PHIL REESE PERCHED on the very point of a cliff hundreds of feet high, in the upper reaches of Wellsville Mountain, north of Brigham City.

His friend Val Gunther worked in a declivity behind him and a little lower. Val's son, Glade, studied rocks nearby.Val Gunther and Reese, residents of Brigham City and Ogden, respectively, were talking happily as they split shale with their rock hammers, crowbars and chisels.

They were figuring how the bluish rock was fracturing, calculating when the giant crowbar was needed, tapping with their rock hammers.

They seemed like shale engineers, understanding almost instinctively how to split the rocks. As they worked, flat sections of rocks flew down the mountain. One rolled on its side far below, then cartwheeled off a bump and sailed toward a lower ledge.

They were cracking their way into another world, the Middle Cambrian Era, an undersea realm that existed 530 million years ago.

That time and ours intersect only at a few rare outcrops throughout the world. Most of the break-outs are hard to reach. But occasionally they have preservation so wonderful that even soft tissues of animals that lived during the Cambrian were fossilized, allowing detailed reconstruction of the animals.

Among the best of these spots are ridges high on Wellsville Mountain and Antelope Spring, 30 miles west of Delta.

The rocks hold the remains of strange animals that thrived in the Cambrian oceans.

That time witnessed the first great explosion of complex life on earth. Within a few million years, life evolved from relatively primitive worms and sea fans into dozens of species and genera. Some had incredibly elaborate body designs.

Although most were just inches long, many of these creatures were capable of a vast range of activities.

THE BEST-KNOWN animals from the Cambrian are the trilobites, arthropods that survived for at least 200 million years before they died off in a mass extinction that hit before the one that killed the dinosaurs. Trilobites crawled along the bottom, feeding on organic matter.

Crablike animals called Cothurnocystis boasted long tails.

On the bottom grew a sponge that looked like a sea urchin, Choia utahensis, which had needle-like spines.

J. Keith Rigby of Brigham Young University, recognized worldwide as an expert on Cambrian sponges, said Choia utahensis was similar to a modern sponge shaped like a saucer. It lived almost buried in soft sediment, "with only those big long tips projected above the surface."

Odaraia was an arthropod that swam on its back, mostly encased in a carapace that was a bivalve, like a clamshell. Its two huge eyes bulged from its front, and its three-pronged tail - shaped like an aircraft tail assembly - stuck out of the shell behind.

Two powerful whiplike legs could suddenly propel the armored arthropod Leanchoilia. Fat, curved worms like thick slugs lay under sediment, their mouths above the surface.

An animal called Canadaspis, a crustacean with a broad head plate over the base of its antennae and legs, swam with a stubby tail hanging behind.

Sponges grew from the sea floor. Worms called Aysheaia lived on them, hooks on their arms allowing them to hang on while they ate the sponges. Algae waved through the water.

Santacaris crawled about the bottom, heavy armor along its long body and flat tail, a set of claws bristling from its mouth.

To protect itself from the predators, Wiwaxia - something like today's sea hare - bristled with a double set of long spikes growing from its back.

The shrimplike Yohoia had bulging eyes, gripping legs, swimming paddle-like gills and claws to grip its food.

Opabinia kept an eye peeled for prey - rather, it kept five big eyes out for prey. It had rows of paddle-gills, a tail made of a double row of plates - and a trunk with a claw on the end. The claw would stuff prey into its mouth, which was in the center of its body.

Sidneyia prowled the bottom looking for trilobites to eat. It was a large, flat, armored customer with little legs, long antennae and tail that ended in a fan. One was discovered with a tiny trilobite in its gullet.

The fiercest predator yet known was Anomalocaris, which at 2 feet long was a monster for the time. It swam by undulating the lobes of its body. Its eyes bulged from the sides of its head, its mouth was a set of circular crushing plates.

Its appendages were two huge curving arms above the mouth, armed with spikes to hold the prey and drag its wriggling victims to the grinding mouth.

SKIP AHEAD 530 million years.

Mountains have risen, oceans have receded, continents have drifted. What were once deep sediments at the bottom of the sea are now layers of shale high on Wellsville Mountain, where the Gunthers, Reese and I worked.

Wellsville Mountain is in one of the steepest ranges in the United States, and reaching its Cambrian layers requires an arduous hike.

The ridges are a couple of miles from the highway and 1,000 feet higher on the rugged face of the mountain.

The immense formations were broken by steep canyons, and boulders littered the foothills. To reach them, the fossil-hunters and I unpacked gear from our car - canteens, cameras, knapsacks, 5-foot-long crowbars, sledgehammers, chisels, rock hammers - and began hiking.

Stickers soon covered pant legs, socks and boot laces. The day was roasting. We crunched uphill along a narrow, gravelly trail, and some in the party began breathing hard before reaching the first giant boulder. Leaning on crowbars, we rested in its shade, a few black files biting.

A farmhouse was visible far below - stacked green hay bales, dirt road, barn and a tiny blue station wagon. Merciless sunlight shifted around the edge of the rock and lit up the weeds and scrub oak.

Splitting open a slab, Reese discovered what is either an appendage or a body of a worm-like creature. The same day he found a large, perfect, beautiful trilobite with long spines called a Zachanthoides; and something like a jellyfish.

Val Gunther discovered a large, well-preserved trilobite that looked like a Glossopleura. Glade Gunther came up with some interesting trilobites.

All obeyed the code of the invertebrate-fossil hunter: Send anything of scientific value to the scientists, like Richard A. Robison of the University of Kansas, or to the Smithsonian.

VERTEBRATE fossils are protected by federal law, so no collector can dig a dinosaur on public land. Invertebrates are a different story. The government encourages rockhounds to explore and collect in its trilobite beds, as long as it's not done commercially.

That's not to say it's illegal to commercially harvest trilobites: Leases have been issued on state land at Antelope Spring, a famous collecting ground west of Delta.

"When we first started collecting, we were interested in building up our own kind of private, amateur collection," Val Gunther said in a later interview. He and his father, Lloyd Gunther, wrote a book about the Cambrian fossils of Utah, published by BYU. They focused on collecting trilobites.

"But our priorities shifted over the years," he added. "We're more interested in collecting the things that are of scientific value. We collect them for the scientific community."

Since the collecting trip described above, Reese has discovered a Sidneyia, he said.

"My dad and I found an Anomalocaris, near complete. . . . We've also found some new arthropods. Some of them were undescribed. One of them that we discovered had 36 segments."

Reese, who has become renowned for his discoveries, said the search for Cambrian fossils is "like an Easter egg hunt . . . like opening up a piece of history.

"Here you're sitting there and going along, and you don't know what you're getting into. And all of a sudden you open a rock and - it's like a painting from the past appears."

What appear are the delicate remains of an animal that lived in the Cambrian era, open to the skies for the first time in more than half a billion years.