The "mass graveyard" of a bird-like dinosaur has been uncovered in Utah, US, Nature magazine reports this week.
Scientists believe the previously unknown species was in the process of converting to "vegetarianism" from a rather more bloodthirsty diet.
Falcarius utahensis seems to represent an intermediate stage between a carnivorous and herbivorous form.
The creature, which lived about 125 million years ago, provides a "missing link" in dinosaur evolution.
"Falcarius represents evolution caught in the act, a primitive form that shares much in common with its carnivorous kin, while possessing a variety of features demonstrating that it had embarked on the path toward more advanced plant-eating forms," said co-author Scott Sampson, of the Utah Museum of Natural History.
Falcarius utahensis belonged to a group of dinosaurs known as therizinosaurs, which were cousins of Velociraptor dinos and the early ancestors of birds.
The bizarre creature appears to sit halfway between nippy carnivores and later, lumbering plant-eating therizinosaurs, although scientists cannot be entirely sure what it ate itself.
"Falcarius shows the beginning of features we associate with plant-eating dinosaurs, including a reduction in size of meat-cutting teeth to leaf-shredding teeth, the expansion of the gut to a size needed to ferment plants and the early stages of changing the legs so they could carry a bulky body instead of running fast after prey," said James Kirkland, of the Utah Geological Survey.
The adult dinosaur walked on two legs, was about 4m long (13ft) and stood 1.4m tall (4.5ft). It also had a woolly feather-like plumage and sharp, curved 10cm-long (4-inch) claws.
These formidable talons were probably a hang-over from the dinosaur's ferocious past, the researchers say, and may not have had a function in its more sedate new lifestyle.
Falcarius shared an - as yet undiscovered - ancestor with the Velociraptor, which was almost certainly a fleet-footed, small-bodied predator, the researchers believe.
At some point, two major groups of dinosaurs split from their carnivorous cousins and shifted into plant-eating. But until now, the intermediate stages of this process remained a mystery.
"With Falcarius, we have actual fossil evidence of a major dietary shift, certainly the best example documented among dinosaurs," said Dr Sampson.
"This little beast is the missing link between small-bodied predatory dinosaurs and the highly specialised and bizarre plant-eating therizinosaurs."
Although the team cannot know whether Falcarius was a committed herbivore - it may have eaten a bit of meat, too - its emergence did coincide neatly with the evolution of flowering plants.
"At the same time Falcarius appeared, the world was changing greatly because flowering plants were appearing," Dr Sampson said. "They would have provided a new food source. It could be that Falcarius was exploiting an open ecological niche."
Researchers were able to get such a complete idea of what Falcarius looked like, because they were lucky enough to find a "mass grave" of the species at the base of the Cedar Mountain rock formation, south of Green River in Utah.
Its formidable talons were probably a hang-over from the dinosaur's more ferocious past
James Kirkland estimates hundreds to thousands of individual dinosaurs - from hatchlings to adults - died at the 8,000 sq m dig site.
No one knows quite what killed them, but mass deaths have appeared in the fossil record before. Scientists have suggested drought, volcanism, fire and botulism poisoning as possible causes.
"Mass mortalities are known in a number of dinosaur groups," said Dr Sampson. "In this case, it is difficult to work out what happened. It could have been a spring which dried up, and the dinosaurs died of thirst.
"Or organic poisons could have contaminated the water - it is hard to know for sure."