SOLOMONS - An amateur fossil hunter strolling the banks of the St. Mary's River after Tropical Storm Isabel stopped in his tracks when he saw a bone protruding from the sandy muck. Beneath his feet, he soon learned, were the skull and jaws of a whale that swam here 8 million years ago, when Southern Maryland was under the Atlantic.

Paleontologists toiled through snow and rain to dig up the bones of the ancient mammal. Then a Navy helicopter hauled the half-ton load to workers from the Calvert Marine Museum, which is to announce the discovery today.

Scientists say the find is likely to deepen understanding of the evolution and diversity of whales in the Miocene epoch, a warm period when subtropical waters lapped across Maryland. Paleontologists say the fossil is the most complete whale skull ever found in an expanse of sedimentary rock in Southern Maryland and northeastern Virginia known as the St. Mary's Formation.

The whales that roamed the Atlantic 8 million to 11 million years ago are an enduring mystery, in part because they were undergoing a critical change. A diverse family of smaller whales called Cetotheres that ruled the oceans for 20 million years were dying out. In their place rose a family of larger mammals related to today's humpback, fin and blue whales.

The fossil "may help answer not only how they were related, but why that change occurred," says Alton C. Dooley Jr., a specialist on Miocene whale fossils at the Virginia Museum of Natural History. "Why do we have a different group of whales now? Why are whales so much bigger than they used to be?"

Another puzzle is what the whale was doing in 50-foot-deep waters. The bulk of whale fossil finds in the state have come from the Calvert and Choptank formations, also in southern Maryland, where waters were many times deeper.

"It's not the type of environment where you expect large whales," Stephen J. Godfrey, the curator of paleontology at the Calvert Marine Museum, said this week in an office cluttered with fossilized crystals, gaping shark jaws and dinosaur replicas. The discovery "will tell us more about whales in this block of time where we were woefully ignorant."

The fossil was found in late September, a few days after Tropical Storm Isabel raked the sea cliffs over the St. Mary's River, tumbling trees and churning up layers of long-buried sediment. Scientists have asked that the exact location be kept secret to discourage unscrupulous collectors.

Jeffrey W. DiMeglio, 22, of Alexandria, Va., a stone carver who has hunted fossils since he was a boy, canoed to the river bank with his girlfriend to look for fossilized shark's teeth and crabs. They had little luck.

But after about an hour of wading barefoot through the soupy sand, DiMeglio stumbled upon the top two inches of what he recognized as a whale rib. "Everything else we were doing at that point came to a halt," recalled DiMeglio, who realized he had just made the find of his life.

He and his girlfriend, Sarah Gulick, hid it under sand and plywood and alerted the museum, a small facility in Solomons known for its collection of marine fossils.

Godfrey and a small crew worked in waders for two weeks, digging with shovels, marsh picks and screwdrivers. The rib bone connected to others, all entombed in grayish sediment a few feet from the water. In time they extracted the nearly 6-foot-long skull and the lower jaws, together the most important find. They also found a shoulder blade, some neck vertebrae, ribs and flipper bones.

The curator could tell from the absence of teeth that the whale was a member of the baleen family. Baleens gulp mouthfuls of water, shut their jaws, then use their tongues to expel the fluid. Hard, hairy plates - baleens - attached to their upper jaws act as a filter, trapping plankton and other food.

After the excavation, Godfrey and his workers wrapped the mud-caked bones in a protective jacket of burlap soaked in plaster of paris. The entire package - which looked like a mummy on a stretcher - weighed about 1,000 pounds.

Museum officials stumped about how to move the fossil got in touch with the Patuxent River Naval Air Station, across the Patuxent River from the museum.

The base's commander, Capt. Dane Swanson, agreed to help after concluding that the fossil "rescue" would count toward mandatory training for its search and rescue team.

On the afternoon of Dec. 16, a UH-3H Sea King helicopter with two pilots and three crewmen lifted off for the dig site, at the bottom of a cliff topped by towering trees. As the helicopter hovered above the tree line, Chief Petty Officer Robert Mirabal, a search and rescue swimmer, slid out the cargo door and rappelled down a rope to shore.

Mirabal clipped a braided nylon rope to the jacketed whale bones. The chopper, with the mummified cargo dangling 120 feet below, flew for 10 minutes before gently landing at a nearby Naval airfield, where museum workers helped load it onto a truck.

"The only difference between this and an actual training flight is we were picking up a whale skull," says Mirabal. "I got bragging rights that I short-hauled the oldest victim in the Navy."

On the museum floor this week, workers carefully cleared sandy mud from around the bones with paint brushes and dental picks. Godfrey estimates it will take two months to clean the fossil. Museum visitors can watch the work.

Until now, St. Mary's Formation has yielded remains of one whale species. The unusually narrow, pointed snout of this fossil leads some paleontologists to speculate that it may be an unknown species of baleen.

Godfrey said it is difficult to say what killed the whale, an adult of undetermined sex that probably spanned 18 feet. One clue: Teeth from extinct mako and cow sharks were found beside the skull.

"It's a whodunnit," said Godfrey, who will begin a close examination of the fossil once it is cleaned. "We're trying to piece together evidence from 8 million years ago."