Before the dig started, it looked like any other patch of dinosaur dirt: gray soil, a few brownish fossilized bones exposed by erosion. Paleontologist Adam Yates thought his diggers would find a few bones from the massospondylus -- South Africa's most common dinosaur.
So the Australian paleontologist at the University of the Witwatersrand in Johannesburg initially assigned a master's student in 2006 to excavate the site and research the story of how the dinosaurs died.
But within days, it was clear that they were on to something big. In about 11 weeks spread over the years since, Yates' team members excavated about 300 bones from a site just over 20 feet long and 9 feet wide.
They discovered three new dinosaurs and the fangs of a mysterious dinosaur eater, a likely fourth new species. The first to be named and researched is Aardonyx celestae. The rest are still under study.
What makes A. celestae so exciting is that the species, like a crucial piece in a complicated jigsaw puzzle, helps explain how some of the earliest dinosaurs, two-legged herbivores known as prosauropods, evolved into the largest creatures that ever walked Earth: the sauropods, four-legged creatures with long necks and small heads that ripped foliage off trees with their cavernous jaws.
Yates doesn't like the term "missing link." It upsets his scientific sensibilities because evolution doesn't unfold in a neat, linear fashion. But he says the term does at least convey the import of the discovery.
"It's one of the dinosaurs in a long, smeary continuum," he said Wednesday. "It shows us what we should already have pretty much guessed, which was that evolution was a messy, complicated affair."
The scientists found two Aardonyx specimens at Spion Kop in the central province of Free State, neither of them adult. The smaller of the two -- a more complete set of bones -- was about 7 years old when it died, about 23 feet long and 6 feet high at the hip. An adult might have grown to 50 feet and weighed half a ton, Yates said.
His eyes lighted up as he spoke about dinosaurs, bones and evolution. Although he was fully in the 21st century at a news conference, wearing a radio mike and using a red laser pointer as he discussed slides, it was easy to imagine him a couple of centuries ago, striding over the limestone and shale cliffs of Lyme Regis, the famous paleontology site in southern England where the novel "The French Lieutenant's Woman" by John Fowles is partly set.
Yates confessed a little sheepishly that he initially overlooked the site. You could barely sink a hammer into the fossil-rich eastern part of South Africa's huge Karoo Basin without hitting massospondylus bones.
"They're very common and I really wasn't interested in digging up a lot of massospondylus bones," Yates recalled. "We had other exciting sites."
But he was there to supervise on Day One in 2006, when the master's student, Marc Blackbeard, and other volunteers started to dig. As they chipped away, they pulled out bones by the dozen, and they were larger than those of a massospondylus.
Yates' voice rose excitedly as he recalled that day: He was rushing around, too busy even to dig, as students kept producing extraordinary bones, asking him what they had found.
"As soon as we started opening up, we realized it was very densely packed," he said. "We kept on finding bone after bone. You start to say, 'This doesn't add up. It's not what I thought it is.' Pretty much within the first few days I was clear that it was a new type of dinosaur."
Aardonyx, or "earth claw," is a reference to the concrete-like stone attached to the claws that were among the first fossils found embedded at the dig at Spion Kop, one of South Africa's richest dinosaur sites. Celeste is Yates' wife, a paleontology preparator who had the tough task of chipping the stone from the fossils.
U.S. paleobiologist and functional morphologist Matthew Bonnan of Western Illinois University, who took part in the project sponsored by National Geographic, studies bones to find out how dinosaurs moved and lived.
"This find is very significant because Aardonyx is a transition animal," he said. "It's a close cousin of the sauropod dinosaurs. It gives us a window on what was happening very early on in the evolution of those giants."
Bonnan describes A. celestae as a lumbering creature with a large belly and chest, like the huge sauropods that came later. Like those animals, it ate huge quantities of foliage. The prosauropods, smaller grazing animals that evolved to run, dominated the landscape when Aardonyx lived.
Aardonyx exemplifies why dinosaurs evolved from bipeds to quadrupeds. Lush vegetation allowed them to eat more; they evolved into larger animals. But their huge bellies made balancing on two legs difficult, so they dropped onto their smaller front legs, eventually evolving into heavy quadrupeds.
The scientists hypothesize that the Spion Kop area was once a lush oasis edged by a vast desert, hence the different kinds of dinosaurs found there. They believe the animals may have died during a drought, possibly at the edge of a dry water hole.
At some point, carnivore X -- the mystery creature -- ate the dead or dying Aardonyx. Several fangs were found at the scene, and they're not like other dinosaur teeth from the same period.
"I'd very much like to find the bones of the mysterious carnivore X," Yates said. "Its teeth are intriguing, teeth like dinosaurs that don't appear until much later."
He hopes more digging will uncover it.
"That's the joy of paleontology," he said. "There's something out there. We have to go out there and find it."