Pterosaur Planet

Picture a world with a warm, humid climate extending as far north as present-day Alaska. In this Mesozoic Eden, luxuriant groves of cycads and tree ferns and seemingly endless forests of conifers rang to the crash of ever-browsing herds of truck-heavy, bus-sized sauropod dinosaurs. Wreaking swathes of destruction, these mountains of flesh were trailed by quick, bright-eyed, hook-clawed theropods, ceaselessly eyeing the sick, the young and the unwary. Offshore, schools of Leedsichthys, each fish the size of a whale, slowly sieved the waters of deep blue ammonite-filed seas, while refracted shadows of snake-necked plesiosaurs and spear-snouted ichthyosaurs played on the sides of their titanic bulk..1

FIGURE 2.1 Predator and prey. Collected from the Karatau region of Kazakhstan in the 1960s by a team of Russian paleontologists led by Alexander Sharov, most of this slab is occupied by a superbly preserved skeleton of the pigeon-sized Late Jurassic pterosaur Sordes pilosus (a restoration of the same pterosaur adorns the cover of this book), surrounded by patches of fossilized skin and hair. This pterosaur was buried in mud at the bottom of a freshwater lake rimmed by dense stands of conifer trees, fragments of which also ended up in the sediment and are now preserved as fine black flecks scattered across the slab. The lakes were also home to Sordes' prey— palaeoniscoid fish— one example of which was, by chance, preserved alongside its predator.

The world in which pterosaurs lived was very different from our own. It was not completely different, though; some things would have looked rather familiar: mosses, ferns, conifers, insects, even turtles and lizards haven't changed all that much, at least in external appearance, since they were perched on, eaten, or flown over by pterosaurs. But in other ways, things were very alien. Life on land was dominated not by mammals, as it is today, but by a remarkable panoply of dinosaurs. Many of these were herbivores that, in turn, were preyed upon by an astonishing array of theropods, not least, the king of them all, Tyrannosaurus rex. Mammals, including the line that eventually culminated in you, me and other members of Homo sapiens, were mostly mouse-sized, while familiar plants such as grasses lay unimaginably far in the future.

This chapter considers how scientists discovered, mapped and reconstructed the world in which pterosaurs lived, and how that world changed, sometimes dramatically, during their 140-million-year tenure. The journey takes in global geography and climate, and along the way, we catch some brief glimpses of the animals and plants that formed the backdrop to the everyday life of pterosaurs. First, though, we will attempt to comprehend the incomprehensible: deep time.

Long, Long Ago Deep time, like quantum physics and cricket,2 is really hard to grasp, and generations of writers have expended considerable amounts of this commodity trying to devise metaphors to help you, the reader, come to terms with the concept. One of the most popular metaphors invites us to consider Earth's entire history, all 4 billion years of it, as a 24-hour clock, a scale within which our own species appears at about 4 seconds before midnight. Impressive, but it doesn't quite convey the sheer inelfableness of deep time, so let's try something else.

If we assume that, on average, humans produce a new generation approximately every 20 years,3 then five human generations equate to a century, 50 generations to a millennium and 500 generations, or 10 millennia, takes us back to the end of the last Ice Age and the dawn of civilization. If we step up a scale and use the entire length of human civilization as our basic unit, then we need at least 100 of these (50,000 human generations) just to reach back a million years. This means that to return to the last moment in time when pterosaurs existed—the final days of the Mesozoic, 65 millions years ago—one would have to experience a time span equivalent to the whole of human civilization repeated 6,500 times, which is equivalent to about 3 million human generations.

For pterosaurs, however, this was the end of what had already been a stupendously long history At this, their final moment on Earth, they had been around (or rather above) for more than 150 million years, having first appeared at least 215 million years ago. Or, to try to frame this in a human context, pterosaurs lived, died and evolved for a period equivalent to more than 15,000 human civilizations. This also means, rather surprisingly, that the last pterosaurs existed much closer to us in time than to their earliest ancestors.

