Skulls and Bones NSF supports paleontologists hunting around the globe for the skulls and bones of dinosaurs and other ancient creatures. >> more

That day, Case almost literally stumbled across something that didn't seem to fit with the other finds the team was making.

"I had worked myself away from the main group and was following up a slope. I started to see a lot of golf ball size rocks that were round and smooth," he said.

Most likely, the stones had been swallowed by marine reptiles to digest their prey, similar to the stones found in the gizzards of modern birds. After the reptiles died, the stones had settled on an ancient ocean bottom in the animals' corpse.

Video Q & A Judd Case, Saint Mary's College of California James E. Martin South Dakota School of Mines & Technology Credit: Dena Headlee, National Science Foundation video Where was the dinosaur discovered? What was so unique about this find? How did you search for other bones once the first discovery was made? Requires Real Player

"Expecting to run into marine-reptile bones, I came across a bone that represented something quite different. It had rounded ends and a groove on it," Case said. "I didn't think it was from a marine reptile because of what I've learned from Jim. I didn't think this is what a marine reptile looks like."

Case then showed his find to Martin, expecting to be convinced they were bones of an extinct ocean creature. "Jim is very good at being skeptical," he noted.

Case, Martin and their colleagues pored over the bone, trying, in a sense, to make its characteristics fit into their preconceived notions.

"We knew we were working in what were once relatively deep marine waters. And we were trying to make it into everything that might be marine reptile," Martin said. "Initially, we were trying to make it anything but a dinosaur."

But as the research team scoured a roughly 40-square-meter (50-square-yard) area, finding a tooth here and a jawbone there, they became more and more convinced the animal they were reconstructing was in fact a dinosaur.

"We just kept going and going until we couldn't find anything more. Until we felt we had really covered the area," Martin said. And gradually, the accumulated evidence showed their predator was a dinosaur.

"As we began to find more pieces throughout that afternoon, Jim said, 'You know, I've only seen these things in meat eating theropods'," Case said. And theropods, or "beast-footed" dinosaurs like Tyrannosaurus rex and smaller carnivores, were not known for frolicking in the deep ocean.

By Peter West