Meanwhile more chicks were pushing out of the ground, hatching all at the same time. It was as if the ground were suddenly swarming with baby dinosaurs. The didelphodon, and more predatory mammals, closed in to feed.

An ancient survival strategy had been operating. Dinosaurs were reptiles who laid their eggs on the ground. Though some parents stayed with their brood, there was no way all the vulnerable eggs and chicks could be protected. So dinosaurs laid many eggs, and their hatchings were synchronized. There should have been dozens of broods hatching right now, scattered through this area of forest: hundreds of chicks. The idea was that suddenly the forest floor would be overwhelmed with baby dinosaurs, far too many for even the hungriest of predators. Most of the chicks would die, but that didn’t matter. It was enough that some would survive.

But here, tonight, the strategy had gone wrong — horribly so for the dinosaur chicks. The mother of these chicks was a hunter isolated from her pack. Confused, hungry, scared of predation herself, she had laid her eggs in the old, familiar place — this rookery was millennia old — and covered them with rotting vegetation for warmth. She had done just what she should have done, save that it was the wrong time, and the eggs had been forced to hatch without the cover of hundreds of others.

The air was filled with the stink of blood, the low growls of the predators, and the sad peeping of the doomed chicks. There were many species of mammals represented here at this grisly banquet. The largest was the big didelphodon. There was a pair of deltatheridiums, ratlike omnivores, neither marsupial nor placental, a unique line that would not outlive the dinosaurs. Many of the creatures here had potential far beyond their present standing; one unprepossessing little creature was an ancestor of the line that would lead to the elephants.

But for now, all that concerned them was their empty bellies. Dissatisfied with the slow emergence of the struggling hatchlings, the mammals had already started to dig into the loose silt, seeking unbroken eggs, scattering the cover of moss laid over the nest by the mother dinosaur.

By the time Purga arrived the rookery had become a killing pit, a squirming mass of feeding mammalian bodies. Purga, late to the fray, burrowed eagerly into the dirt. Soon tiny bones crunched in her mouth. And, so deeply did she immerse her head in search of the deep-buried goodies, she was the last to sense the return of the mother dinosaur.

She heard an angry bellow, felt the ground shudder.

Her snout sticky with yolk, Purga pulled her head out of the dirt. The other mammals were already vanishing into the forest’s welcoming green black. For one instant Purga saw the whole creature, an unlikely feathered monster suspended in the air, limbs splayed, mouth gaping. Then a vast clawed hand flashed out of the sky.

Purga hissed and rolled. Too late she learned that this was the nest of a troodon: an agile, fast-moving killer — and a specialist hunter of mammals.

The troodon’s name meant “Wounding Tooth.”

Wounding Tooth, the size of a dog, was not the largest of dinosaurs, but she was intelligent and agile. Her brain compared in size to that of the flightless birds of later eras she somewhat resembled. Her eyes were as large and as well night-adapted as Purga’s, and they could see forward, giving her binocular vision, the better to triangulate on her small, fast-moving targets. She had legs that enabled her to spring like a kangaroo, a long sicklelike claw on the second toe of each foot, and hands like spades evolved specifically to dig out and crush scuttling mammals.

She was coated in small sleek feathers, an elaborate development of scales. The feathers weren’t meant for flying, but for warmth during the night’s chill. In the equable climate that swathed the Earth in these times, you didn’t need a hot-blooded metabolic engine to keep warm: If you were big enough, your cold-blooded body would retain its heat right through the night, even if you lived at Earth’s extremes, at the poles. But smaller dinosaurs, like the troodon, needed a little extra insulation.

Small or not, she had one of the largest brains of all dinosaurs. All in all, she was a well-equipped hunter. But Wounding Tooth had problems of her own.

She could not know it, but they had been caused by the widening of the Atlantic, the huge geological event that had dominated the whole of this Cretaceous period. As the Americas were pushed west, North America’s huge inland seaway had shallowed and drained, and close to the western coast — just a few hundred kilometers from the troodon’s hatching site — that line of new volcanoes had erupted like an angry wound. The volcanism had disturbed the complex web of life in many ways. The young volcanoes were almost continually active, belching out smoke and ash ladened with sulfur that, mixing with the rain, turned to acid. Many species of plants had vanished, and trees on the higher ground had been reduced to bare trunks. Elsewhere the destruction had been more direct, with vast fingers of cold lava reaching deep into the forest.

The troodon’s mammalian food, relatively close to the base of the food chain, had been less disturbed than most of the larger species of predatory dinosaurs. In fact, with their tiny bodies, deep burrows, and fast reproductive rate, the mammals were better equipped to live through such times of stress than the land’s grander overlords.

But troodons were pack hunters. And this female had, some days ago, become isolated from her pack by a spectacular venting of hot steam from a fissure. Even though she was alone, Wounding Tooth was carrying eggs from her last fertilization. So she had come to the herd’s ancient roosting site. A deep part of her had hoped to find others of her kind here. But there was no other here, only herself.

Wounding Tooth was growing older. At fifty, she found many of her much-stressed joints were racked with the pain of arthritis. And, because of her age and loss of strength and flexibility, she herself was under threat: This was, after all, a time of predators powerful enough to justify armor plating on creatures bigger than elephants. She had to reproduce; every instinct demanded it.

She had laid her eggs, as she had before. What else could she do?

The nest itself was a circular pit scraped out of the dirt, and she had arranged the eggs with an odd, almost surgical precision. She made sure the twenty eggs were not too close together, and that the top of each elongated egg pointed to the center so the emerging chicks would have a good chance of digging their way out. Then she had covered over the eggs with dirt and moss. She had returned several times to the nest, probing with her claws to tap the shells. The eggs had developed well; she could see it. But now the eggs had hatched — her young had emerged — but nothing was left of them except scattered bits of red flesh and gnawed bone. And here, in the center of the smashed nest, was a mammal, her face stained by blood and yolk and dirt.

Which was why Wounding Tooth leapt.

Purga helplessly squirted urine and musk, leaving a scent warning: Beware! Mammal hunter about! Then she ran out of the forest, back to the clearing of the ankylosaurs.

But on the edge of the clearing Purga hesitated. She had a choice to make, a choice of dangers. She had to get away from the pursuing troodon. She was heading back toward her burrow, where her pups waited. But by crossing the clearing again she was leaving the safety of the trees. The unconscious calculus rapidly produced a result. She took the gamble; she raced across the clearing.

A sleepy infant giant raised one bony eyelid.

The light seemed brighter than ever now, exposing Purga clearly. But there was no dawn; it was only the comet, its nucleus huge, blurred, and bright, the gas jets that erupted from it clearly visible, even through the haze of air. It was an eerie, extraordinary sight that sparked a dim curiosity in her agile mind, even as she ran.