Jurassic

Cycad Continental Drift

main continental land masses and oceans are shown for the Upper Triassic (top), Upper Jurassic (middle) and Upper Cretaceous (bottom). The red dots pinpoint locations where pterosaurs were fossilized.

cycads and conifers, some reaching 30 or 40 meters (120 feet) high. Among them was a new type of cycad, the bennettitalean, with a short trunk and a well-developed bush-like crown. This particular cycad was so extraordinarily successful that it ended up completely dominating the flora and waxed so luxuriant that vast accumulations of its rotting remains eventually led to the formation of Jurassic coals.

As new kinds of short-tailed pterosaurs spread across the world at the start of the Cretaceous, the breakup of Pangaea progressed even further. Australasia and Antarctica broke away from the rest of Gondwanaland, and the remaining land mass, South America and Africa, had also completely separated by the Late Cretaceous (Figure 2.3, bottom). The rift that split them apart continued northward and had sundered North America and Greenland from Europe by the end of the Mesozoic. In addition to all this continental drift, sea levels rose to some of their highest levels ever in the early Late Cretaceous, flooding over the continents and creating a Pteranodon-haunted midcontinental seaway down through North America and an archipelago of islands where Europe stands today.

Initially as warm as in the Jurassic, temperatures cooled toward the middle of this period and seem to have become more variable toward the end of the Cretaceous. The vegetation also underwent some major changes. Early Cretaceous floras were similar to those of the Jurassic, except for the appearance of a new group: angiosperms, or flowering plants. At first, these newcomers remained small and shrubby, but in the Late Cretaceous they grew much bigger and evolved into many new types of plants, prominent among them magnolias, sycamores and oaks, which formed forests that largely replaced those of cycads and ginkgoes.

Meet the Neighbors Like birds today, pterosaurs were widespread and lived as members of many different communities, not only in mountains, forests or on the plains, but also in coastal regions and even far out to sea. Many of the simpler types of animals that formed the bulk of these communities— worms, spiders, crabs, shellfish, corals and sponges— are little different from their living descendants that surround us now. Indeed, the single most important group by almost any measure, insects, was already well-established,8 and the pterosaur world hummed, buzzed and whirred with dragonflies, beetles, bugs and cockroaches, just as our world does today. Not everything was the same, though. One type of shellfish common in the Mesozoic, the brachiopod, though still with us, has dwindled to but a shadow of its former glory, and the molluscan epitome of the Mesozoic, the ammonites, probably one of the commonest animals in the seas over which pterosaurs soared, died out, together with the dinosaurs.

Some of the backboned animals that roamed the Mesozoic lands or swam in the seas would also have seemed rather familiar. Modern sharks and rays and the so called ray-finned fish (actinopterygians), which include early forms such as sturgeon and paddle fish, were common in the later Mesozoic. Actinopterygians also include the greatest of all the finny races, teleosts— bony fish. The bewildering variety of modern teleosts— ranging from seahorses to sail fish— is only the latest development of a group that first rose to dominance during the Cretaceous (Figure 2.4)■ In the Jurassic and earlier, the seas, lakes and rivers that teleosts later filled were occupied by older, more ancient groups of fish. Prominent among them were hybodontids, a group of early, spiny-finned sharks, and a large heterogeneous association of fish called the paleonisciforms, often of rather small size, that were common in the Triassic and Jurassic and probably figured largely in the diet of many pterosaurs.

The top predators in the seas were several groups of reptiles: ichthyosaurs, plesiosaurs, and arriving somewhat later, mosasaurs.9 Originally descended from small lizard-like animals that lived on land, these creatures took to the water very seriously. Ichthyosaurs became completely adapted for a life under the ocean waves. Like modern whales and dolphins, even their young were born in the water, as revealed by several fossilized remains of pregnant mothers that died while giving birth. Generally rather dolphin-shaped, but with vertical fish-like tails, ichthyosaurs were typically about 2 to 3 meters (6 to 9 feet) long, though some whale-sized forms reached lengths of 15 meters (about 50 feet) or more. Most species had long jaws that were crammed with simple, sharp-pointed teeth that enabled ichthyosaurs to get a good grip on their prey, which, if their fossilized stomach contents are anything to go by, was primarily squid.

