Second was dropped. He clattered against bare rock. He glimpsed the adult azhdarchid flap away. He scrabbled at the rock, but it was too hard to burrow into.

And little monsters closed in on him, blue-black in the comet light. Fed by their parents' proteinaceous offerings of fish and meat, the chicks were growing quickly. But their wings were still undeveloped, and their bodies and heads were relatively large. They looked like miniature dinosaurs.

The first beak nipped at Second's hind leg, almost playfully. The scent of his own blood evoked sudden memories of the burrow. He experienced a kind of regret. He bared his teeth. The ravenous chicks closed around him. It was over in a heartbeat, his warm body torn apart.

But now something moved, far above the mother azhdarchid. She twisted her sketch of a head to peer upwards. In these tall Cretaceous skies, fueled by the oxygen-rich air, a pyramid of predators had erected itself, with all the savagery of its landbound analogues. But when she saw the vast sprawled shadow, skimming over the comet-bright sky above above the lowest of the clouds, she knew she was in no danger. the lowest of the clouds, she knew she was in no danger.

It was only an air whale.

The largest flying animal ever discovered by humans was a type of azhdarchid christened Quetzalcoatlus. Its wingspan of fifteen meters was four times that of the largest birds, condors; it had looked like a light airplane.

But the greatest pterosaur of all was an order of magnitude larger again.

The air whale's tremendous, delicate wings were one hundred hundred meters across. His bones were little more than sketches, strut-filled and hollow, astonishingly light. His mouth was vast, a translucent cavern. His main danger was overheating in the unfiltered sunlight of the high air, but his body had a number of mechanisms to compensate, including the capacity to vary the flow of blood in his tremendous wings, and air sacs placed in his body that enabled his internal organs to lose heat. meters across. His bones were little more than sketches, strut-filled and hollow, astonishingly light. His mouth was vast, a translucent cavern. His main danger was overheating in the unfiltered sunlight of the high air, but his body had a number of mechanisms to compensate, including the capacity to vary the flow of blood in his tremendous wings, and air sacs placed in his body that enabled his internal organs to lose heat.

He lived his life in that thin, high layer of air called the stratosphere, higher than the mountains, above most of the clouds. But even this far from the ground there was life: a thin ethereal plankton of insects and spiders, windblown. Sometimes mating swarms of midges, or even locusts, could be blown up into this lofty realm. This was the whale's thin bounty, which he scooped endlessly into his vast mouth.

Far below, if he had chosen to look, the air whale might have glimpsed the little drama of Second, the azhdarchid chicks, and the pterosaur. But from up here such remote events were of little interest. When he looked down over his airy domain, the whale could see the curve of the Earth: the fat blue band of thicker air that marked the horizon, and the glimmering of the sea in the light of the comet. The sky above faded to violet at the zenith. He was so high that there was too little air to scatter the light effectively; despite the brightness of the comet, he could see stars.

The air whale was capable of circumnavigating the globe, following the stratospheric winds and seeking updrafts, without once touching the ground. His kind made up a thin population- the aerial plankton could sustain no more- but they were scattered all over the planet. Three or four times in his life he had mated, summoned to the planet's highest mountain peaks by innate timing mechanisms triggered by the motion of the sun. Mating was perfunctory and uninteresting; such huge, delicate creatures couldn't afford the displays and courtship rituals of more terrestrial species. Nevertheless ancient instincts did sometimes come to the fore. There could be fights, often savage, almost always lethal, and when that happened, huge flimsy bodies would rain out of the skies, to baffle ground-based scavengers.

The whale was the end product of a brutal evolutionary competition, mostly aimed at removing weight; everything that had been surplus to requirements had, over the generations, been selected out or shriveled to insignificance. And, since nothing ever happened up here in the cool stratosphere, those diminished organs included the whale's brain. The whale was at once the most spectacular but among the most stupid of his great family; his brain, though a fine control center for his elaborate flight systems, was little more than an organic adding machine. So the magnificent astronaut's eye view before him meant nothing.

Only the warm oxygen-laden air of the late Cretaceous would allow such immense, delicate creatures to escape from gravity's clutches, and never again would there be a gene bank like the pterosaur's to supply raw material for similar evolutionary experiments. Never again would any creature fill this particular ecological niche, and in the future windblown insects would sail in peace.

And human paleontologists, piecing together this remote era from fragments of bone and fossilized plant, would learn little of the true giants. Most pterosaur bones found would be of marine and lakeland species, because that was where fossils were most easily preserved. By comparison the creatures that dominated the roof of the world, the upland areas and mountaintops, left few traces, for their habitats were subject to ferocious uplift and erosion. The highest mountains of the human era, the Himalayas, did not even exist in the Cretaceous.

