The Ocucaje Desert holds some of the most important fossils in the world. And their only defender is a renegade guide with an eye for shark teeth.

I am investigating the skull of a huge toothy beast on the rocky slope of a dun mountain when I hear Roberto cry out. I look over to see him doing a shuffle in the dust, tan arms held above his head—the lone spot of motion on the mountain’s stony face. He gathers us around so that we can all see what he has found: the fossilized tooth of a megalodon shark, one of the most fearsome killers in the planet’s history. The tooth is a six-inch dagger, gleaming white.

It’s our second day deep in the Ocucaje Desert in the Ica region of southern Peru. Photographer Morgan Stetler, Sergio Tueros Grimaldo (a 17-year-old spending his summer vacation studying extinct sharks), and I have traveled here with our guide, Roberto Penny Cabrera, in search of fossils.

For eons, this land was the bed of a shallow bay off the coast of South America. The water teemed with life: whales, dolphins, giant penguins, crocodiles, and the megalodon, a 45-foot-long whale-eating shark, which disappeared some 1.5 million years ago. As the Andes rose, they pushed the land out of the water. A million years ago it stopped raining. Now the landscape looks like Mars. Wind-formed hills of crumbling stone and dunes of fine sand enclose basins of shiny polished pebbles that appear to have been sorted by size: small ones here, larger ones over there. And the remains of all the creatures that once swam now lie buried in the stone.

The fossils, particularly those of the megalodon, make the region one of the most important paleontological areas in the world. And yet the Peruvian government offers the desert no legal protection. Most of the land is available for mining, but fossils—taken by unscrupulous scientists and local scavengers and sold to collectors and museums—have become lucrative exports.

Our guide, Roberto, sees himself as the desert’s only protector, and for the most part he’s right. Behind the wheel of Hermelinda, his trusty black-and-olive ’83 Datsun diesel pickup, he cruises the trackless desert sands like one of the lonely, indefatigable desert foxes that live there.

Roberto is a descendant of Capitán Doctor Jerónimo Luis de Cabrera y Toledo, a conquistador who, in 1563, founded the city of Ica on the banks of a river 160 miles south of Lima. Roberto favors desert fatigues, a khaki shirt, and tan boots. He has a thick gray moustache, and over his shirt he wears a steel-ribbed tan corset that he cinches tight each morning to keep his back straight. A broad-brimmed hat shades his blue eyes, and he is never without the knife on his wide leather belt or the cell phone strapped to his thick wrist. At 54, he maintains the natural good looks of a rakish man of action: a Burt Reynolds or a Douglas Fairbanks Jr.

We had begun our trip at Roberto’s home in Ica’s city center. He lives on the main square, in the thick-walled yellow mansion built by his grandfather a century ago. From the facade of the house, under the Cabrera coat of arms, two huge, dark doors swing into a courtyard. Inside, the bustling traffic in the plaza fades to a distant echo. In narrow, dim hallways with dark wood wainscoting, cracks in the plaster snake off into the gloom. A piano sits askew in the middle of a room, where an earthquake shoved it. In a grand dining room, a massive mahogany table is set for a feast of dust. In the rear courtyard, the entire back of the house has collapsed.

The front rooms of the mansion, facing the plaza, house the Museum of the Engraved Stones of Ica. The stones were collected by Roberto’s late uncle, Javier Cabrera Darquea. A prominent physician in the 1960s and ’70s, he spent his later years taking trips to the desert. He came back bearing rocks engraved with images of people of, as he told it, extraterrestrial origin, commingling with dinosaurs. Above Javier’s desk is a framed photo of him presenting a black, carved stone to actress Shirley MacLaine.

This is Roberto’s birthright: The Cabrera name is equal parts proud legacy and burden. The family is given to big ideas both practical (bringing irrigation to the valley; planting the first olive trees in the area) and eccentric (communing with aliens; ranting in the public square, as one of Roberto’s brothers has been known to do). For every noble event commemorated in the bright oil paintings hung above Javier Cabrera’s stones, the clan hides a darker tale. When I told our cab driver we were going to the Cabrera mansion, he looked at me over his shoulder and said, “They’re all crazy.”