On the hunt for dinosaur fossils in Patagonia in 1999, Apesteguia and his assistant were scouring the rocky, barren landscape on a January afternoon when a 12-year-old local boy led them to a gorge framed by red sandstone cliffs.
What they found took even them by surprise: a set of astonishingly well-preserved fossils of a species of lizard-like reptiles - called sphenodontians - that disappeared from the fossil record 120 million years ago.
"It was fantastic," Apesteguia said. "We had no idea that sphenodontians could possibly be found there."
The 90-million-year-old fossils date from the Late Cretaceous period, and their discovery punctures a long-held assumption among paleontologists that sphenodontians disappeared from both North and South America 120 million years ago.
Sphenodontians in North America gave way to lizards at about that time. But in South America, where the reptiles had no such competition, scientists now believe they flourished for at least another 55 million years.
"Practically nothing was known about the last 120 million years of this species," said Fernando Novas, an Argentine paleontologist who secured funding for the expedition. "This discovery will modify the history written in all the paleontology textbooks."
Sphenodontians shared their habitat with dinosaurs and crocodiles, both of which occasionally fed on their much-smaller neighbors. The fossils prove they were also more numerous than any other terrestrial vertebrate in the region.
"This group was very abundant and very prosperous," said Guillermo Rougier, a paleontologist at the University of Louisville who studies mammal fossils in the same region of Rio Negro. "This discovery illustrates ... the diversity of these creatures, of which we had no idea."
Today the only living sphenodontian descendant is the tuatara, found only in New Zealand.
Apesteguia's finding also illuminates the differences in how animal life in North and South America evolved.
Traditionally the southern continent has not received as much study as the north, Rougier said, and the sphenodontian discovery points to a unique history that scientists have only begun to uncover.
"The two continents had very few things in common," Rougier said. "Most of our ideas are based on records from the north. Research in the south is showing a very different picture."
Apesteguia unearthed the fossils in a rock quarry in the Patagonian province of Rio Negro two days before the end of a monthlong expedition for dinosaur remains.
After laboring all day with his assistant, Jorge Gonzalez, under the blazing summer sun, Apesteguia agreed to follow Miguel Avelas and his sister, Estela, to a site two miles distant where the children insisted they had seen broken bones.
As they ran after their guides, Apesteguia and Gonzalez stumbled across small bones scattered across the ground.
"We left some clothes to mark the place and followed the children ahead," Apesteguia said. They then reached the high cliffs, a place of "astonishing beauty" as Apesteguia recalled.
"I stopped just to see the landscape," he said. "I had no eyes for other things. So Miguel told me, 'there.'
"I looked down," he continued. "In a step in the wall three meters ahead of me, two large bones were jutting out of the red stone."
Just then heavy raindrops began to fall, so Apesteguia and Gonzalez rushed to cover the bones with clothing and returned to their jeep, parked in what had been a dry riverbed but was now quickly flooding.
When the two men returned the next day, they found several fossilized sphenodontian skeletons, all nearly complete. They also collected pieces of hundreds of others and took them back to Buenos Aires for study.
Apesteguia and Novas published their discovery in the Oct. 9 issue of Nature, a top science journal.
The narrow, elongated skull of one of the sphenodontians fits neatly into Apesteguia's palm. But despite the skull's small size, the reptiles he discovered ranged up to just over 3 feet in length - much bigger than any sphenodontian unearthed so far.
Most notable is the creature's small, pointed, downward-facing beak and sharp teeth. Apesteguia said the reptiles probably ate insects, and possibly plants and other animals. They likely had spiny backs and ridged tails and resembled modern-day lizards.
The Patagonia region of Argentina has been a veritable treasure trove for paleontologists in recent years. In 1997 an American-led team of scientists discovered a square-mile dinosaur nesting site that yielded thousands of eggs.
The previous year Argentine museum workers unearthed a virtually complete titanosaur skeleton in Rincon de los Sauces. And paleontologist Jose Bonaparte, who has been exploring the area since 1980, has discovered a carnotaurus and patagosaurus, along with dozens of other species. Numerous other teams, drawn by such scientific riches, have been working in the region over the past several years.
La Buitrera, where Apesteguia found the sphenodontians, is an arid region where summer temperatures often climb above 100 degrees - a hot, dry climate ideal for the preservation of fossils.
"La Buitrera is a fantastic deposit," Novas said. "Pieces of many sizes of sphenodontians, serpents and small mammals almost the size of a mouse abound, so many that they allow us to understand ... an unknown ecosystem."
By Alexa Stanard