CHICAGO (Reuters) - A fossil found in South Dakota is that of a never before seen species of dinosaur, a horse-sized plant eater with spikes on its bony flat head, scientists said on Monday.


"When my colleagues saw a CAT scan of the new fossil, they tore up their family tree diagrams and said, 'Back to the drawing board!' ... We never suspected such a creature existed," said paleontologist Robert Bakker.

Discovery of the flat-headed member of the pachycephalosaur family changes the view of dinosaur history during the final days of the Cretaceous Period 66 million years ago, showing that family trees were still evolving even as the entire dinosaur world was about to go extinct, the Children's Museum of Indianapolis said in announcing the find.

The nearly complete pachycephalosaur skull was donated to the museum by three amateur fossil hunters from Iowa who found it in 2003 while exploring the Hell Creek Formation in central South Dakota.

The discovery was announced in Indianapolis in conjunction with the annual meeting of the American Association of Museums.

The museum said the pachycephalosaur family is marked by dragon-like heads covered with horns, knobs and bumps. The most famous family member, Pachycephalosaurus, had a solid, domed bone helmet up to eight inches (20 cm) thick used to ram other dinosaurs in their sides, it said.

The new species has a flat head with no bone dome. The only other flat-headed pachycephalosaurs discovered were found in China and Mongolia but all of those had had short muzzles and no long horns anywhere on the skull, the announcement said.

The pachycephalosaurs in general all had massive necks and could inflict significant "blunt force trauma" on other dinosaurs, Bakker said.

"This new species ... likely pressed their foreheads together and shoved one another really hard," he added.

The museum, billed as the largest of its kind in the United States, said the fossil would become part of its dinosaur exhibit.