey.)

Burial Irrespective of how pterosaurs died, the chances of them becoming fossilized were vanishingly small— on the order of winning the main lottery prize twice in the same month. The reason is that in the Mesozoic, as they do today, after death, almost all organisms, including pterosaurs, immediately began to decay and were broken down or devoured by bacteria, scavengers or even predators long before they had any chance of becoming entombed in sediment. Then, as now, this was generally a good thing, because it ensured that many elements vital for life, such as carbon, nitrogen, potassium and phosphorus, were recycled. It also saved the world from being buried beneath an ever-deepening layer of insect corpses interspersed with the odd dead dinosaur.

The secret to immortality through fossilization is to make sure that after death, one's carcass is buried as rapidly as possible in a place where conditions are so extreme that they prevent living organisms from reaching the carcass and breaking it down, or even accidentally dismembering it, merely by ploughing through the sediment on which it sits or in which it was buried. For the lucky few who get that far, the next step is to ensure that the key process of fossilization, the replacement or replication of organic tissues by minerals, actually takes place. Then, if not already buried, the body must be interred in sediment that, over the millennia, slowly becomes rock. Barring the odd geological accident, such as disturbances by movements of the land masses or volcanism, or the exposure and destruction of the rock at Earth's surface, the enclosed and protected fossil should last almost indefinitely.

This then, very briefly, is the typical path of fossilization along which pterosaurs and all other fossils traveled. Now, we need to take a closer look at this sequence of events to see exactly how they led to the different kinds of pterosaur fossils that we have today: some flattened, some not, a few with soft parts, the vast majority with only their bones and teeth.

As we have seen, the best spots for getting fossilized should, if possible, have a complete absence of living creatures of any kind and, preferably, a plentiful supply of sediment, the finer-grained, the better. It also helps if the water is still, or nearly so, because strong currents can damage the cadaver or wash it away altogether. Such fossil "traps" are not that common, but they do exist. The bottom of stagnant lakes, or very salty lagoons and even shallow land-locked seas are perhaps obvious examples, but "events" such as underwater mud flows, volcanic eruptions, or even a sediment-laden river in flood could also do the job, although there is a much greater risk that the carcass will be damaged or destroyed. All these and many other kinds of fossil "traps" also existed in the Mesozoic. The problem was getting dead pterosaurs into them. Fortunately (at least from the pterosaurologist's point ofview), pterosaurs' main means of locomotion— flight— meant that occasionally they found themselves over such "traps" into which they fell, or were blown, from the air. Indeed, at one locality in Zhejiang, China,9 they may even have been "downed" by volcanic eruptions. Aside from an aerial delivery, most pterosaurs probably reached fossilization traps by floating in, carried by currents.

The Solnhofen Limestones "trap" that we first met earlier in this chapter provides a good example. Recall that these rocks formed as a result of very fine-grained limey muds settling out at the bottom of lagoons. Conditions on the lagoon floor seem to have been extremely unpleasant, possibly because the stagnant water contained little or no oxygen and had become very salty, and no organism larger than bacteria could live there. Consequently, nothing disturbed the sediments, which thus retained their fine lamination, and any animals that did accidentally wander in did not last long, as the bodies of horseshoe crabs preserved at the end of their tracks (so-called death marches) eloquently show.

So many pterosaur remains have been recovered from the Solnhofen Limestone that it seems reasonable to conclude they must have lived in the vicinity of these lagoons of death, but the speed at which their bodies arrived in these cemeteries seems to have varied. Many, including the broken-winged pterosaur mentioned earlier, may have been killed in storms and sent to the bottom almost immediately— the rapidity of their arrival and entombment reflected in the condition of their skeletons: complete and often undisturbed, as can be seen from the examples in Figures 1.1 and 1.2. Other individuals may have floated for days and weeks, buoyed up by their light, air-filled skeletons and shedding odd pieces such as head, wings, legs or even feet, until finally the water-logged carcass, now lacking most of its soft parts, sank to the bottom. Once on the lagoon floor, most, but not all, pterosaurs were quickly buried in limy mud. Some carcasses, such as that shown in Figure 3.4, seem to have lain uncovered for months or perhaps even years, slowly decaying,10 the skeleton becoming increasingly jumbled up by water currents, until everything was buried by the next storm-generated influx of mud.

