There was no simple cause for rises in numbers like this. The burrowers were locked into intricate ecological cycles involving the abundance of the vegetation and insects they browsed, and the carnivores who preyed on them in turn. At times of excess bodies it was the burrowers’ instinct to get away, to strike out blindly over the green land in search of empty places to establish new burrows. Many of them fell to predation, but that was the way of things: Enough of them would survive.

At least, that was how it had happened in the past. But now, as the ice advanced and the tundra shriveled back, there was nowhere left to go that wasn’t already colonized. And so there were always great crowds like this, and you always had to fight.

Of course, it was bad for the mutta who had laid these eggs. The muttas hatched their eggs on the ground, as their ancestors had always done, which made them vulnerable to opportunistic predators like the burrowers. Indeed, the key cause of the decline in the muttas’ numbers was the increasing competition for the protein locked up in their huge eggs. Giant mammalian herbivores, like mammoths or caribou, would have fared better here, as their young would have been safer at such a crucial moment in their lives. But the muttas, stranded like the rest when Antarctica had sailed away from the other continents, had had no choice in the matter.

Suddenly a claw came sweeping out of the sky. With an instinct more than two hundred million years old, Dig flattened herself against the ground, while burrowers squeaked and scrambled over each other.

The claw grabbed a small, immature burrower and popped her whole into a gaping mouth. Again the claw burned through the air, grasping as if frustrated. But the mammals had scattered. And after a time Dig heard the unmistakable sound of chewing, as a toothed beak crushed one mutta embryo after another.

This bandit was a leaellyn. Another dinosaur, it looked like an athletic chicken. Not equipped to be effective hunters of large prey, the leaellyns were mainly opportunistic scavengers. For this leaellyn, like the mammals, a mutta egg this late in the season was a rare treat.

As the leaellyn fed, Dig tried to lie still to avoid the killer’s attention. But she was hungry. It had been a short, poor summer, and it had been impossible for her to fatten herself up as much as she needed to face the privations of winter. And the leaellyn was taking the eggs — taking all of her eggs.

Anger and desperation at last overrode caution. She raised herself up on her hind legs, hissing, her paws spread.

The leaellyn, blood and yolk smeared around its mouth, flinched back, startled by this sudden apparition. But, its small reptilian mind soon told it, this was not a threat to a leaellyn. In fact, this warm furry ball, for all its unusual posture, was good to eat, better than embryos and yolk.

The leaellyn opened its mouth and leaned forward.

Dig evaded and escaped. But she had to abandon the nest, and hunger burned inside her belly.

Dig could have traced her lineage back to Plesi, the little carpolestid who had inhabited the warming world a few million years after the fall of the Devil’s Tail. Plesi’s offspring had wandered the planet, using land bridges, islands, and rafts to cross from one island continent to another. One branch of the ancient family had crossed a land bridge between South America and Antarctica at a time when the southern continent had yet to settle over the pole.

And here it had encountered dinosaurs.

Even during the warm Cretaceous the dinosaurs of Antarctica had had to endure long months of polar darkness. So those chance survivors who had lived through the global catastrophe here had been well equipped to endure the comet winter that had followed while their contemporaries in the warmer latitudes had perished.

But the continents had drifted further apart, fragments of the wreckage of the ancient supercontinent still separating. Antarctica had spun away from the other pieces of southern Pangaea, soon traveling so far that no land bridge, no rafting was possible. And as the world recovered from the impact, the flora and fauna of Antarctica had begun to explore their own unique evolutionary destiny. Here the ancient game of dinosaur versus mammal had been granted a long, drawn-out coda — and here, still, thanks to the twin ferocity of the dinosaurs and the cold, the mammals had remained trapped in their humiliating Cretaceous niches.

But at last Antarctica had settled over the southern pole, and the great ice cap had slowly grown.

The days grew short, the crimson sun arcing only briefly over the horizon. The ground hardened with frost. Many plant species died back to ground level, their spores waiting out the return of the summer’s brief warmth.

There was little fresh snow. In fact, much of the continent was technically a semidesert: What snow did fall came as hard crystalline flakes that rested on the ground like rock until the wind gathered it into banks and drifts.

But the snow, sparse as it was, was essential for the burrowers.

Those who had survived the summer and autumn began to dig into the snow drifts, constructing intricate tunnel systems beneath the hard-crusted upper layers. The tunnels were elaborate, humid, nivean cities, the walls hardened by the passage of many small, warm bodies, the air filled with a warm, damp-fur smell. The burrows were not exactly warm inside, but the temperature never dropped below freezing.

Outside, auroras flapped silently across the star-stained winter sky.

The leaellyn who had stolen the eggs from Dig was one of a pack, mostly siblings, who had hunted together in a small group centered on one dominant breeding pair. In the winter, as they felt their customary cold-weather torpor come on, the leaellyn pack huddled together.

The leaellyns were descended from small, agile herbivorous dinosaurs that had once swarmed in great nervous clans over the floor of the Antarctic forest. In those days the leaellyns could grow as large as an adult human, and they had big eyes well adapted to the darkness of the polar forests. But with the great chill the leaellyns had become dwarfed, fatter, and covered with scaly feathers for insulation.

And, as the megayears had worn away, they had learned to eat meat.

As the cold deepened, the pack members slid into unconsciousness. Their metabolisms slowed to a crawl, astonishingly slow, just enough to keep their flesh from freezing. It was an ancient strategy, shaped by millions of years of habitation in these polar regions, and it had always proven effective.

But not this time. For there had never been a winter as cold as this. In the worst of it the leaellyn group was overwhelmed by a storm. The savage wind took away too much of their body heat. Ice formed inside the leaellyns’ flesh, shattering the structure of their cells; gradually the frostbite extended cold daggers deep into their small bodies.

But the leaellyns felt no pain. Their slumber was a silent, dreamless, reptilian sleep, deeper than any mammal would ever know, and it segued smoothly into death.

Every year the summers were shorter, the onset of winter harder. Each spring the great ice cap that lay over the center of the continent, a place where nothing could live, advanced a little further. Once there had been tall trees here: conifers, tree ferns, and the ancient podocarps, with clusters of heavy fruit at their bases. It had been a forest where Noth would have been at home. But now those trees existed only as seams of coal buried deep beneath Dig’s feet, long since felled by the cold. It had been many millions of years since any of Dig’s ancestors had climbed off the ground.

The primates of Antarctica had had to become adapted to the cold. They could not grow larger; competition with the dinosaurs saw to that. But they developed layers of insulation, fat and fur, designed to trap their body heat. Dig’s feet were kept so cold there was little temperature difference between them and the ground, and little heat was lost. Cold blood coming up into her torso from her feet was pushed through blood vessels containing warm blood that ran the other way. So the descending blood was cooled before it ever got to her feet. The fat in her legs and feet was a special kind, made of shorter hydrocarbon chains, with a low melting point. Otherwise it would have hardened, like chilled butter. And so on.