On the day her strength finally ran out, it took only a few minutes of the herd’s steady, ground-covering trot for her to become separated from the pack.

The orniths were waiting. They had been waiting for days. They reacted immediately.

Three males moved in first, all of them sons of Listener. They stalked around the matriarch, cracking their whips, flimsy bits of treated leather that emulated the supersonic crackle of the diplos’ tails.

Some of the diplo herd looked back dimly. They made out the matriarch, and her tiny predators. Even now the million-year programming of the diplos’ small brains could not accept that these skinny carnivores presented any threat. The diplos turned away, and continued their relentless feeding.

The matriarch could see the capering, diminutive figures before her. She rumbled her irritation, the great boulders grinding in her stomach. She tried to lift her head, to bring her own tail to bear, but too many joints had fused to painful immobility.

Now the second wave of hunters moved in. Armed with poison-tipped spears, and using the claws of their hands and feet, they attacked the matriarch as allosaurs once had, striking and retreating.

But the matriarch had not survived more than a century by chance. Summoning up the last of her energy, ignoring the hot aches that spread from the pinpricks in her side, she reared up on her hind feet. Like a falling building, she towered over the band of carnivores, and they fled before her. She crashed back to the earth with an impact like a sharp earthquake, her slamming forefeet sending waves of pain through every major joint in her body.

If she had fled then, if she had hurried after her herd, she might have survived, even thrown off the effects of the spears. But that last monumental effort had briefly exhausted her. And she was not given time to recover. Again the hunters closed in, striking at her with their spears and claws and teeth.

And here came Listener.

Listener had stripped naked, discarding even the whip around her waist. Now she flew at the diplo’s flank, which quivered mountainously. The hide itself was like thick leather, resistant even to her powerful claws, and it was crisscrossed by gullies, the scars of ancient wounds, within which parasitic growths blossomed, lurid red and green. The stink of rotten flesh was almost overwhelming. But she clung there, digging in her claws. She climbed until she had reached the spines that lined the matriarch’s back. Here, Listener bit into the diplo’s flesh and began to rip away at the horny plates embedded beneath.

Perhaps in some dark corner of her antique mind the diplo remembered the day she had ruined this little ornith’s life. Now, aware of new pains on her back, she tried to turn her neck, if not to swipe away the irritation, at least to see the perpetrator. But she could not turn.

Listener did not stop her frantic, gruesome excavation until she had dug down to the spinal cord itself, which she severed with a harsh bite.

For long days the mountain of meat served to sustain the nation of hunters, even as the young played in the cavernous hall of the matriarch’s great ribs.

But Listener was criticized, in angry head bobs, dances, and gestures. This is a mistake. She was the matriarch. We should have spared her until another emerged. See how the herd is becoming scattered, ill-disciplined, its numbers falling further. For now we eat. Soon we may starve. You were blinded by your rage. We were foolish to follow you. And so on.

Listener kept her own counsel. For she knew the damage the loss of the matriarch had done to the herd, how badly it had been weakened, how much less were its chances of survival. And she knew it did not matter, not anymore. For she had smelled the salt.

When the matriarch was consumed, the hunting nation moved on, following the savannah corridor to the east as it had always done, walking in the herd’s unmistakable wake of trampled ground and crashed trees.

Until they ran out of continent. Beyond a final belt of forest — beyond a shallow sandstone cliff — an ocean lay shining. The giant diplos milled, confused, in this unfamiliar place, with its peculiar electric stink of ozone and salt.

The herd had reached the eastern coast of what would become Spain. They were facing the mighty Tethys Sea, which had forced its way westward between the separating continental blocks. Soon the Tethyan waters would break through all the way to the west coast, sundering a supercontinent.

Listener stood on the edge of the cliff, her forest-adapted eyes dazzled by its light, and smelled the ozone and salt she had detected so many days ago. The matriarch was dead, destroyed, but it did not matter. For, after walking across a supercontinent, the diplodocus herd had nowhere to go.

The orniths might have fared better had they had a more flexible culture. Perhaps if they had learned to farm the great sauropods — or even simply not to pressure them so hard in this time of change — they might have survived longer. But everything about them was shaped by their origins as carnivorous hunters. Even their rudimentary mythos was dominated by the hunt, by legends of a kind of ornitholestes Valhalla. They were hunters who could make tools: that was all they would ever be, until there was nothing left to hunt.

The whole of the orniths’ rise and fall was contained in a few thousand years, a thin slice of time compared to the eighty million years the dinosaur empire would yet persist. They made tools only of perishable materials — wood, vegetable fiber, leather. They never discovered metals, or learned how to shape stone. They didn’t even build fires, which might have left hearths. Their stay had been too brief; the thin strata would not preserve their inflated skulls. When they were gone the orniths would leave no trace for human archaeologists to ponder, none but the puzzle of the great sauropods’ abrupt extinction. Listener and her culture would vanish. Like the great air whale and innumerable other fabulous beasts, they would vanish forever.

With a sudden stab of loss, Listener hurled her spear into the ocean. It disappeared into the water’s glimmering mass.

Once interplanetary impacts had been constructive, a force for good.

Earth had formed close to the brightening sun. Water and other volatiles had quickly boiled away, leaving the young world an empty theater of rock. But the comets, falling in from the outer system, delivered substances that had coalesced in that cooler region: especially the water that would fill Earth’s oceans, and compounds of carbon, whose chain-based chemistry would lie at the heart of all life. Earth settled down to a long chemical age in which complex organic molecules were manufactured in the mindless churning of the new oceans. It was a long prelude to life. It would not have come about without the comets.

But now the time of the impacts was done, so it seemed. In the new solar system, the remaining planets and moons followed nearly circular orbits, like a vast piece of clockwork. Any objects following more disorderly paths had mostly been removed.

Mostly.

The thing that now came out of the dark, its surface of dirty slush sputtering in the sun’s heat, was like a memory of Earth’s traumatic formation.

Or a bad dream.

In human times, the Yucatan Peninsula was a tongue of land that pushed north out of Mexico into the gulf. On the peninsula’s northern coast there was a small fishing harbor called Puerto Chicxulub (Chic-shoe-lube). It was an unprepossessing place, a limestone plain littered with sinkholes and freshwater springs, agave plantations, and brush.