Scientists have caught a dinosaur in the act of evolving from meat-eater to vegetarian.
The two-legged, feathered creature ate plants, but its bones show that the transition from its carnivorous ancestors was still in progress.
All plant-eating dinosaurs were ultimately descended from a meat-eater, and switchovers to plant-eating occurred several times. The newly discovered species, which lived 125 million years ago in what is now Utah, could help scientists understand details of how the changeovers took place.
It's "our first really good case of a dinosaur in the midst of shifting from the meat-eating body to a plant-eating one," said an expert not involved in the discovery, Thomas R. Holtz Jr. of the University of Maryland.
"It's definitely eating a substantial amount of plants, (but) we still see the original imprint of meat-eating upon it."
The creature, with 12.7 centimeters (5-inch) claws on its outsized hands, measured some 3.7 meters (12 feet) from its snout to the tip of its long skinny tail. It stood just over 91 centimeters (3 feet) tall at the hip and could apparently reach about 1.5 meters (5 feet) off the ground with its long neck to munch leaves or fruit, said Utah state paleontologist James Kirkland.
He describes the creature in Thursday's issue of the British journal Nature with Lindsay Zanno and Scott Sampson of the Utah Museum of Natural History at the University of Utah, among others.
They dubbed it Falcarius utahensis. Bones from hundreds or maybe thousands of these dinosaurs were discovered at a two-acre dig site in east-central Utah, south of the town of Green River. Nobody knows why they gathered there or what killed them, Kirkland said.
But analysis revealed that Falcarius was the earliest known member of a bizarre-looking group of plant-eaters called therizinosaurs. Found mostly in Asia, the barrel-bodied creatures waddled upright like Godzilla or "a pot-bellied bear," Kirkland said.
Falcarius, very early in its evolution into the therizinosaur body type, retained the rather horizontal posture and built-for-speed legs of its meat-eating ancestors. But it had already lost the flattened and serrated teeth used to tear meat and acquired the smaller, more densely packed teeth of a vegetarian, Zanno said.
It also showed some change toward the larger gut needed to digest plant material rather than meat, as well as a lengthened neck and smaller head associated with eating plants, she said.
Holtz said Falcarius still had fairly slender proportions overall rather than the barrel body of later therizinosaurs. "This one could probably move fairly quickly," he said, whereas its more evolved relatives "would have had problems hunting things faster than a tree."
Kirkland and Zanno said they suspect Falcarius probably ate some meat in addition to plants.
"I wouldn't doubt this thing would eat a lizard or two in a pinch," Kirkland said.
MALCOLM RITTER, AP Science Writer