Falcarius utahensis was adapted for both ripping up plants and chasing down prey.© PaleoForms LLC, Provo, Utah

Fossil-hunters working in the dusty Utah desert have caught a dinosaur in the act of going vegetarian. The newly discovered species, which lived about 130 million years ago, displays the hallmarks of adapting to a leafy diet.

The species, christened Falcarius utahensis, belongs to a dinosaur group called the therizinosauroids. These are mostly thought to have been plant eaters. But the recently discovered fossil, the most primitive therizinosauroid found so far, seems to have survived on a mixed diet of meat and veg.

Researchers, led by James Kirkland of the Utah Geological Survey in Salt Lake City, uncovered a skull, pelvis and limb bones belonging to the species at Cedar Mountain in eastern Utah¹. From the fossils they conclude that F. utahensis walked upright, standing more than a metre high and measuring some 4 metres from tip to tail.

Meat of the matter

The creature's teeth have a shape that seems to be adapted to leaf shredding, the researchers report. Similar teeth can be found in modern iguanas, for example, a reptilian family that also has a varied diet.

Falcarius utahensis also has a slightly widened pelvis, Kirkland's team points out, which would have been necessary to accommodate the longer gut needed to extract nutrients from plants.

But the dinosaur's legs reveal that it still has adaptations suited for meat eating as well. The creature's thigh bones were longer than its shin bones, suggesting that it could run at an impressive pace. "The legs are still adapted for running after prey," says Kirkland. Later therizinosauroids have longer shin bones, which suggests that they waddled around like long-legged birds.

It's not easy being green

The switch to vegetarianism is surprising, says Paul Barrett, who studies dinosaurs at the Natural History Museum in London. The therizinosauroids belong to a larger group of dinosaurs known as theropods, and many of these are known to have been excellent at catching a meaty meal.

"Nobody understands why theropods should revert to herbivory when they're such excellent predators," Barrett says. "It's a mystery." Perhaps certain dinosaurs were pushed along the evolutionary route to vegetarianism because they lived in an area where there was no other plant-eating competitor, he suggests.

Falcarius utahensis's diet is not its only noteworthy feature, Kirkland's team adds; its North American home is also a surprise. Until now, therizinosauroids have been found almost exclusively in China, which led experts to believe the group arose there.

"This was considered a nearly pure Asian group," Kirkland says. "Finding the most primitive member of the group in Utah throws that into question." The team now suspects that therizinosauroids once roamed over most of the Northern Hemisphere. 

• References

  1. Kirkland J. I., Zanno L. E., Sampson S. D., Clark J. M. & Beblieux D. D. Nature, 435. 84 - 87 (2005). | Article | PubMed | ChemPort |