The fossil record was patchy and selective. All through time there had been monsters and wonders that no human being would ever know had existed- like this immense flyer.

With the most delicate of touches from his immense extended forefingers the whale banked his wings and soared toward a particularly rich layer of aerial plankton.

The cruel night was not yet done with Purga.

Despite the loss of Second, she continued to forage. There was no choice. Death was common; life continued. There was no time to grieve.

But when she returned to her burrow a small, narrow face came pushing out of the dark toward her, a twitching, mobile snout, bright black eyes, quivering whiskers: one of her kind, another male.

She hissed and backed out of the burrow entrance. She could smell blood. The blood of her pups.

It had happened again. Without hesitation Purga launched herself at the male. But he was fat and strong- evidently a good forager- and he pushed her away easily. Without hesitation Purga launched herself at the male. But he was fat and strong- evidently a good forager- and he pushed her away easily.

In despair she ran out into the dangerous dawn, where mountainous dinosaurs were starting to stir, the air resonating to the first long-distance calls of the hadrosaurs. She made for an old fern she knew, around whose roots the ground was dry and crumbling. Quickly she dug herself in, ignoring the moist squirms of the worms and beetles. Once she was safe in her cocoon of soil she lay there trembling, trying to shut out of her head the dread stink of her pups' blood.

The strange male, on discovering Purga's scent marks- the scent of a fertile female- had followed them back to the burrow, carefully covering over her marks with his own in order to hide her from any other males.

When he had entered the burrow the pups had clustered around the stranger, his same-species smell overwhelming his warning aura of not-of-my-family. He could smell from the traces of fur and dung that a healthy, fertile female lived here. The female was of use to him, but not the pups. They did not smell of him; they were nothing to do with him. Without them, the female would have that much more incentive to raise the litter he would give her.

For the male, it was all utterly logical. The two larger pups had mouthed his belly, seeking milk, even as the male had consumed their younger sister.

The night after that the male found her again, having tracked her scent. He still stank of her dead babies, of the lost part of her. She fought him off savagely.

It took her two more nights before she accepted his courtship. Soon her body would begin to incubate his young.

It was hard.

It was life.

It would have been of no consolation to Purga to know that this brutal landscape, which had swallowed up two of her broods, would soon be overwhelmed by a wave of suffering and death to dwarf anything she had endured.

IV.

Earth was now inside inside the comet's swelling coma, the loose cloud of gases that swathed the nucleus itself. the comet's swelling coma, the loose cloud of gases that swathed the nucleus itself.

All over the night side of Earth the tail could be seen stretching away from the sun. It was as if the planet had drifted into a sparkling tunnel. The sky glittered with meteors, tiny bits of comet falling harmlessly into the high atmosphere, creating a light show glimpsed by uncaring dinosaurs.

But the comet's nucleus was bigger than any meteor. It moved at twenty kilometers per second, at interplanetary speeds. It had already crossed the orbit of the Moon.

From where it would take just five more hours to reach Earth.

All night long, the birds and pterosaurs sang their confusion; during the day, they slumped with exhaustion. There was no room in their neural programming for a new light in the sky, and they were disturbed at a deep cellular level. In the shallow seas, too, the unending light had disturbed the plankton and larger creatures like crab and shrimp; the cynical hunters of the reef fed well.

Only the great dinosaurs were unperturbed. The comet light made no difference to the air temperature, and when true night had fallen they slipped into their usual torpor. On the last night of a reign that had lasted nearly two hundred million years, the rulers of the Earth slept untroubled.

If not for the tyrannosaur eggs, the young giganotosaur would have spotted the disturbed troodon earlier. In the lee of the mountains, he stalked silently through the green shadows. His name meant Giant.

The forest here was sparse, spindly araucaria and tree ferns, scattered over a ground littered with sharp-edged volcanic rocks. Nothing moved. Anything that could hide had already hidden; anything else lay still, hoping for the shadow of death to pass by.

He came to a pile of moss and lichen. Superficially it looked like a heap of debris piled up at random by wind or the passage of animals. But Giant recognized the characteristic scrapings, the lingering smell of meat eater.

It was a nest.

With a rumble of anticipation, he fell on the nest and began to dismantle it with his stubby forearms. When he had exposed the eggs, Giant dug his clawed thumb, with a surgical precision, into the top of the largest. He pulled out the embryo head first. As the mucus and yolk drained from it, lurid colors bright, Giant saw the chick squirm feebly, even saw its tiny heart beat.