The time interval in which pterosaurs lived is referred to as the Mesozoic (Figure 2.2) and divided by geologists into three periods: the Triassic, toward the end of which pterosaurs first appeared; the Jurassic, during which, apart from insects and the original early bird, Arcbaeopteryx, pterosaurs were practically the only creatures to be seen in the skies; and the Cretaceous, when the heavens must have thrummed with multitudes of pterosaurs and birds. Each period is subdivided into Early, Middle and Late (apart from the Cretaceous, which has no Middle), and each of these is more finely divided into units of time called stages, usually named after a region or location where the stage was first defined.4

Stages, and the longer intervals to which they belong, such as periods, are firmly embedded in a relative time scale tied to the fossils of what were abundant, rapidly evolving creatures such as ammonites (which floated by the millions in Mesozoic seas) and an absolute time scale based primarily on the slow decay of certain radioactive isotopes.5 The significance of the stage here is that the most accurate geological date we can obtain for most pterosaur fossils is only to the stage level.6 Since, typically, stages are about 6 million years in length (equivalent to about 600 human civilizations), this means that, as a rule, it is rather difficult to resolve any "short-term" events in pterosaur history, unless they lasted at least a few million years.

An Ever-Changing World By pooling information from a wide range of disciplines, scientists have been able to develop a surprisingly detailed picture of the geography, climate, vegetation and faunas of the Mesozoic worlds in which pterosaurs lived.6 Matching the fit of coastlines and using other types of information such as paleomagnetic data,7 which give away the positions of continents in the past, scientists are able to track the drift of land masses and reconstruct the geographical history of our world. Features of sediments that make up the rock record, such as the size, shape and composition of the individual grains from which they are formed and the fossils they contain, give geologists clues about the local geography and climate.

Prehistoric Life

FIGURE 2 . 2 Pterosaurs and time. The history of the world from its inception around 4,500 million years ago to the present is depicted in the left column. Life is thought to have first appeared some 700 million to 800 million years later, but only really got going at the beginning of the Phanerozoic, shown in the middle column, about 542 million years ago. The 150 million year long history of pterosaurs, together with some of the more important pterosaur fossil localities, is illustrated in the right column.

FIGURE 2 . 2 Pterosaurs and time. The history of the world from its inception around 4,500 million years ago to the present is depicted in the left column. Life is thought to have first appeared some 700 million to 800 million years later, but only really got going at the beginning of the Phanerozoic, shown in the middle column, about 542 million years ago. The 150 million year long history of pterosaurs, together with some of the more important pterosaur fossil localities, is illustrated in the right column.

Slice up samples of fossilized wood into sections so thin that you can see right through them and, under the microscope, variations in cell size can be used to detect wet and dry seasons and their relative intensity. Keep collecting more plant fossils, and eventually paleobotanists (paleontologists who specialize in the study of fossil plants) will be able to reconstruct much of the ancient flora of the region.

If we could go back 215 million years to the Late Triassic, a time when the first rather primitive-looking rhamphorhynchoid pterosaurs were beginning to flap through the skies, we would encounter a world very different from that of today. Rather than being spread all over the globe, as they are now, most of the continents were fused together in a single huge land mass called Pangaea (Figure 2.3, top). This had a profound effect on the climate: There were no ice caps, average global temperatures were much higher than today, and in general, the differences between seasons were far less marked. As a consequence, much of the interior of Pangaea was dry and arid and seems to have been dominated by extensive deserts.

Out on the coastal plains, conditions were more humid, especially in the vicinity of the main waterways. Here, plants could grow in abundance, but the Late Triassic flora over which early pterosaurs flitted would have looked quite strange to our eyes. Tree ferns, ginkgoes, cycads and conifers dominated the vegetation, forming extensive forests whose shady understory was carpeted by myriad ferns and mosses. Elsewhere, immense drifts of reed ferns and horsetails fringed swampy regions and colonized the edges of lakes and rivers.

The start of the Jurassic, approximately 199 million years ago, saw several well-established groups of long-tailed pterosaurs living around the margins of a supercontinent that was beginning to break up and whose coastal regions had been flooded by rising sea levels. By the end of this period (Figure 2.3, middle), Pangaea had split right through the middle, forming two land masses: Laurasia in the north, composed of modern-day North America, Europe and Asia, and Gondwanaland, formed from Antarctica, South America, Africa, Australasia and India, in the south. This new configuration also brought about important climatic changes. Tropical or subtropical conditions extended across both land masses but, crucially, it was more humid than before the split and the differences between the seasons were more pronounced, with hot, dry summers and cooler, wetter winters.

The vegetation was rather like that in the Late Triassic and, as the humidity increased, ferns, in particular, prospered in the lowland areas, forming vast, dense tracts. Drier upland environments were dominated by

Prehistoric Life
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