GLEN CANYON NATIONAL RECREATION AREA — To stare at the cracked, brownish bones embedded in gray shale is to travel back in time 93 million years.

The towering walls of nearby cliff walls are replaced by a vast, shallow sea. A rocky outcrop reverts to a sandy shoreline. The Tropic Shale is gone, the rock once again the sea's mud floor. Suddenly, small fish scatter as a giant, robust reptile hurtles after them, arms and legs shaped like paddles propelling it as fast as a shark can swim, its mouth wide open and 100 sharp teeth showing.

It is a plesiosaur, a top predator in that long-disappeared ocean.

The best-preserved plesiosaur ever discovered in the region is being removed from the shale and taken to a museum in Flagstaff, Ariz. Scientists say it's a new species, judging by its morphology.

On Tuesday, officials of Glen Canyon National Recreation Area, where it was found two years ago, led reporters to the site. They asked that details about the location not be published, other than the fact that it is on the Utah side of Lake Powell. About five paleontologists and volunteers are working here at any time, and as many as 15 have helped since the dig started in earnest two weeks ago.

"The sea extended from about Cedar City in Utah to Kansas," said Dave Gillette, chief of the excavation. "It was warm water, and it had lots of life in it."

The former Utah state paleontologist, Gillette is now Colbert curator of paleontology at the Museum of Northern Arizona, Flagstaff. He hopes the last massive slab of rock and fossils will be carried out before the Fourth of July, when visitors may interrupt the work. It will have to be moved by litter so that vehicle tracks don't mar the desert landscape.

The plesiosaur is an extinct reptile whose family died out when the dinosaurs did, 65 million years ago — long after the newly found animal was swimming here. It was related to both dinosaurs and crocodiles. While many may imagine plesiosaurs as looking like the mythical "Nessie," with its long neck, this one happens to be a short-neck variety.

Gillette said its general body plan was something like that of a porpoise or seal. The reptiles were big, with some from this area reaching 20 feet long.

Lex Newcomb, an official at GCNRA, has been putting together a paleontological management plan for the recreation area, which is a unit of the National Park Service. Park officials commissioned the Museum of Northern Arizona for the project. A survey crew member ran across the plesiosaur two years ago, and because it is on an eroding hillside, saving the fragile fossil is deemed an emergency project.