FIGURE 3.4 Death and decay in the Solnhofen lagoons. Above left: the snapped wing-finger bone, seen in the lower left region of this photograph, and in greater detail in the photograph to the right, must have been almost instantly fatal for this individual of Pterodactylus from the Solnhofen Limestone. In another skeleton of a similar sized 20 inch (50 centimeter) wingspan Pterodactylus (below) many of the original bones have been dissolved away over the millennia, leaving empty cavities. The jumbled-up nature of the skeleton suggests that the carcass of this individual had been decaying for months or years on the lagoon floor before it was finally buried. (Photographs courtesy of Peter Wellnhofer, above, and Carola Radke, below.)

FIGURE 3.4 Death and decay in the Solnhofen lagoons. Above left: the snapped wing-finger bone, seen in the lower left region of this photograph, and in greater detail in the photograph to the right, must have been almost instantly fatal for this individual of Pterodactylus from the Solnhofen Limestone. In another skeleton of a similar sized 20 inch (50 centimeter) wingspan Pterodactylus (below) many of the original bones have been dissolved away over the millennia, leaving empty cavities. The jumbled-up nature of the skeleton suggests that the carcass of this individual had been decaying for months or years on the lagoon floor before it was finally buried. (Photographs courtesy of Peter Wellnhofer, above, and Carola Radke, below.)

From Bone to Stone The fine details of the actual process of fossilization, converting organic material to stone, are still not fully understood, but the preservation of bones and teeth— which form 99 percent of the pterosaur fossil record— is fairly straightforward. Pterosaur hard parts (like our own skeletons) were largely composed of the relatively inert mineral apatite (calcium phosphate), and thus were already well on the way to being fossils even before their owner was dead. The main event during fossilization, as these hard parts lay shrouded in sediment, appears to have been an enrichment of their mineral component by the addition of further calcium phosphate or a similar substance, such as calcium carbonate. Both these and other minerals could have crystallized out from the water that percolated through the sediment.

The preservation of soft parts is more complicated and can occur in different ways. Internal soft parts, such as major organs (heart, liver, lungs), the blood system or nerves, were literally soft and decayed and degraded extremely rapidly. Not surprisingly, they are almost unknown in pterosaurs. External soft parts, that is, the skin and its various derivatives, such as "hair," wing membranes and foot webs, all discussed in more detail in Chapter 6, are a lot tougher and, on one or two rare occasions, were preserved, although usually only in small patches.

The most common type of pterosaur soft-part preservation, illustrated for two different species in Figure 3.5, consists of impressions. If they survived long enough, patches of skin or wing membranes, for example, could leave indentations (forming a negative image of the imprinting surface) on the sediment that over- or underlay them. Ideally, the sediment should have been extremely fine-grained (i.e., mud) and of the right consistency. The Solnhofen Limestone corresponded exactly to these requirements (for which generations of paleontologists have been eternally thankful) and has yielded numerous examples of impressions with superbly preserved copies of pterosaur wing membranes and other structures, some showing incredibly fine detail, such as thread-like lineations of individual "hairs."

Sometimes, rather than an impression, the actual soft parts themselves are preserved. Usually, in this case, the result is a fine black film that consists of partially decayed organic remains that have reacted with minerals in the surrounding sediments or groundwater to form a complex but relatively inert substance. The effect is rather like a photograph or a painting, essentially two-dimensional but, as in the Karatau pterosaur depicted in Figure 3.6, subtle variations in the color and texture of the fossilized soft parts can pick out fine details only fractions of a millimeter in width.

FIGURE 3«5 Above: superb wing impression of the so-called Zittel wing specimen of Rhamphorhytichus (about 40 inches [ 1 meter] in wingspan), preserved in the Upper Jurassic Solnhofen Limestone of southern Bavaria. Below: Pterodactylus (about 16 inches [40 centimeter] in wingspan) from the same rock sequence, also with well-preserved wing impressions partially picked out by the orange-red mineral goethite. (Photographs courtesy of Peter Wellnhofer.)

FIGURE 3«5 Above: superb wing impression of the so-called Zittel wing specimen of Rhamphorhytichus (about 40 inches [ 1 meter] in wingspan), preserved in the Upper Jurassic Solnhofen Limestone of southern Bavaria. Below: Pterodactylus (about 16 inches [40 centimeter] in wingspan) from the same rock sequence, also with well-preserved wing impressions partially picked out by the orange-red mineral goethite. (Photographs courtesy of Peter Wellnhofer.)

