The Bureau of Land Management asked Kirkland to make a deposition about the find, and Kirkland told what he knew. As a result, the Moab man pleaded guilty to theft of government property and was sentenced to five months in prison, 36 months of supervised release and a $15,000 fine.

"He served his prison time in 2003 and then returned home to Moab," a U. release says. Kirkland said he still considers the man his friend, adding he wants to help with the continuing dig at the Crystal Geyser site.

The article in Nature says Falcarius utahensis was a small- to medium-sized dinosaur. It was about 40 inches high at the hips and 13 feet long, including the tail. It had vicious hooked claws yet a long neck and many small teeth that seem better suited to shredding leaves than attack.

During a press conference Wednesday at the Utah Division of Natural Resources headquarters, Sampson said Falcarius utahensis represents the first documentation of the type of dinosaur called therizinosauroids in North America during the early Cretaceous era. Other early examples came from Asia, where fossils of related species were discovered with feather imprints.

The closest relatives, which are from China, are "preserved with feathers," said Kirkland, who has traveled to that country to examine specimens.

Later examples of the type are known with feathers. The Utah fossils are not well enough preserved to show evidence of feathers.

Zanno summarized the scientists' thinking about Falcarius: "So we make the assumption that if your parents and kids have it (a characteristic), you have it as well."

The most surprising aspect of the discovery is that it is a transition form between savage meat-eaters and vegetarians.

"Falcarius does have an extremely well-developed forelimb, with sharp claws, like raptors," she said.

"Perhaps these claws were also used for defensive purposes," Kirkland said.

Yet its teeth are not those of a hunter and its hips indicate the development of the kind of big gut needed to digest large amounts of vegetation. Also, the huge number of animals in the quarry may indicate they depended on plants. Predators tend to be rare, preying on more numerous herbivores.

Why would a dinosaur switch from meat to plants? Sampson said the period saw the sudden appearance of flowering plants, more nutritious than the pines and gingkos that were common previously. Maybe flowering plants opened a new ecological niche for dinosaurs.

"All of a sudden there was a new food source that spread widely," he said.

"I doubt seriously this animal could cut a steak with that mouth," Kirkland said. But it might have been able to eat lizards and other small animals, he said, although there is no proof of what it ate.

Kirkland said he thinks plants would have been part of the dinosaur's diet.

Why did the mass deaths happen? No evidence was found for volcanoes in the area, and there were no rivers that could have drowned Falcarius herds.

Maybe the animals congregated at spring, which could have dried up, killing many, he said. Or maybe botulism or some other organic contaminant in the springs killed them the way thousands of birds sometimes die at Bear River Migratory Bird Refuge, he said.

"These things do happen," Kirkland said. "They happen today."

A mounted skeleton replica and a smaller model of what the animal looked like in life will be on display at the museum, probably in June.

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E-mail: bau@desnews.com