A shadow fled at the corner of her vision.

Instinctively she darted sideways — just as a dinosaur hand slammed into the ground where she had stood. She ran back into the ankylosaur herd, darting this way and that, seeking cover in the shadow of the lethargic dinosaurs.

The troodon chased her around immense legs. But even the enraged mammal hunter was reluctant to disturb these huge armored beasts. Their clubbed tails could have crushed her in a second. Purga even slithered perilously under one ankylosaur’s huge raised foot, which hovered like a falling moon above her, while Wounding Tooth, frustrated, hissed and scraped at the ground.

At last Purga reached the far side of the clearing. Her scent and instinct guiding her unerringly, she raced into the undergrowth.

Her burrow was pitch dark, too dark even for her huge eyes to make out anything. It was like entering a mouth set in the warm earth. But the burrow was full of the consanguineous smell of her family, and she could hear the snuffle of her two young as they squirmed blindly out of the dark. Soon their warm, tiny mouths were nipping at her belly, seeking her nipples. Her mate was not here; he was out foraging himself in this clear Cretaceous night.

But Wounding Tooth must be close; the scent of warm flesh, fur, and milk that had helped bring Purga home would draw the hunter here too.

The imperatives in her head shifted again. She tucked her infants behind her and pushed her way to the back of the burrow, away from its entrance. Purga, unlike the troodon, was young — just a few months old, in fact — and this was her first brood. And unlike the prolific dinosaurs, Purga’s kind bore few young. She could not afford to lose her brood. And now she prepared to fight for them.

There was a crash behind her.

The roof of packed earth imploded, showering Purga and the pups with dirt. Comet light flooded in, startlingly bright after the few seconds of darkness. It was as if a bomb had fallen. A huge grasping hand reached out of the sky into the burrow. The pups squirmed and squealed, but one of them was impaled on a bloody claw. In an instant its life was over. It was lifted up and out of the burrow, a naked, lifeless scrap, and out of Purga’s life.

Purga hissed her distress. She ran toward the burrow entrance, away from the claw. She could sense the remaining infant, naked and stumbling, hurrying after her. But the wily troodon had foreseen this. That claw now pushed into the entrance, breaking open its earthen walls. Reptile fingers closed, and the second pup’s life was squeezed out, its skull and tiny bones crushed, its organs pulped.

Purga, her world broken apart in a few heartbeats, scrabbled away from the debris of the entrance, away from the broken roof, back to the deepest recesses of the burrow. But again and again that machinelike clawed paw slammed through the roof, breaking it down and admitting more milky comet light.

Purga’s body urged her to flee, to find darkness, a new burrow, shelter — to be anywhere but here. She was even hungry; for such a fast-metabolizing creature as Purga, it had been a long time since she had supped the yolk of Wounding Tooth’s eggs.

But suddenly the strength drained out of her.

She huddled at the rear of her ruined burrow, shivering, folding her paws over her face as if to clean her fur of mites. From the moment of her birth into this world of huge teeth and claws that could flash from the sky without warning, she had struggled to survive by instinct and agility. But now her young were gone. The innate imperatives dissolved, and something like despair settled on her.

And while Purga trembled in the remains of her burrow, a world trembled with her.

If she submitted now, she would leave no living descendants: The molecular river of inheritance would be blocked, here, forever. Others of her kind would breed, of course; other lines would go on into far distant futurity, to grow, to evolve, but not Purga’s line, not her genes.

And not Joan Useb.

Life always had been chancy.

The great clawed hand slammed down one more time, centimeters from Purga. And now Wounding Tooth, impatient, rammed her great head into the burrow. Purga quailed before a wall of snapping teeth.

But as the dinosaur pressed closer, screeching, Purga smelled meat, and crushed bones, and a lingering sweetness of milk. The monster’s hot breath smelled of Purga’s babies.

With a spasm of rage Purga threw herself forward.

The great teeth snapped, scything through the air around Purga like some vast piece of machinery. But Purga squirmed to avoid their flashing arcs, and sank her own teeth into the corner of the dinosaur’s lips. The scaly skin was tough, but she felt her lower incisors sink into the warm, softer flesh inside the creature’s mouth.

Wounding Tooth bellowed and pulled back. Purga, hooked by her own teeth, was dragged out of her burrow and hauled up into the air, up through many times her own body height, up past the scaly belly of Wounding Tooth and into the cold night.

Her mist of rage faded. She twisted her head, ripping away a scrap of dinosaur flesh, and she tumbled backward through the misty air. Even as she fell a great clawed hand swept sideways at her, seeking to grab her. But Purga was a creature of the trees, and she twisted as she fell. Again luck favored her — though the grasping claw came close enough to make a breeze that ruffled the downy hairs of her belly.

She fell onto a patch of trampled dirt. She was momentarily winded. But already teeth and claws were descending again, painted silver by the eerie comet light. With a lithe wriggle Purga rolled over, got her feet under her, and ran into the roots of the nearest tree. Alone, eyes wide, mouth gaping, she huddled there, panting, twitching at every leaf that stirred.

There was a scrap of meat in Purga’s mouth. She had forgotten that it had come from the dinosaur. She chewed it quickly and swallowed, for a moment assuaging the hunger that clamored at her even now. Then she peered around, seeking a safer refuge.

Wounding Tooth paced and bellowed out her frustration.

Purga had chosen life. But she had found an enemy.

II

The Devil’s Tail was as old as the sun.

The solar system had been born out of a rich, spinning cloud of rock and volatiles. Battered by a supernova shock, the cloud quickly coalesced into planetesimals: loosely aggregated lumps of rock and ice that swam chaotically through the dark, like blind fish.

The planetesimals collided. Often they were destroyed, their substance returning to the cloud. But some of them merged. Out of this clattering violence, the planets grew.

Close to the center, the new planets were rocky balls like Earth, baked by the sun’s fire. Farther out, huge misty worlds were born, globes stuffed with gases — even the lightest gases of all, hydrogen and helium, gases manufactured in the first few moments of the universe itself.

And around these growing gas giants, the comets — the last of the icy planetesimals — swarmed like flies.

For the comets it was a dangerous time. Many of them were dragged into the gravity wells of Jupiter and the other giants, their masses feeding those growing monsters. Others were hurled inward by the giants’ gravitational slingshots to the warm, crowded center, there to batter the inner planets.

But a few lucky survivors were hurled the other way, away from the sun and into the huge, cold spaces of the outer dark. Soon a loose cloud of comets formed out here, all of them following vast, slow orbits that could reach halfway to the sun’s nearest stellar neighbor.

One such was the Devil’s Tail.

Out here the comet was safe. For most of its long lifetime its nearest neighbor was as remote as Jupiter was from Earth. And at the farthest point of its orbit, the Devil’s Tail sailed all of a third of the way to the nearest star, reaching at last a place where the sun itself was lost against the star fields, its huddled planets invisible. Away from the heat, the comet quickly cooled and froze hard. Its surface was made black by silicaceous dust, and an epochal frost carved exotic, fragile ice sculptures on its low-gravity surface, a meaningless wonderland that no eye would ever see.