The refuge adjacent to the Missouri River is popular with hunters for its big game and remote setting.

David Bradt, a ranch manager from Florence, Montana, said he was hunting elk unsuccessfully in November 2010 when he walked into a canyon to splash some water on his face.

In the creek, the water ran over what he thought was petrified wood sticking out of a rock. He pulled back the brush, saw vertebrae and knew it was fossilized bones.

He thought it was a dinosaur and was floored when he learned it was a sea creature.

"It's about the size of a cow, and I'm thinking it's a triceratops," he said. "I didn't know there was an ocean there."

Bradt took photographs and reported the find to the U.S. Fish and Wildlife Service and the Museum of the Rockies in Bozeman.

It took three days to excavate the fossil, but much longer to clean and study it before the determination could be made that it was a new species, Druckenmiller said.

He said the inland sea that stretched the width of Montana to Minnesota and from Canada to the Gulf of Mexico was teeming with marine reptiles, but relatively few of their fossils have been excavated.

"It's a total bias — just more people out there are interested in land-living dinosaurs than marine reptiles," he said. "There would be a lot more known if more people were studying them."

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