Sauropods were big dinosaurs with little heads, like the sort known as brontosaurus. Their heads were so small and flimsy, in fact, that their skulls are rarely found.

About a dozen sauropod skulls are known from the Jurassic era, the great middle period of dinosaur life. But for the Cretaceous, the final 80 million years of the rule of dinosaurs, no sauropod skulls have been known from North America.

Until now.

Over the past few years, experts at Dinosaur National Monument on the Utah-Colorado border and at Brigham Young University have quietly worked on an astonishing four sauropod skulls or parts of skulls, found close to each other at the monument.

"We've really got a remarkable — it's almost mind-boggling — new discovery," said Dan Chure, Dinosaur's paleontologist. "If there's one thing you would not expect to find . . . it's sauropod skulls, because they're so rare."

Also, the fossils have fine preservation, he said in a telephone interview. "It's kind of hard to overstate how amazing this is."

All four are the same type, a new species and genera, says Chure. They lived around 100 million years ago, or possibly a little earlier.

The sauropod may have been 25 feet long with an 18-inch skull.

The animals, which do not yet have a formal scientific name, were not as gigantic as some sauropods. But like all creatures of their family, at the end of their long necks were heads that seem absurdly small.

Actually, a tiny head makes sense. If this animal had a noggin the size of a T. rex's, rather than rise to the top of trees to munch on the leaves, that heavy head would be dragging along the ground.

A sauropod skull is not a single bone but a series of delicate bones. "It seems that as soon as they die, the head falls off," Chure said. The bones fall apart and the pieces may wash downstream or become scattered by scavengers. They rot away because they are too thin to be easily fossilized.

"This has been very frustrating to people who work on sauropods," Chure said. Sauropod excavators might haul out 500-pound leg bones but nothing from the ruminating end.

About a dozen sauropod skulls have been recovered from Jurassic layers, "when sauropods are kind of at their zenith in terms of diversity and abundance," said Chure.

But in the next era, the 80 million years of the Cretaceous period, sauropod skulls are exceedingly rare, he said. One was found in Madagascar, two in Africa and one in South America, an animal which has not been described yet.

"And until recently, there were none from North America," Chure said.