Pictures Pinworms

FIGURE 3.6 The wing membranes of Sordes pilosus, from the Upper Jurassic Karatau beds of Kazakhstan. The black coloring, which represents patches of soft tissues, appears to be a complex mixture of decayed organic remains and several minerals based around the element manganese. Inset, microscopic detail of individual wing fibers, some of which have pulled apart into fine strands only six-thousandths of a millimeter wide.

FIGURE 3.6 The wing membranes of Sordes pilosus, from the Upper Jurassic Karatau beds of Kazakhstan. The black coloring, which represents patches of soft tissues, appears to be a complex mixture of decayed organic remains and several minerals based around the element manganese. Inset, microscopic detail of individual wing fibers, some of which have pulled apart into fine strands only six-thousandths of a millimeter wide.

Going a step further, there are one or two pterosaurs in which small patches of soft parts were mineralized in their original condition, before any significant decay could take place, where even the three-dimensional details are fossilized. So far, this so-called exceptional preservation has been reported in only a single specimen," part of which is illustrated in Figure 3.7, showing one of many pterosaur fossils from the Lower Cretaceous Santana Formation, which crops out around the edges ofthe Araripe Plateau in Brazil.

Two critical steps fostered the extraordinary preservation seen in this Santana fossil. First, within hours or even minutes of death, the pterosaur cadaver, now sinking toward the bottom of a largely land-locked Early Cretaceous lagoon, encountered a region that was saturated in phosphate.12 This mineral precipitated out on the bacteria that were by now furiously breaking down the pterosaur soft parts, resulting in a film of mineralized bacteria that replicated even the finest details, such as individual muscle fibers. This explanation was proposed by David Martill, a paleontologist at Portsmouth University in England, and he coined for it the delightfully appropriate term "The Medusa Effect."13 The second step, ensuring the long-term survival of this apparently instantaneously petrified pterosaur, was the development of a hard stony casing, termed a "concretion," around the pterosaur cadaver as it lay in the sediment on the floor of the lagoon. Critically, this protected the three-dimensionally preserved soft parts from getting crushed or from other damage due to geological perturbations, such as earthquakes.

The processes we have dealt with so far concern only the original tissues, hard and soft, from which pterosaurs were constructed and result in what

FIGURE 3*7 The extraordinarily well-preserved wing membrane of an Early Cretaceous Brazilian pterosaur from the Santana Formation of the Araripe Plateau. Several different layers, including a sheet of muscle fibers (lowermost), are seen in this cross-sectional view of the membrane which, as preserved, is about 1 millimeter in thickness. (Photograph courtesy of David Martill.)

FIGURE 3*7 The extraordinarily well-preserved wing membrane of an Early Cretaceous Brazilian pterosaur from the Santana Formation of the Araripe Plateau. Several different layers, including a sheet of muscle fibers (lowermost), are seen in this cross-sectional view of the membrane which, as preserved, is about 1 millimeter in thickness. (Photograph courtesy of David Martill.)

are generally referred to as "body fossils." This is not, however, the only fossil evidence we have of these creatures. Handprints and footprints left by pterosaurs when they walked or ran over a soft surface, such as the mud on a seashore, could ultimately become trace fossils. Normally, of course, such traces were erased by the next tide or the destruction of the track surface by erosion, but occasionally, events conspired to bring about their preservation. Prolonged drying out, followed by sudden burial under another layer of sediment and then further layers, as a sea gradually flooded over a coastal plain, is just one plausible scenario among many, all of which removed this fleeting moment to the depths.

In Pluto's Realm This brings us neatly to the next stage in our fossil journey, a sojourn lasting many millions of years in the geological underworld. Once petrified and buried, one might expect that little else could happen to a pterosaur fossil, snug in its rocky tomb. But even here, it wasn't safe.

