And one of the allosaurs, not far away, turned with glassy interest. Listener stood stock still, shocked.

With a single bound the allo reached Stego. Stego screamed and scrabbled at the mud. The allo poked him curiously, almost gently, with her muzzle.

Then, with astonishing speed, the allo’s head shot forward and delivered a single clean bite, all but severing Stego’s neck. She grabbed him by the shoulder, lifting him high. His head dangled by a few threads of skin, but his body twitched still. She carried him to the edge of the forest, away from the herd, where she began to feed. The process was efficient. The allo had joints within her jaw and skull, so that like a python she could open her mouth wide and position her teeth, the better to consume her prey.

Listener found herself staring stupidly at an allosaur track, a three-toed crater firmly planted in the trampled mud. A hunter without her mate is like a herd without its matriarch: The ornith proverb sounded in her head, over and over.

The big matriarch diplo swung her head around to stare directly at Listener. Listener understood. The orniths’ antics had given the allos their chance to attack. So, with her whiplash, the matriarch had exposed Stego. She had given him to the allos. It had been revenge.

The matriarch turned away, lowing, as if contented.

Something hardened, a dark core, in Listener’s mind.

She knew she would spend the rest of her life with this herd. And she knew that the matriarch was its most important individual; providing protection to the rest with her sheer bulk, leading them with her wisdom acquired over long years. Without her the herd would be much less well coordinated, much more under threat. In a way, this matriarch was the most important individual creature in Listener’s life. In that moment, she swore vengeance of her own.

Each night the orniths retreated to their ancestral forest, where once they had hunted mammals, insects, and the nests of diplodocus. They scattered in little pockets, and surrounded the area with heavily armed sentries. That evening, the mourning was extensive. This ornith nation was only a few hundred strong, and could ill afford to lose a strong, intelligent young male like Stego.

Even as the cold of night drew in, Listener found it hard to rest.

She gazed up at a sky across which auroras flapped, steep three-dimensional sculptures of light, green and purple. In this age Earth’s magnetic field was three times the strength it would be in the human era, and, as it trapped the wind streaming from the sun, the shining auroras would sometimes blanket the planet from pole to pole. But the lights in the sky meant nothing to Listener, and brought no comfort or distraction.

She sought refuge in memories of happier, simpler times when she and Stego, emulating their distant ancestors, had hunted for diplo eggs. The trick was to seek out a patch of forest floor, not too far from the edge, that looked apparently lifeless, strewn with leaves and dirt. If you put your sensitive ear to the ground you could hear, if you were lucky, the telltale scratching of diplo chicks in their eggs. Listener had always preferred to wait, to guard “her” nest from others, until the diplo chicks began to break out of their eggs and stick their tiny heads out of the scattered dirt.

For an inventive mind like Listener’s, there was no end to the games you could play.

You could try to guess which chick would come up next. You could see how quickly you could kill a new emergent, snuffing it out within a heartbeat of its first glimpse of daylight. You could even let the chicks come out of their shells altogether. Already a meter long, with their flimsy tails and necks dangling, the chicks’ only priority was to escape to the deeper forest. You could let a chick get all the way to a patch of scrub — almost — and then haul it back. You could nip off its legs one by one, or bits of its tail, and, crunching the little morsels, see how it still struggled, as long as its brief life lasted, to get away.

All smart carnivores played. It was a way of learning about the world, of how prey animals behaved, of honing reflexes. For their time, orniths had been very smart carnivores indeed.

Once, not more than twenty thousand years ago, a new game had occurred to one of them. She had picked up a handy stick in her grasping hand, and she used that to probe for unbroken eggs.

By the next generation the sticks had become hooks to drag out the embryos, and sharpened spears to stab them.

And by the next, the new weapons were being trialed on bigger game: juvenile diplos, younger than five or six years, not yet part of a herd but already a meat haul worth hundreds of embryonic chicks. Meanwhile a rudimentary language was born, of the subtle communications of pack hunters.

A kind of arms race followed. In this age of immense prey, the orniths’ better tools, more sophisticated communication, and complex structures were quickly rewarded by bigger and better hauls of meat. Ornith brains rapidly expanded, the better to make the tools, and sustain societies, and process language — but there was a need for more meat to feed the big expensive brains, requiring better tools yet. It was a virtuous spiral that would operate again, much later in Earth’s long history.

The orniths had spread all over Pangaea, following their prey herds as they crisscrossed the supercontinent along their vast ancestral corridors of parkway.

But now conditions were changing. Pangaea was breaking up, its backbone weakening. Rift valleys, immense troughs littered with ash and lava, were starting to open. New oceans would be born in a great cross shape: Eventually the Atlantic would separate the Americas from Africa and Eurasia, while the mighty equatorial Tethys would separate Europe and Asia from Africa, India, Australasia. Thus Pangaea would be quartered.

It was a time of rapid and dramatic climate change. The drift of continental fragments created new mountains which, in turn, cast rain shadows across the lands; the forests died back, and immense dune fields spread. Generation by generation — as their range disintegrated, and the vegetation no longer had time to recover from their devastating passage — the great sauropod herds were diminishing.

Still, if not for the orniths, the sauropods might have lingered much longer, even surviving into the great high summer of dinosaur evolution, the Cretaceous.

If not for the orniths.

Though Listener went on to take more mates and to raise proud clutches of healthy and savage young, she never forgot what had become of her first mate, Stego. Listener did not dare challenge the matriarch. Everyone knew that the best chance of the herd’s survival was for the powerful old female to continue her long life; after all, no new matriarch had emerged to replace her.

But, slowly and surely, she drew up her plans.

It took her a decade. Over that time the numbers of diplos in the herd halved. The allosaurs too went into steep decline across the supercontinent as their prey animals became scarce.

At last, after a particularly harsh and dry season, the old one was observed to limp. Perhaps there was arthritis in her hips, as there evidently was in her long neck and tail.

The time was close.

Then Listener smelled something in the wind from the east, a taste she had not known for a long time. It was salt. And she realized that the fate of the matriarch was no longer important.

At last she achieved a consensus among the hunters.

The great diplo cow was now 120 years old. Her hide bore the scars of failed predator attacks, and many of the bony spines on her back had snapped off. Still she was growing, now massing a remarkable twenty-three tons. But the degeneration of her bones, after their heroic lifetime of load-bearing, had slowed her cruelly.