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Utah's Cedar Mountain Formation has yielded two new species of iguanodont, cousins of the famous iguanodon, the plant-eating, beaked-mouthed dinosaur known for its ability to walk on its hind legs.

Working on federal land in two eastern Utah sites, teams led by the Utah Geological Survey discovered the two specimens in 2004. It took years of careful fieldwork to extract the bones, which include a nearly complete skull, and prepare them for study.

The most carefully dated of the two was discovered near Arches National Park by Andrew Milner, of the St. George Dinosaur Discovery Site. It was estimated to be 124 million years old and dubbed Hippodraco.

The survey's Don DeBlieux discovered the other specimen near Green River. This one was dubbed Iguanacolossus.

"It's a very big animal — 30 to 35 feet long. For bipedal plant eaters, that's a big one. We have most of the backbone, tail, ribs, hips and shoulder, but we're missing the legs. We're thinking the legs were dragged off by a predatory dinosaur," said state paleontologist Jim Kirkland, a senior author of a new report on the finds.

The new species were identified in an article published this week in the online journal PLoS ONE . Lead author Andrew McDonald, a doctoral student at the University of Pennsylvania who is regarded as a rising star in dinosaur paleontology, analyzed the bones and characterized the new species.

The scientists were lucky with Hippodraco because the specimen came with a near complete skull that revealed the fine detail of its head.

"It's kind of mushed, but it has the stapes in the ear, a few of the bony plates in the eye sockets," Kirkland said.

Several other specimens that probably represent new dinosaur species have been recovered from Cedar Mountain Formation and are awaiting the reconstruction work necessary to characterize them. Iguanodonts are among those bones and more are on the way, Kirkland said.

"We are getting a lot more of these guys. We have a site that is a bed full of these animals. We think we have a couple new ones out there," Kirkland said.

They both date to the early Cretaceous Period. Iguanacolossus was found at the very bottom of the Cedar Mountain Formation, where it meets the Jurassic Period Morrison Formation, another rich source of dinosaur bones. Scientists weren't able to date the bones, but they suspect the Iguanacolossus is a few million years older than the Hippodraco, making it one of the earliest known Cretaceous dinosaurs in North America.

North America's record of iguanodonts is far from complete, so this week's announcement helps fill in crucial gaps in how these animals evolved. At the time, Europe and North America were joined, but North America's iguanodonts were more primitive than their European counterparts, indicating the populations didn't interact.

"It suggests the Appalachia [mountains] were more like the Himalaya back then. It must have been a more formidable barrier than I previously thought," Kirkland said.