Over the millennia, as the weight of overlying sediment built up, the underlying rock layers, in one of which lay our pterosaur fossil, were slowly squashed down and down until, in many cases, they were only one-tenth or less of their original thickness. Three-dimensional parts of the pterosaur skeleton, such as the skull, shoulder girdle and pelvis, and even the individual, hollow, thin-walled bones, were usually quite incapable of resisting such compression and, as a consequence, the vast majority of pterosaurs ended up as picture-fossils, crushed completely flat. The most memorable example that I have encountered consisted of an incomplete skeleton of a small Upper Triassic pterosaur from Austria that had been reduced to a vanishingly thin film of bones probably less than a tenth of a millimeter thick.14

Not all pterosaurs suffered this fate, however. Sometimes, the sediments seem to have been compressed at an early stage when they were still very soft, so that the relatively hard skeletons "floated" within them and were not crushed. This appears to have happened in several Solnhofen Limestone pterosaurs. A similar process also seems to have occurred in the Santana Formation, although in this case, the fossils were encased in concretions, which then "floated" in the surrounding sediment.

Another major geological danger to fossils was chemical in origin. In particular, variations in the acidity or alkalinity (pH), or other chemical properties of the groundwater percolating through the sediments could lead to the fossil being dissolved away, leaving mere holes in the rock where the bones formerly lay, as the example in Figure 3.4 demonstrates. Alternatively, the precipitation of minerals around the fossil skeleton can lead to its encrustation and, in some extreme cases, even its destruction. A common feature of Solnhofen Limestone pterosaurs, for example, is the presence of calcite crystals, which look a bit like granulated sugar, around the ends of limb bones. Sometimes they are so profuse that they completely obscure and even obliterate parts of the skeleton and, because they often merge into the bone itself, they are damned difficult to remove without damaging the fossil.

Escape From the Underworld Having survived the rigors of fossil-ization and several eons of entombment in rock, our pterosaur fossil now approached one of the most dangerous moments in its journey. In order to be found and collected, a fossil must be on or very near Earth's surface, but as soon as it is exposed, for example, by natural erosion or by quarrying, it and the rock in which it is embedded immediately begin to weather away. If not rescued quickly, the fossil can be lost forever. Hard as it is for a ptero-saurologist to cope with, this is what happens to the vast majority of fossils. Let us pause here for a minute's silence, dedicated to the remembrance of all those pterosaurs that, having survived the almost impossible journey to our modern world, were reduced to dust in some remote, ever-windy, Mongolian landscape or, bitterer still, were fed into the maw of a colossal earth-moving machine as it clawed its way along the bottom of an Oxfordshire clay pit.

Surprising as it may seem, most of the pterosaur fossils now housed in museum collections, from Brighton, England, to Beijing, China, were not found as a result of paleontological expeditions or searches specially sent out to look for them, but were accidental discoveries made during other activities, usually quarrying for stone or minerals. The fine, platy Solnhofen Limestone, which splits in a most satisfying way into sheet-like slabs, was, and still is, used both for printing and for building and decorating. Ubiquitous fossils such as Saccocoma, a floating crinoid,^ and much rarer items— crabs, fish, pterosaurs, even Archaeopteryx— found while working the stone, were traditionally sold by the quarrymen for "beer money," although the high prices commanded by rarities such as pterosaurs mean that most are now traded for large sums by the quarry owners and fossil dealers. Many other "classic" locations that have yielded pterosaurs, for example, quarries in the Posidonia shales at Holzmaden, southeast of Stuttgart, Germany,16 and strip mines in the Cambridge Greensand around Cambridge, England,1' also developed for purely commercial reasons.

Some locations that began as stone quarries produced fossils in such quantity and of such value that work eventually switched largely to fossil collection as the main source ofincome. Two ofthe most important sources for Lower Cretaceous pterosaurs developed in this way. The Santana Formation of Brazil,18 whose nodules were originally burnt to produce lime, now generates its wealth and fame by producing thousands of superb fossils. Among these are some of the best pterosaur skeletons ever found and crateloads of fossil fish, occasionally with astonishingly well-preserved soft parts, such as eyes, muscles and guts.

Similarly, rocks in northeast China that belong to the Jehol Group, long exploited by local farmers for stone and originally formed from sediments that accumulated in large freshwater lakes, have recently achieved worldwide fame by yielding huge numbers of exquisitely preserved remains of the animals and plants— the so-called Jehol Biota— that lived in, over and around the lakes in the Early Cretaceous.19 The fossils range from early flowering plants to complete skeletons of early mammals and, most sensational of all, feathered dinosaurs. These ancient lake beds have also yielded a jaw-dropping array of new pterosaurs, several with fossilized soft parts, including one of the most astounding pterosaurian discoveries of all time— eggs containing embryos. More on these can be found in Chapter '.

