Evolution

The raptor erupted from cover. It spun in the air, and its grime-crusted hind claws flashed cruelly. The strikes were made well.

Blood gushed. Bellowing, the anatotitan tried to back away from the water. But the titan’s black entrails slid out of immense, deep belly wounds, steaming. At last she caught her forefeet in the slippery mess. With a sound like thunder she slid forward on her chest. And then, with a spasm, the great hind legs collapsed, rolling the great bulk of her body onto its side.

One of the other anatotitans looked back and lowed mournfully, a deep noise that made the ground under Purga tremble. But the herd was already moving on.

The raptor, panting rapidly, waited for the titan to weaken.

The dinosaurs had first emerged more than a hundred and fifty million years ago, in a time of hot dry climates more welcoming to reptiles than mammals. In those days the continents were fused into the single vast Pangaean landmass, and the dinosaurs had been able to spread across the planet. Since then, continents had fissioned, danced, and whirled, and bands of climate had shifted across the planet. And the dinosaurs had evolved in response.

Dinosaurs were different.

They did not hunt like the mammalian killers of later times. Their cold blood meant they were poor at sustaining speed for long distances; they could never be endurance hunters, running down their prey like wolves. But they had versatile, high-pressure hearts. And the design of their bodies had much in common with birds’: This raptor’s neck bones and torso contained a duct system that drew the air through its lungs, and oxygen could be supplied to its tissues at a tremendous rate. It was capable of short sprints, and could pour a great deal of energy into its attacks.

Dinosaur hunts were events of stillness, of ambush and silence and motionlessness, broken by brief bursts of savage violence.

Mammals were not poorly evolved compared to the dinosaurs. The product of her own track of tens of millions of years of evolution, Purga was exquisitely adapted for the niche in which she made her living. But the brutal facts of energy economics kept mammals caged in the neglected corners of a dinosaur world. Overall, a dinosaur killer made better use of energy than mammals: This raptor could run like a gazelle but it rested like a lizard. It was that combination of energy efficiency and lethal effectiveness that had kept the dinosaurs supreme for so long.

The raptor was something like a huge, ferocious bird, perhaps. Or something like a souped-up crocodile. But it was not truly like those animals. It was like nothing seen on Earth in human times, something no human eye would ever witness.

It was a dinosaur.

This raptor’s preferred way of killing was to burst out of cover and slash at its prey, inflicting wounds that were savage but often nonlethal. The prey might flee, but it would be weakened by raking wounds to its legs and flanks — or hamstringing — blood loss and shock would result. The raptor had poor dental hygiene — its breath stank ferociously — and its bite passed on a mouthful of bacteria. The raptor would follow, perhaps attacking again, perhaps just following the scent of the stinking, infected wounds, until weakness disabled the prey.

Today this raptor had been lucky; it had disabled its victim with a single blow. All it had to do now was wait until the titan was too weak to do the raptor any harm. It could even take its food while its prey was still alive.

The raptor would not trouble with such small fry as Purga while such a giant meal awaited it. Moving cautiously, watchfully, Purga left the shelter of the fern, and scurried across the scrubby floodplain, through the devastated track left by the anatotitan herd, until she reached the security of the trees.

For the first time in four billion years, heat had touched the Devil’s Tail. Fragile ice sculptures older than Earth were quickly lost.

Gases boiled through fissures in the crust. Soon a shining cloud of dust and gas the size of the Moon had gathered around the comet. The wind from the sun, of light and sleeting particles, made the gas and dust stream behind the falling comet nucleus in tails millions of kilometers long. The twin tails were extremely tenuous, but they caught the light and began to shine.

For the first time, uncomprehending eyes on Earth made out the approaching comet.

Spitting, rotating, its dark nucleus founting gases with ever greater vigor, the Devil’s Tail swam on.

III

Another long, hot Cretaceous day wore away.

Purga slept through the day, her new family curled around her. She slept even when her pups suckled. The snug burrow floor was littered with the primates’ soft fur — and it smelled, indubitably, of Purga, of her new mate, and of the three pups who were half of herself.