On the far side of a quiet green valley, where ferns curled like sleepy fingers and warm mist floated above a silver river, lived two young dinosaurs named Sato and Mireya.

Sato was a little horned dinosaur with a broad nose and three small bumps above his eyes. Mireya was a long-necked dinosaur with a tail that swept gentle lines in the sand whenever she walked.

They lived beneath a cliff striped with orange, gray, and purple stone. Every evening, the valley filled with familiar sounds. Water whispered over pebbles. Small winged lizards clicked from the trees. Giant dragonflies hummed above the reeds.

But on this particular evening, just as the sun turned the clouds the color of ripe peaches, Sato noticed something unusual near the riverbank.

“Mireya,” he called softly. “Come and look.”

Mireya lowered her long neck beside him.

In the smooth mud were footprints.

At first, they looked like ordinary dinosaur tracks. There were round ones from duck-billed dinosaurs, little three-toed prints from quick runners, and wide, heavy marks from someone with very large feet.

But these footprints were strange.

They changed.

One set began as three neat toes pressed into the mud. A few steps later, the toes were longer. Then they became wider. Then, farther along, they looked almost like the prints of another creature entirely.

Sato leaned close, his tiny frill nearly touching the ground.

“Perhaps a dinosaur was changing as it walked,” he said.

Mireya gave a thoughtful little rumble. “Dinosaurs do not change feet while taking a stroll.”

“Perhaps it had mud on its toes.”

“Then why did the mud change the shape of every single print?”

Sato looked again. Mireya was right.

The tracks seemed to be telling a secret.

They began near the river, crossed a patch of damp grass, curved around a round black boulder, and then disappeared into a narrow path between tall horsetail plants.

The sky was growing softer. Soon the first stars would shine.

“We should go home before moonrise,” said Mireya.

“Yes,” said Sato.

Neither of them moved.

Then Sato said, “We could follow them just a little way.”

Mireya looked toward home. Their families would be gathering near the cliff, sharing leaves and resting beneath the warm rocks.

“Just a little way,” she agreed.

So Sato and Mireya followed the changing footprints.

The path led them past the river reeds, where little fish flashed beneath the surface like moving pieces of sky. It led them under the low branches of ancient trees with bark as wrinkled as an old turtle’s shell.

Soon they came to the first change.

The valley floor, usually soft with grass and moss, became cracked and dry.

Sato stopped in surprise.

“This place is thirsty,” he said.

The footprints here were narrow and sharp. Each toe point pressed deeply into the dusty ground.

Mireya sniffed the air. It smelled warm and faintly spicy, like sun-baked stone.

“I have never seen this part of the valley so dry,” she said.

They continued. The cracked ground stretched farther than either dinosaur expected. Where there were usually pools full of floating leaves, there were only shallow bowls of dry mud.

Near one empty pool, they found a pile of pale shells.

“What are those?” asked Sato.

Mireya nudged one gently with her nose.

“River shells,” she said. “My grandmother says small creatures once lived inside them.”

“Why are they here if there is no water?”

Mireya thought for a moment. “Maybe there used to be water.”

A breeze moved through the dry grass. It made a soft shushing sound, as if the valley itself were trying to remember.

The footprints changed again.

They were no longer sharp and narrow. Now they were broad, with soft edges, as though the creature who made them had stepped through something squishy.

Sato’s eyes grew wide.

“Maybe there is mud ahead!”

“Or maybe,” said Mireya, “we are walking into another old secret.”

They followed the tracks over a low rise.

On the other side, both dinosaurs froze.

Before them stood a forest.

Not the green, leafy forest they knew.

This forest was made of stone.

Tall tree trunks rose from the earth, gray and hard. Their branches reached upward, but there were no leaves. Some were broken. Some leaned against one another. Some had strange ripples across them, as if they had once been soft and flowing.

Sato stepped carefully between two stone roots.

“Were these trees?” he whispered.

“I think so,” Mireya whispered back.

She ran the tip of her tail along one trunk. It made a quiet scraping sound.

“Why are they stone now?” Sato asked.

Mireya did not know.

They walked deeper into the stone forest. The changing footprints passed between the trees, though here they were difficult to see. Instead of pressed marks, they appeared as little dips in the rock, smooth and worn.

Then Sato noticed something glittering.

In a crack between two stone roots lay a small, curled shape.

It looked like a sleeping snail, except it was made of shining blue-gray rock.

Mireya carefully lifted it with her mouth and placed it in Sato’s palm.

“It is very old,” she said.

“How old?”

“Older than my grandmother’s grandmother’s grandmother.”

Sato stared at the tiny curled stone.

“That is very old.”

“Yes.”

He put it back exactly where he had found it.