The general rarity of pterosaur fossils, even in rocks of the kind suitable for preserving pterosaurs, means that, usually, they are remarkably difficult to find out in the field, and even highly experienced collectors may make only one or two discoveries in a whole lifetime of work.20 For this reason, very few scientific expeditions have set out with the specific intention of finding pterosaurs, and those that have went to locations that had already produced at least one or two remains and were thought to have at least some chance of finding more.

One of the most arduous but ultimately successful expeditions ever made to collect pterosaurs set out from Ulan-bataar, Mongolia, in the summer of 1981. Spurred by just a handful of peculiar-looking wrist bones from the collections of the Paleontological Institute in Moscow that Natasha Bakhurina, the leader of the expedition, felt sure were pterosaurian despite her colleagues' doubts, a small team of Russian and Mongolian paleontologists made for the remote region of Tatal, five days west of Ulan-bataar. Enduring stormy weather, a daily drive of 30 kilometers (nearly 19 miles) to obtain fresh water, a brush with the Black Death, and no means of contacting the outside world,

Bakhurina and her crew (pictured in Figure 3.8) eventually found the locality. At first, their searches were fruitless, but then, as they were about to give up in despair and move on to another region, they found a tiny fragment of pterosaur bone, then another and another— and then they hit the jackpot: a rock layer full of bones. Weeks of painstaking and back-breaking work collecting the fragile fossils and transporting them back to civilization were eventually rewarded with a fabulous prize: an almost complete set of skulls, vertebrae and limb bones of a brand new heron-sized, Lower Cretaceous, lake-dwelling, clam-eating dsungaripterid pterosaur.21

ReleasedF r o m the Rock Getting the fossil back to the museum or research laboratory is still far from the end of the story. Except for those rare occasions when the sediment is very soft and can be blown or brushed away when first found, the fossil remains of most pterosaurs are deeply embedded in the millennium-hardened sediment within which they were originally interred. Sometimes, a lucky split might have exposed much of the fossil, but usually, it has to be carefully freed (the technical term is "prepared") from the rock that still surrounds it in order to reveal anatomical details and render the fossil suitable for study or display. More often than not, this is a

FIGURE 3.8 Russian paleontologists in the summer of 1981 searching Lower Cretaceous beds in the Tatal region of Western Mongolia For signs of pterosaurs. (Photograph courtesy of Natasha Bakhurina.)

difficult, time-consuming and very labor-intensive process that has to be carried out in specially equipped laboratories.

Most pterosaurs are prepared using extremely sharp needles, usually mounted in a chuck. Dental picks are also very effective, and various power tools that vibrate needles or chisels at high speed or blast away rock using abrasive powders carried in an airstream can help to winkle out details or clear away large areas of stone rapidly. Acid preparation, which involves immersing the whole specimen in a bath of very weak acid for hours or days, followed by prolonged washing in pure water, then painting over exposed bone to protect it from the next acid bath, is slow, but can be very effective. Indeed, it can be too effective. A beautiful pterosaur skull from the Santana Formation prepared in this way was absolutely fabulous to behold, but so fragile that it was almost impossible to handle.

The main aim of preparation, of course, is to expose the fossil as far as possible. In the case of pterosaurs, this generally means revealing as much of the skeleton as one can. Problems arise, however, when soft parts are preserved alongside the bones. Recent investigations of older specimens, mostly from the Solnhofen Limestones, collected and prepared mainly in the 19th century, reveal that fossilized soft parts were much more common than previously realized. Unfortunately, the existence of impressions of wing membranes, skin or claw sheaths often seems to have been overlooked and, as a specimen of Pterodactylus in the collections of the American Museum of Natural History in New York dramatically shows, in several cases they were partially or completely destroyed during work to expose the skeleton.22

Even when fossilized soft tissues were recognized, they occasionally had to suffer the indignity of being "cleaned up," probably to improve the fossil's appearance so that it could be sold for a higher price. A classic example of this practice can be seen in the so-called "Zittel wing," pictured in Figure 3.5, which originally belonged to an individual of Rhamphorhynchus, one of the long-tailed pterosaurs from the Solnhofen Limestone.23 This fossil, collected in the mid-1800s, is famous for having some of the best-preserved impressions of the flight membranes of any pterosaur. In particular, the rear edge of the main flight membrane, the so-called cheiropatagium (see Chapter 8) is remarkably (one might say suspiciously) straight and even— almost certainly not because this is how it was in a living Rhamphorhynchus, but because at some point a scalpel or knife blade was run along this edge to tidy it up. Now, sadly, we will never know the exact shape of the main wing membrane in this particular fossil.