Just as the embryos of chimps, gorillas, humans would all be disturbingly similar, so dinosaur fetuses all looked alike. There was no way to know that this chick would have been a female tyrannosaur. Blind, deaf, immature, the embryo struggled to open her mouth, dimly imagining the hulking shape of a mother who would feed her. Giant flicked the embryo into his mouth and swallowed it without chewing. The chick's life ended in crushing, acidic darkness.

It didn't matter. Even if no predator had come this way, her egg would have been destroyed before she could have hatched by a monster even more terrible than a giganotosaur.

Giant was descended from South American stock that had crossed a temporary land bridge into this continent a thousand years earlier.

In a world of slowly separating island continents, the dinosaur fauna had become diverse. In Africa there were archaic-looking long-necked giant herbivores and creatures like hippos with fat, low-slung bodies and powerfully clawed thumbs. In Asia there were small, fast-running horned dinosaurs with noses like parrots' beaks. And in South America large sauropods were hunted by giant pack-hunting predators; there it was like a throwback to earlier times, to Pangaea. The giganotosaurs had cut their evolutionary teeth hunting the great South American titanosaurs.

Giant was an immature male, and yet he already outmassed all but the very largest carnivores of the era. Giant's head, in proportion to his body, was larger than a tyrannosaur's- and yet his brain was smaller. The giganotosaurs were less agile, less fast, less bright; they had more in common with the ancient allosaurs, equipped to kill with teeth and hands, whereas the tyrannosaurs, all their evolutionary energy funneled into their huge heads, specialized in immense, sharklike bites. Where the tyrannosaurs were solitary ambush hunters, the giganotosaurs were pack animals. To bring down a sauropod fifty meters long and weighing a hundred tons, you didn't need brains as much as raw strength, rudimentary teamwork- and a kind of reckless frenzy.

But, coming across that land bridge into a new country, the giganotosaurs had been forced to confront an established order of predators. The invaders had quickly learned that their takeover of a region could not succeed unless they first mounted a bloody coup against the ruling carnivore.

Which was why this young male giganotosaur was munching slippery tyrannosaur embryos. Resolutely Giant cracked one egg after another. The carefully constructed nest turned into a mess of shattered eggs, scattered moss, and chunks of dismembered chick. Giant was feeding well- and issuing a challenge.

It would be a transfer of power. The tyrannosaur had been the top predator, mistress of the land for a hundred kilometers around, as if the whole elaborate ecosystem was a vast farm run for her benefit. The prey species had come to terms with the formidable presence that lived amongst them: with their armor or weapons or evasive strategies, each of the hunted had reached a point where their losses to predators were not a threat to the endurance of the herd.

Given time, all that would have changed. The impact of the invaders' hunger would have rippled down the food chain, disturbing creatures large and small, before a new equilibrium could be established. It would have taken longer still for the prey species to learn new behaviors, or even evolve new coping systems or armor to deal with the giganotosaurs.

But none of that would happen. There would not be time for the giganotosaur clan to exploit their triumph. Not in the few hours left.

The nest destroyed, Giant wandered away. He was still hungry, as always.

He could smell putrefaction in the still, misty air. Something huge had died: easy meat, perhaps. He pushed through a bank of tree ferns and emerged into another small clearing. Beyond, through a screen of greenery, he could dimly see the black flank of a young volcanic mountain.

And there, in the middle of the clearing, was a dinosaur- a troodon- standing quite still above a scraping of earth.

Giant froze. The troodon had not seen him. And she was alone; there were none of the watchful companions he associated with the packs of this particular agile little dinosaur.

There was something wrong with the way she was behaving. And that, so the grim predatory calculus of his mind prompted him, gave him an opportunity.

Wounding Tooth should have been able to overcome the loss of a clutch of eggs.

This was a savage time, after all. Infant mortality rates were high; and at any time of life, sudden death was the way of things. This was the world the troodon had been evolved to cope with.

But she could not cope, not anymore.

She had always been the weakest of her brood. She would not even have survived the first days after her hatching if not for the chance decimation of her siblings by a roaming marsupial predator. She had grown to overcome her physical weakness, and had become an effective hunter. But in a dark part of her mind she had always remained the weakest, robbed of food by her siblings, even eyed as a cannibalistic snack.

Add to that a slow poisoning by the fumes and dust of the volcanoes in the west. Add to that an awareness of her own aging. Add to that the hammer blow of her lost brood. She hadn't been able to get Purga's scent out of her head.

It had not been hard to pursue that scent out of her home range, across the floodplain to the ocean shore, and now to this new place where the scent of Purga was strong.

Wounding Tooth stood still and silent. The burrow, her nose told her, was right under her feet. She bent and pressed one side of her head against the ground. But she heard nothing. The primates were very still.