Plesiosaurs first appeared in the Late Triassic, some time after the ich-thyosaurs, but whereas the latter seem to have died out at the start of the Late Cretaceous, plesiosaurs, like ammonites, dinosaurs and pterosaurs, survived to the very end of the Mesozoic (Figure 2.4). Famous for their peculiar appearance "like a snake threaded through a barrel," these animals had a long neck, a short tail and two pairs of flippers, with which they propelled

FIGURE 2.4 Meet the neighbors. A selection of the most important groups of backboned animals that shared the Mesozoic world with pterosaurs. The dagger symbol indicates the point in time when a particular group is thought to have become extinct.

Paleonisciforms

themselves through the water. Such an arrangement does not seem well-designed for high-speed swimming, especially because it is not at all clear how the flippers were deployed so that they did not interfere with one another. Consequently, it is thought that plesiosaurs are more likely to have been ambush predators, lurking in the murky depths waiting for their prey to swim by, rather than pursuit predators like the ichthyosaurs, which chased down their dinners. Smaller forms probably lived on fish, but the big plesio-saurs, such as Liopleurodon, with a skull 2 meters (6 feet) long, a total body length of 15 meters (45 feet), and some of the biggest teeth in the animal kingdom, were probably capable of killing and eating anything else in the sea, including sharks, ichthyosaurs and other plesiosaurs.

Propelled by a powerful tail and stabilized by paddle-shaped limbs, mo-sasaurs, like ichthyosaurs and plesiosaurs, were superbly adapted to a life in the water, even giving birth there. Descended from true lizards that took to the seas in the early Late Cretaceous, mosasaurs evolved several different lifestyles: Some developed large, globular teeth and went in for feeding on shellfish while others, among them huge 15 meter-long (50 foot-long) leviathans such as Mosasaurus, preyed on fish or perhaps even other mosasaurs.

Several important groups of land-living backboned animals, still alive today also first appeared at about the same time as pterosaurs. Mesozoic lakes and ponds were home to the earliest frogs and salamanders and many, but not all, crocodiles. Some small, agile crocodiles roamed the land, while several Cretaceous species seem to have become vegetarian and taken to chomping the undergrowth.10 Yet another group of crocodiles, the thalat-tosuchians, evolved paddle-like limbs and became completely adapted to a life in the seas, presumably only returning to land to breed. Plodding along patiently, protected by their shells, turtles were there, too. Among the most common of vertebrate fossils, they were to be found worldwide in the Jurassic and Cretaceous. They also took to the seas, where some species became giants, reaching the size of a family car— 4 meters (13 feet) in length and up to a ton in weight.

Not all groups opted for gigantism, though. Mammals were highly successful (literally so in some cases, where the design of the limbs shows that they must have been completely at home up in the cycads and tree ferns), spreading across the entire Mesozoic world and evolving into many different types and kinds, but with one proviso— they always stayed small.11 Rarely larger than a rabbit, and usually smaller than a rat, Mesozoic mammals lived in the shadows, feasting on insects, shoots or seeds, and we, their descendants, might still be there today if all but one lineage of dinosaurs (the birds), had not eventually become extinct.12

Dinosaurs, of course, are what made the Mesozoic really different from our modern mammal-dominated world. They seem to have risen to prominence at about the same time as pterosaurs; they rapidly became the dominant land animal and stayed on top for the next 150 million years.13 Often large, or even gigantic when compared with today's mammals, their complex communities developed on every land mass and must have had a profound impact on both the flora and fauna of the Mesozoic world.

The majority of dinosaurs were plant-eaters. The most successful, and from the flora's point of view the most dangerous, were the sauropods. These huge, long-necked, four-legged behemoths, typified by Diplodocus, a dinosaur familiar to all, must have wreaked havoc on the vegetation, either by consuming it or by trampling it underfoot. Worse still for the landscape, fossilized tracks suggest that sauropods congregated in herds and migrated long distances to find fresh fodder.