Pencils, Paper, and Pixels Much of the basic information that we have for pterosaurs— anatomical details, measurements, drawings— was collected by previous generations of pterosaurologists using just pencils, paper, sharp eyes and, if they could gain access to one, a good binocular microscope. This was more than adequate when the first pterosaur was found 200 years ago and pretty much remained so until quite recently. Nowadays, laptop computers and digital cameras have largely replaced the paper and pencil, but sharp eyes and a microscope are still obligatory.

Even in the brightest daylight, however, you cannot always see everything. Fortunately for pterosaurologists there are other kinds of light; the most useful of these is ultraviolet. Bathe bones or patches of fossilized soft tissues in its skin-burning rays and, if certain minerals are there, the fossil will fluoresce. With the judicious use of color filters and plenty of patience, one can produce high-contrast photographs that show a fluorescing fossil in brilliant detail. Helmut Tischlinger, a pterosaurologist from Stammham in Bavaria, who has spent a lifetime studying, preparing and photographing Solnhofen Limestone fossils, is the master of this technique.24 Several of his superb photographs adorn this book (for example, the first and last illustrations in Chapter 1) and reveal the incredible sharpness of detail visible in ultraviolet light. Features that normally are barely discernible suddenly leap out at the viewer. In another of Tischlinger's photos (Figure 8.1), the magic of ultraviolet light brings forth the delicate tracery of blood vessels in a pterosaur wing membrane.

Those who would like to see even further can turn to several different pieces of modern equipment. Ultrafine anatomical details, such as the minutiae of bone cells and the microstructure of the protective layer of enamel that coated pterosaur teeth, can be viewed with a scanning electron microscope. Another heavy-duty piece of equipment, the CAT scanner, has also been of much aid to pterosaurologists. Larry Wimer and his group from Ohio State University linked up with the CAT scanning team at Austin, Texas, to peer inside the skulls of two different pterosaurs.25 Using computers to render the internal volume of the braincase as a "solid" three-dimensional image revealed some previously unsuspected details of the brain, not least the remarkably large size of the organs for balance— which as Chapter 8 explains, led to some surprisingly new ideas about how a pterosaur's wings worked.

While scrutinizing fossil remains in any and every possible way is the starting point for investigating pterosaurs, many other techniques and approaches have enabled pterosaurologists to take this understanding further.

There is a long tradition of model making, much of which has been aimed at trying to determine how pterosaurs flew. One of the earliest attempts in this direction was made by Erich von Hoist, a renowned German scientist who was passionately interested in animal flight. At the German Paleontological Society meeting in Wilhelmshaven in 1956, he demonstrated a rubberband-powered model of Rhamphorhynchus that, according to eyewitness accounts,26 flew most elegantly. More recently, pterosaurologists have turned, with some success, to testing models in wind tunnels, investigating the behavior of the extraordinary head crest of Pteranodon and experimenting with different configurations of the wings.2'

All the tools we have seen so far, however, are overshadowed by a technological development that has dramatically stepped up the pace of research on pterosaurs (and other fossils), and has reshaped the way we do that work and communicate it to one another: It is also the same tool on which I am doggedly tapping out these words— the desktop computer.28 Computers provide an extremely effective way of organizing information about pterosaurs, whether it concerns their anatomy, the dimensions of their bones, where they were found, or the age of the rocks from which they were collected. These machines also save huge amounts of time when it comes to analyzing information; in a few microseconds, they can identify and illustrate growth patterns that in the past took weeks to calculate and graph with a pencil and paper. Perhaps even more importantly, computers now permit us to communicate our findings far and wide (and very fast), and to participate in virtual research teams whose members live on different continents and might actually meet each other only once or twice in a lifetime.

Computers have already had a huge impact on pterosaur research, but this is only the start. Try to trace in your mind's eye the exact three-dimensional trajectory of each and every bone in a pterosaur leg as it extends and flexes through a single step. Now try to do this for all four limbs at once— a walking pterosaur. Not easy, is it? But it can be with a computer. Using measurements from a superbly preserved, uncrushed skeleton of a Santana pterosaur and a piece of software downloaded for free from the World Wide Web, Don Henderson, a computer-literate physicist turned paleobiologist now working at the University of Calgary, Canada, developed a computer model that could be used to test pterosaurs' walking ability. How the first virtual pterosaur, which Henderson nicknamed "Robodactylus," fell over, flew apart and eventually performed is revealed in Chapter 9.

