The forest of this age was dominated by tall araucaria and ginkgoes. In the open spaces there was a ground cover of ground ferns, saplings, and pineapple-shrub cycadeoids. But there were no flowering plants. This was a rather drab, unfinished-looking world, a world of gray-green and brown, a world without color, through which the hunters stalked.

Listener was first to hear the approach of the diplo herd. She felt it as a gentle thrumming in her bones. She immediately dropped to the ground, scraping away ferns and conifer needles, and pressed her head against the compacted soil.

The noise was a deep rumble, like a remote earthquake. These were the deepest voices of the diplos — what Listener thought of as belly-voices, a low-frequency contact rumble that could carry for kilometers. The diplo herd must have abandoned the grove where it had spent the chill night, those long hours of truce when hunters and hunted alike slid into dreamless immobility. It was when the diplos moved that you had a chance to harass the herd, perhaps to pick off a vulnerable youngster or invalid.

Listener’s mate was called Stego, for he was stubborn, as hard to deflect from his course as a mighty — but notoriously tiny-brained — stegosaur. He asked, They are moving?

Yes, she replied. They are moving.

Hunting carnivores were accustomed to working silently. So their language was a composite of soft clicks, hand signals, and a ducking body posture — no facial expressions, for the faces of these orniths were as rigid as any dinosaur’s.

As they approached the herd, the noise of the great animals’ belly-voices became obvious. It made the very ground shake: The languid fronds of ferns vibrated, and dust danced up, as if in anticipation. And soon the orniths could hear the footfalls of the mighty animals, tremendous, remote impacts that sounded like boulders tumbling down a hillside.

The orniths reached the very edge of the forest. And there, before them, was the herd.

When diplodocus walked, it was as if the landscape were shifting, as if the hills had been uprooted and were moving liquidly over the land. A human observer might have found it difficult to comprehend what she saw. The scale was wrong: Surely these great sliding masses must be something geological, not animal.

The largest of this forty-strong herd was an immense cow, a diplo matriarch who had been the center of this herd for over a century. She was fully thirty meters long, five meters tall at the hips, and she weighed twenty tons — but then even the youngsters of the herd, some as young as ten years old, were more massive than the largest African elephant. The matriarch walked with her immense neck and tail held almost horizontal, running parallel to the ground for tens of meters. The weight of her immense gut was supported by her mighty hips and broad, elephantine legs. Thick ropelike ligaments ran up her neck, over her back, and along her tail, all supported in canals along the top of her backbone. The weight of her neck and tail tensed the ligaments over her neck, thereby balancing the weight of her torso. Thus she was constructed like a biological suspension bridge.

The matriarch’s head looked almost absurdly small, as if it belonged to another animal entirely. Nevertheless this was the conduit through which all her food had to pass. She fed constantly; her powerful jaws were capable of taking bites out of tree trunks, huge muscles flowing as the low-quality food was briskly processed. She even cropped in her sleep. In a world as lush as this late Jurassic, finding food wasn’t a problem.

Such a large animal could move only with a chthonic slowness. But the matriarch had nothing to fear. She was protected by her immense size, and by a row of toothlike spines and crude armor plates on her back. She did not need to be smart, agile, to have fast reactions; her small brain was mostly devoted to the biomechanics of her immense body, to balance, posture, and movement. For all her bulk the matriarch was oddly graceful. She was a twenty-ton ballerina.

As the herd progressed the herbivores snorted and growled, lowing irritably where one mighty body impeded another. Under this was the grinding, mechanical noise of the diplos’ stomachs. Rocks rumbled and ground continually within those mighty gizzards to help with the shredding of material, making a diplo’s gut a highly efficient processor of variable, low-quality fodder that was barely chewed by the small head and muscleless cheeks. It sounded like heavy machinery at work.

Surrounding this immense parade were the great herbivores’ camp followers. Insects hovered around the diplos themselves and their immense piles of waste. Through their swarms dove a variety of small, insectivorous pterosaurs. Some of the pterosaurs rode on the diplos’ huge uncaring backs. There was even a pair of ungainly protobirds, flapping like chickens, running around the feet of the diplos, snapping enthusiastically at grubs, ticks, and beetles. And then there were the carnivorous dinosaurs, who hunted the hunters in turn. Listener spotted a gaggle of juvenile coelurosaurs, gamely stalking their prey among the tree-trunk legs of the herbivores, at every moment risking death from a carelessly placed footfall or tail twitch.

It was a vast, mobile community, a city that marched endlessly through the world forest. And it was a community of which Listener was part — where she had spent all her life, which she would follow until she died.

Now the diplo matriarch came to a grove of ginkgoes, quite tall, ripe with green growth. She raised her head on its cable neck for a closer inspection. Then she dipped her head into the leaves and began to browse, tearing at the leaves with her stubby teeth. The other adults joined her. The animals began simply to barge down the trees, snapping trunks and even ripping roots out of the ground. Soon the grove was flattened; it would take decades for the ginkgoes to recover from this brief visit. Thus the diplos shaped the landscape. They left behind a great scribble of openness, a corridor of green savannah in a world otherwise dominated by forest, for the herd so ravaged the vegetation of any area that it had to keep moving, like a rampaging army.

These were not the mightiest herbivores — that honor went to the giant, tree-cropping brachiosaurs, who could grow as massive as seventy tons — but the brachiosaurs were solitary, or moved in small groups. The diplo herds, sometimes a hundred strong, had shaped the land as no animal had before or since.

This loose herd had been together — traveling forever east, its members changing, its structure continual — for ten thousand years. But there was room for such titanic journeys.

Jurassic Earth was dominated by a single immense continent: Pangaea, which meant “the land of all Earth.” It was a mighty land. South America and Africa had docked to form a part of the mighty rock platform, and a titanic river drained the heart of the supercontinent — a river of which the Amazon and Congo were both mere tributaries.

As the continents had coalesced there had been a great pulse of death. The removal of barriers of mountain and ocean had forced species of plants and animals to mix. Now a uniformity of flora and fauna sprawled across all of Pangaea, from ocean to ocean, pole to pole — a uniformity sustained even though vast tectonic forces were already laboring to shatter the immense landmass. Only a handful of animal species had survived the great joining: insects, amphibians, reptiles — and protomammals, reptilian creatures with mammalian features, a lumpen, ugly, unfinished lot. But that handful of species would ultimately give rise to all the mammals — including humans — and to the great lineages of birds, crocodiles, and dinosaurs.

As if in response to the vast landscape in which they found themselves, the diplos had grown huge. Certainly their immensity was suitable for these times of unpredictable, mixed vegetation. With her long neck a diplo could work methodically across a wide area without even needing to move, taking whatever ground cover was available, even the lower branches of trees.