Other vegetarians included the armored dinosaurs, such as stegosaurs, whose spikes and plates attest to the need for a really effective defense. In the Cretaceous, other groups of herbivorous dinosaurs came to the fore: had-rosaurs and their relatives the iguanodontians, the ceratopsians (led by the king of the spiky heads, Triceratops) and the pachycephalosaurs— the thick heads. Apart from some of the ceratopsians, who returned to a life on all fours, most of these dinosaurs ran around on long powerful hind limbs, only supporting themselves with their arms when they needed to feed, rest or move slowly.

Inevitably, where there are herbivores there are carnivores— in this case, theropods— some of the biggest and most dangerous predators that ever lived. Theropods were killers. Their jaws were filled, as a rule, with dagger-shaped teeth equipped with sharp cutting edges; many had wickedly hooked claws on their fingers; and some even bore large killing claws on their toes. Their long, powerful hind limbs tucked under the body enabled them to move quickly and efficiently. To cap it off, they were at least as intelligent as any of their potential victims, big or small, and might even have ganged up on their prey by hunting in groups.

Pterosaurs were never free of the menace posed by these predators, as some dramatic fossils emphasize— pterosaur bones with theropod teeth still embedded in them.14 Theropods of all kinds and sizes from tiny killers no bigger than a chicken up to 4- or 5-ton, 12-meter-long (39 feet) "you name it and we've eaten it" monsters such as Tyrannosaurus rex were at the top of the food chain throughout the Jurassic and Cretaceous. They even did something that only pterosaurs had ever done before: took to the air, disguised as birds.

Feathered Friends, or Foes? The earliest birds, represented by the most famous fossil in the world, Archaeopteryx," first appeared on the scene in the Late Jurassic, approximately halfway through the reign of the pterosaurs. The question of the origin of birds, one of the most important and fiercely debated scientific issues of modern times, was recently answered in a most dramatic fashion. Complete, undisturbed skeletons of small theropods from Lower Cretaceous lake sediments of northeast China, surrounded by halos of beautifully preserved feathers, indistinguishable from the feathers of crows, ducks or pigeons living today, show beyond any possible doubt that birds are the direct descendants of meat-eating dinosaurs.16 In fact, so many fossils have been found that the pathway from small, ground-living, fast-running Velociraptor-like theropods to the early birds that swooped and screeched through the Cretaceous skies can now be traced in astonishing detail, even down to the origin and evolution of one of nature's greatest inventions: the leather.1'

Although sharing some basic similarities in body design, birds differ in many important ways from pterosaurs. Most obviously, the wings were composed of feathers, not membranes, and, perhaps even more significantly, the wings of birds were only supported by the arms and had no connection to the legs at all. Freed from any major role in flight, the legs and feet were able to evolve and adapt to doing many other things, among them perching, running and grabbing prey.

These and many other features of birds, such as their warm-blooded physiology and care of the young, seem to have ensured the extraordinary success of this group. After their origin in the Jurassic, many different lineages of birds seem to have appeared quite rapidly, and by the Early Cretaceous, birds had become firmly established worldwide.18 Specialized, flightless diving birds, completely unrelated but remarkably similar to today's loons, fol-lowed,19 and by the Late Cretaceous, the ancestors of at least some modern groups of birds, such as gulls and ducks, had also appeared.

So, it seems that the Cretaceous world was full of birds, which raises an interesting question: How did they achieve this, if pterosaurs were already there? After all, the two living groups of fliers, birds and bats, are pretty well segregated between day and night, with just a few specialists such as owls or fruit bats that trespass in each other's realm. I believe that the answers to this question and to its logical follow-up (Were birds responsible for pterosaurs' extinction?) lie in the different design, construction and function of these two animal aeronauts. This issue is explored in later chapters, the last of which (Chapter 11) returns to the main question: Was the Cretaceous a long, slow, showdown between birds and pterosaurs?

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