Thousands of fossils unearthed in Utah have provided scientists with a missing link between carnivores and herbivores, as well as offering insights into how meat-eating dinosaurs gave rise to modern birds, with their broad range of diets.

The new species, Falcarius utahensis, or “sickle-maker of Utah”, lived in the early Cretaceous period, about 125 million years ago, and stood 4.5ft (1.4m) tall and 13ft (4m) long. It had sharp, curved 4in (10cm) claws, from which it takes its name. Its anatomical features suggest that it represents an intermediate stage in the evolution of a group of herbivorous dinosaurs known as therizinosaurs, which developed from the theropod family of predators.

Scientists have long known that the first dinosaurs were carnivorous and that plant-eating groups, such as the therizinosaurs and the long-necked sauropods, had ancestors that hunted for meat.

The research team that discovered Falcarius, led by James Kirkland, of the Utah Geological Survey, is not certain whether the new dinosaur ate meat, plants or both. Its body, however, reveals important details of how this evolutionary process developed.

“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,” Dr Kirkland said.

Lindsay Zanno, a colleague, said: “Falcarius is the most primitive known therizinosaur, demonstrating unequivocally that this large-bodied group of bizarre herbivorous dinosaurs came from Velociraptor-like ancestors.” Falcarius is not directly descended from Velociraptor, but it shares a common ancestor that has yet to be discovered.

Scott Sampson, the chief curator of the Utah Museum of Natural History and another member of the study team, said: “We know that the first dinosaur was a small-bodied, lightly built, fleet-footed predator,” he said. “Early on, two major groups of dinosaurs shifted to plant-eating, but we have virtually no record of those transitions. With Falcarius, we have actual fossil evidence of a major dietary shift.” Details of the find are published today in the journal Nature.

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