Petrified footprints: a puzzling parade of Permian beasts.

It's a clear, chilly morning in southern New Mexico's desolate Robledo Mountains, and Jerry MacDonald, panting as much from excitement as exertion, is hot on the trail of wild game. He's not quite sure what he's tracking but he knows he want it.

"Look at that little guy go!"

Too late. MacDonald's four-footed prey has vanished. It seems to have scurried straight into the sheer rock cliff rising up next to us. Only its tracks remain in the mud behind it. MacDonald, sweating in the 40-degree desert air, is off in pursuit.

"He's realy moving. Let's see if we run into him up this way."

That the trail is some 280 million years old, and the mud now mudstone, doesn't dampen the thrill of the chase for MacDonald--a thrill that verges on bloodlust. The spritely creature he's hunting is an amphibian whose tracks he's never seen before; it may well be that no one has ever seen them before. With a loud grunt, he jams a six-foot pry bar into a fissure between two layers of red mudstone at knee level and starts rocking the end of the bar. Then he clasps his two meaty hands on the broken-off slab and begins to yank.

"I want this," he whispers to himself, breathing hard, as he struggles to free the rock. "Come on!"

At layer 25 of excavation, MacDonald brushes off some unidentified prints that appear curiously bipedal.

The slab finally comes free, a smooth chunk of heavy red-brown mudstone about the size of a small suitcase. Gasping, MacDonald heaves the slab up onto one side. Running the length of the top edge is a series of fine, parallel cracks. He pokes a chisel into the most prominent of the cracks and thwunks it with a small sledgehammer. The stone pops open.

The early-morning sun highlights a crisp series of footprints much larger than we were anticipating. They're almost an inch deep and several inches long from the back of the heel to the vicious-looking ends of the widely splayed toes. On this exact spot, long before the age of dinosaurs, a sail-backed pelycosaur known as Dimetrodon grandis, king carnivore of the Permian period, swaggered through this patch of now petrified mud. To MacDonald, panting and chortling as he sweeps rock debris from the impressions with his fingers, it's as though the reptile had just crawled out of sight, snorting and hissing.

Most museums would be happy with a single slab bearing a half-dozen Permian footprints. MacDonald has dug up and assembled trackways up to 27 feet long that contain an entire food chain's worth of footprints, whole feeding frenzies in stone. Until this morning, I've always regarded fossil hunting as slow, tedious and uneventful work. No longer. MacDonald is digging up footprints faster than I can jot down their descriptions in my notebook.

"It's so rich they wouldn't believe me"

"Jeez Louise! Another pelycosaur--a big one!" We're both laughing now. "Man, look at those claws! This layer is going to be spectacular." MacDonald wasn't expecting this; he was just clearing rock to get down to his target layer. I comment that he doesn't have to wait long for gratification. "Uh-uh. Not her." He stops prying and straightens up. "That's why people wouldn't believe me," he says, fixing me with an exhilarated glare. "It's so incredibly rich, they just wouldn't believe me!"

Jerry Paul…