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Some 125 million years ago, a bizarre — and probably feathered — dinosaur roamed Utah, a creature considered a key "missing link" that may have marked a transition from meat-eating predators to vegetarians.

The discovery of the new species, Falcarius utahensis, was announced by scientists Wednesday. It is in the family of maniraptorans, a dinosaur group that included predators such as the Utahraptor and the Velociraptor — the vicious dinosaurs in "Jurassic Park." Falcarius had characteristics of both meat- and plant-eaters.

Two mass boneyards of the creatures were discovered near evidence for ancient springs south of Green River and close to the present-day Crystal Geyser in Grand County.

State paleontologist James Kirkland said Wednesday that bones from hundreds or thousands of dinosaurs are believed preserved at the quarry. About 99 percent are Falcarius, with specimens of all ages, from babies to adults. That indicates mass die-offs happened rather than bones accumulating over time, because it's hard to believe so few other types would be in the vicinity over the years, he said.

Falcarius is a former missing link, "which, of course, now is no longer missing," said Scott Sampson, chief curator at the Utah Museum of Natural History on the University of Utah campus.

A scientific paper describing the new animal was being published in today's edition of the scientific journal Nature. The authors are Kirkland; Lindsay E. Zanno, a U. doctoral student; Sampson; James M. Clark of George Washington University in Washington, D.C.; and Donald D. DeBlieux of the Utah Geological Survey.

The dinosaur dates to the early Cretaceous era, a period about which little is known in this country because of the scarcity of remains. But the Cedar Mountain Formation that curves through much of east-central Utah preserves important dinosaur fossils from the period, such as the carnivorous Allosaurus (Utah's "state dinosaur"), the Utahraptor and the herbivorous Gastonia.

Kirkland said the newly discovered Falcarius utahensis ("sickle-maker from Utah," referring to its claws) first came to his attention when he realized fossils were being sold on the black market. After he tried and failed to find the site, a Moab man who had illegally removed the vertebrate fossils from the federal land "took me directly to the site," he said. He told the man that if he were asked he'd have to say where the bones came from.

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.

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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.

E-mail: bau@desnews.com