The footprints led onward, out of the stone forest and toward the base of a hill.

There, something even stranger waited.

A wall of earth had been split open, perhaps long ago by rain or shaking ground. Its sides showed many layers, one above another.

At the bottom was dark brown soil.

Above that was a band of smooth gray mud.

Above the mud was yellow sand.

Then a layer of tiny white shells.

Then red dust.

Then black stone.

It was like the valley had been stacking blankets for a very, very long time.

Sato tilted his head.

“Do you think the footprints are inside that hill?”

Mireya looked at the layers.

“Maybe not the footprints we are following. But perhaps other footprints. Footprints from old days.”

Sato imagined dinosaurs walking where they stood, but before the hill had become a hill. He imagined rain falling and rivers swelling. He imagined muddy ponds, dry plains, forests, and fields of ferns.

“The valley did not always look like it does now,” he said.

“No,” said Mireya. “It changed.”

Just then, a low sound rolled through the air.

Hooooo.

Sato jumped close to Mireya’s side.

“What was that?”

Another sound came from above.

Hooooo.

It was not a roar. It was not thunder. It was the wind blowing through a hollow in the hill.

But as it sighed, a small tumble of sand slipped down the layered wall.

Under the sand, Sato saw something.

Three long grooves.

Then another three.

And another.

Footprints.

They were much larger than Sato’s feet, and much wider than Mireya’s. They crossed the gray mud layer inside the hill, as clear as if they had been made that morning.

Sato forgot to be afraid.

“Look,” he breathed.

Mireya’s eyes were soft and round in the twilight.

“A dinosaur walked there,” she said. “Long, long ago.”

“Was it someone we know?”

“Not someone we know. But perhaps it had friends. Perhaps it liked the river. Perhaps it searched for sweet leaves at sunset.”

Sato looked at the old tracks. He could almost see the ancient dinosaur walking through mud while rain clouds gathered above its head.

Then he looked down at the changing footprints they had followed.

At last, he understood.

“They are not from one dinosaur,” he said.

Mireya smiled. “No. I do not think they are.”

The prints had changed because the ground had changed. In dry earth, the tracks were sharp. In wet mud, they were wide. In stone, they had become shallow dents. Each kind of ground had kept the footsteps differently.

And the places around them had changed too.

The dry pool had once been a riverbed. The stone forest had once been living trees. The hill had once been mud beneath an ancient sky.

The valley was like a great storybook, Sato decided.

Only instead of pages, it had rocks.

Instead of words, it had shells, roots, ripples, and tracks.

The moon rose over the hill, bright and round. It poured pale light across the valley.

“We should go home now,” said Mireya gently.

Sato nodded.

They turned back, but this time they did not hurry. As they walked, the path seemed less mysterious and more wonderful.

They crossed the stone forest, which now looked peaceful beneath the moon. They passed the dry pools, where Sato imagined fish once swimming. They returned through the tall horsetail plants and found the river shining as it always had.

At the riverbank, the changing footprints ended.

Or perhaps, Sato thought, they did not end at all.

Perhaps every creature made marks on the world. Sometimes the marks lasted only until rain washed them away. Sometimes they stayed hidden inside hills for longer than anyone could count.

When they reached home, their families were waiting beneath the striped cliff.

Sato’s mother asked, “Where have you two been?”

Mireya opened her mouth, ready to explain.

But Sato spoke first.

“We went for a walk through an old season.”

His mother blinked.

Mireya’s father gave a quiet laugh. “Did you?”

“We saw a dry valley where there should have been pools,” said Sato. “And trees that had turned to stone. And tracks inside a hill.”

The grown dinosaurs listened. No one laughed this time.

Mireya’s grandmother, who had scales the color of mossy rocks, looked toward the moonlit hill in the distance.

“The valley has many memories,” she said. “It is good to notice them.”

That night, Sato and Mireya curled up beneath the warm cliff.

Sato rested his head on a bed of soft fern fronds. Mireya folded her long neck beside him, her tail wrapped in a gentle curve around her feet.

Above them, the stars blinked one by one.

Sato thought about the old footprints under the hill. He thought about rain from ages ago, forests from ages ago, and dinosaurs whose names no one remembered.

Then he thought about tomorrow.

Tomorrow, the river would sparkle. The ferns would sway. New footprints would appear in the mud.

And someday, perhaps, far in the future, another curious pair of young dinosaurs might find them.

With that happy thought, Sato’s eyes grew heavy.

Mireya gave a soft, sleepy rumble.

The river whispered beyond the reeds.

The ancient valley held its secrets close.

And beneath the wide, starry sky, the two young discoverers dreamed of walking through seasons that had not yet happened.