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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.

Parts of sauropod headgear had been recovered from this continent, however. BYU researchers found some brain cases earlier at another site in Utah but not full skulls.

Brooks Britt, assistant professor of geology at BYU, noted, "Sauropod skulls are among the rarest of dinosaur finds because they have the thinnest bones, the most delicate skulls."

Recently, he and his lab teased the second skull and the snout of a third specimen from a large block of sandstone sent there from the monument. Also recovered was the brain case of a fourth animal.

The second skull was disarticulated, meaning the pieces had fallen apart. But the bones were there, and they are especially valuable because they can be examined from all sides.

In the 1970s, visiting paleontologists discovered the site where the sauropod skulls were later uncovered, which is on the Utah side of the monument in the general vicinity of the monument visitors center. In the 1980s, said Chure, "we relocated the site and collected some bones that were sticking out of the ground and weathering."

The following decade, monument staff members worked at the quarry. About the year 2000, they dug up the first stunning find, the beautifully preserved and articulated skull.

By articulated, paleontologists mean it is together, not separated in pieces. "It's slightly distorted, but it's certainly an outstanding specimen," Chure said.

About a year and a half ago, crews dug out a giant slab of sandstone from the quarry, because they could see traces of fossilized bones in the rock. The slab was around 6 or 7 feet long, 4 feet wide and 3 feet thick. It weighed thousands of pounds.

"It was a whoppin' big block," Chure said.

It was lifted from the quarry by helicopter and taken to the visitors center. Early last year, a truck carried the slab to BYU, where Britt's team worked on it, painstakingly removing rock.

They extracted sauropod body bones and essentially a whole second skull, which was in pieces.

"Those guys did an amazing job," Chure said. "Some of the bones are paper thin, and they got all of the bones out of the rock."

They found the snout of a third sauropod of the same species, and at the quarry, scientists recovered the brain case of a fourth.

More specimens may await discovery in the new quarry. "The producing layer goes for probably at least as long an area as the quarry we have inside the building, and we've only excavated a small part of it," Chure said.

The articulated skull is so well preserved that eventually it may be used to make a mold, which could be cast. The cast could be placed on display.

How did four skulls end up together? Chure can't tell for certain.

The remains were in an ancient river or stream environment. Perhaps a herd of the animals drowned crossing a river. Or maybe they died in a drought, waiting beside a river that had gone dry, and a later flash flood washed the carcasses together.

He doesn't know what else might be discovered at the new site.

"We could have used up all our luck right away," he said. But he doesn't really think that and quickly adds that there's a lot more digging and chipping to do.

"Further work there is likely to turn up additional specimens," he said.

Chure hopes that he and Britt can get funding to support a concerted, longer effort. The new quarry, he said, might turn out to be as important as the famous one at the visitors center.

E-mail: bau@desnews.com