So she waited, through the long hours, as the sun rose higher on this last day, as the comet light grew subtly brighter. She did not even flinch when meteors flared overhead.

If she had known about the giganotosaur that watched her she would not have cared. Even if she could have understood the meaning of the comet light, she would not have cared. Let her have Purga; that was all.

It was a peculiar irony that her high intelligence had brought Wounding Tooth to this. She was one of the few dinosaur types smart enough to have gone insane.

It was not yet dark. Purga could tell that from the glint of light at the rough portal to the burrow. But what was day, what was night in these strange times?

After several nights bathed in comet light, she was exhausted, fractious, hungry- and so was her mate, Third, and her two surviving pups. The pups were just about large enough to hunt for themselves now, and therefore dangerous. If there was not enough food, the family, pent up in this burrow, might turn on one another.

The imperatives slid through her mind, and a new decision was reached. She would have to go out, even if the time felt wrong, even if the land was flooded with light. Hesitantly she moved toward the burrow entrance.

Once outside, she stopped to listen. She could hear no earth-shaking footsteps. She stepped forward, muzzle twitching, whiskers exploring.

The light was strong, strange. In the sky cometary matter continued to fall, streaking across the dome of the sky like silent fireworks. It was extraordinary, somehow compelling- too remote to be frightening.

An immense cage plunged out of the sky. She scrabbled back toward the burrow. But those great hands were faster, thick ropy muscles pulling the fingers closed around her.

And now she faced a picket fence of teeth, hundreds of them, a tremendous face, reptilian eyes as big as her head. A giant mouth opened, and Purga smelled meat.

The dinosaur's face, with its great, thin-skinned snout, had none of the muscular mobility of Purga's. Wounding Tooth's head was rigid, expressionless, like a robot's. But though she could not show it, all of Wounding Tooth's being was focused on the tiny warm mammal in her grip.

Her limbs pinned against her belly, Purga stopped struggling.

Oddly, Purga, in this ultimate moment, knew a certain peace that Wounding Tooth would have envied. Purga was already in her middle age, already slowing in her movements and thought. And she had, after all, achieved as much as a creature like her could have hoped for. She had produced young. Even encased in the troodon's cold reptilian grip, she could smell her young on her own fur. In her way she was content. She would die- here and now, in heartbeats- but the species would go on.

But something moved beyond the troodon's bulky body, something even more massive, a gliding mountain, utterly silent.

The troodon was unbelievably careless. Giant didn't care why. And he didn't care about the warm scrap she held in her hands.

His attack was fast, silent, and utterly savage, a single bite to her neck. Wounding Tooth had time for a moment of shock, of unbelievable pain- and then, as whiteness enfolded her, a peculiar relief.

Her hands opened. A ball of fur tumbled through the air.

Before Wounding Tooth's body fell Giant had renewed his attack. Briskly he slit open the belly cavity and began pulling out entrails. He expelled their contents by shaking them from side to side; bloody, half-digested food showered the area.

Soon his two brothers came racing across the clearing. Giganotosaurs hunted together, but their society was fragile at the best of times. Giant knew he couldn't defend his kill, but he was determined not to lose it all. Even as he chewed on the liver of Wounding Tooth, he turned to kick and bite.

Purga found herself on the ground. Above her, mountains battled with ferocious savagery. A rain of blood and saliva fell all around her. She had no idea what had happened. She had been ready for death. Now here she was in the dirt, free again.

And the light in the sky grew stranger yet.

The comet nucleus could have passed through the volume of space occupied by Earth in just ten minutes.

In the great boiling it had endured the comet had lost a great deal of mass, but not a catastrophic amount. If it had been able to complete its skim around the sun, it would have soared back out to the cometary cloud, quickly cooling, the lovely coma and tail dispersing into the dark, to resume its aeonic dreaming.

If.

For days, weeks, the great comet had worked its way across the sky- but slowly, its hour-by-hour motion imperceptible to any creature who glanced up at it, uncomprehending. But now the bright-glowing head was sliding sliding: sliding down the sky like a setting sun, sinking toward the southern horizon.

All across the daylit side of the planet, silence fell. Around the drying lakes the crowding duckbills looked up. Raptors ceased their stalking and pursuit, just for a moment, their clever brains struggling to interpret this unprecedented spectacle. Birds and pterosaurs flew from their nests and roosting places, already startled by a threat they could not understand, seeking the comfort of the air.

Even the warring giganotosaurs paused in their brutal feeding.

Purga bolted for the darkness of her burrow. The disembodied head of the troodon fell behind her, lodging in the burrow's entrance, following Purga with a grotesque, empty stare as the light continued to shift.

CHAPTER 2.