Computers have also been busy elsewhere— right at the very heart of 21st century pterosaurology. Modern methods of discovering how species are related to one another— and this goes for all organisms, not just pterosaurs— use a basketful of techniques that are collectively known as phylogenetic systematics.29 This work is founded on tables of data that consist of tens or even hundreds of species that form the tables' rows, and the hundreds or even thousands of characters— shape of the teeth, number of toes— that vary among these species and that make up the tables' columns. Where each row and column intersect lies a number, a single data point, that tells you exactly what kind of character was present in a particular species— the large crest on the skull of Pteranodon, or the spiked prow on the lower jaw of Rhamphorhjnchus?" Each data point provides a tiny clue as to how species were originally related to one another, so phylogenetic systematics takes advantage of this and tries to fit all the thousands ofbits of data together in as harmonious a fashion as possible, thereby revealing the pattern of relationships among species.31 The problem is that even with small tables that have relatively few rows and columns there can be millions or even billions of ways of fitting together the data points, many of them only very slightly less harmonious than the most harmonious solution. It would take humans several lifetimes to do this work by hand, which is where computers come in: the bigger and faster, the better.

Tables ofphylogenetic data now exist for pterosaurs, too.32 They are still modest, with rows of species numbering in the tens and columns of characters at only 100 or so, but they still need a computer to search for the most harmonious combination and reveal how pterosaurs are related to one another. This type of research only really got under way in the 1990s, but most of the main branches of the pterosaur evolutionary tree (we will meet them in the next chapter) have already been sketched out, although, just as for many other groups of organisms, the exact arrangement of many of the twigs continues to be hotly disputed. Still, the importance of this tree cannot be overemphasized: It forms the fundamental framework upon which our understanding of pterosaur evolutionary history is being built, and it also lies behind much of what is written in this book— yet without computers, most of this tree would still be invisible.

On the Record Our journey from living pterosaur, via death, fossilization, burial, discovery, preparation and study, back to a pterosaur that lives, at least in the human intellect, is done. But that still leaves one important question: What, in fact, do we have, in terms of pterosaur fossils? The answer would fill an entire book. That book was written by Peter Wellnhofer and is called The Illustrated Encyclopedia of Pterosaurs. All we require here is an up-to-date summary of the pterosaur fossil record, while those who desire a fuller account should turn to the appendix at the back of this book or get a copy of Wellnhofer's Encyclopedia.

As you will have already deduced from this chapter, pterosaurs have what can only be described as a modest fossil record, at best.33 In terms of sheer numbers of individuals in museum collections, common fossils such as ammonites can be counted in the millions, and even groups such as fish number in the tens of thousands. By contrast, only about 5,000 to 6,000 pterosaur individuals have been collected so far (Figure 3.9). These fossils range from a few hundred complete skeletons, through all possible combinations of incompleteness, to several thousand single, isolated bones, or even just bone fragments. Unsurprisingly, fossilized soft parts of pterosaurs are rare. To date, they have been reported in just over 100 individuals, are often rather patchy, and, as a rule, are usually associated with relatively complete skeletons. In addition to these body fossils (where original remains are preserved), thousands of footprints and trackways made by pterosaurs wandering along the edges of rivers, lakes and seashores have recently come to light at several sites around the world.34

As Figure 3.9 illustrates, most of the "good" fossils, by which I mean those that are sufficiently complete and well enough preserved to tell us something about the biology of pterosaurs, have been recovered from just a small number of fossil localities. These sources, such as the Solnhofen Limestones of Bavaria, the Jehol Group of northeast China and the Santana Formation of Brazil, are separated by long periods of time and large geographical distances. Thus, while some evidence of pterosaurs has been reported from almost all the 24 stages (those 6-million year or so blocks of time that geologists use to slice up the past) that lie between the oldest records in the Upper Triassic and the youngest at the end of the Cretaceous, the quality of the evidence is very uneven.

FIGURE 3.9 An overview of the fossil record of pterosaurs. (F, flapling; J, juvenile; A, adult)
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