Scientists digging at a highway construction site south of Smyrna, Del., have turned up the 17-million-year-old remains of a hornless rhinoceros and two species of early horses the size of an Irish setter. They are calling it one of the richest deposits of land-mammal fossils on the East Coast north of Florida.
"It's a once-in-a-lifetime opportunity for us," said Dr. Kelvin W. Ramsey, a scientist with the Delaware Geological Survey in Newark. "It's the best fossil site north of Florida for mammals . . . I think for any period."
Other mammal remains found in the 50-foot-deep excavation have included foot bones and a claw from a chalicothere (pronounced calico-theer), a horselike creature with long front legs, short rear legs and a sloping back. Instead of hoofs, the animal had three-toed feet with large claws.
It is believed to be the first chalicothere fossil found east of the Mississippi outside Florida. Chalicothere fossils also have been found in Asia, Europe and Africa.
David J. Bohaska, a paleobiologist at the Smithsonian Institution in Washington, said paleontologists and construction workers also have found fossils identified by the Smithsonian as the remains of early beaver, rabbit, a small peccary and an extinct deerlike animal.
One partial limb bone may be that of a large saber-toothed cat, Dr. Ramsey said.
The dig has also turned up the shells of more than 100 marine mollusks, including about two dozen species never seen before.
Experts at the Virginia Museum of Natural History in Martinsville have also identified the earliest known example of Crassostrea virginica oysters, "the same ones you dredge out of the Chesapeake today," Dr. Ramsey said. "But some of these are 10 to 12 inches long."
Dr. Ramsey reported the find Saturday in Harrisburg, Pa., at a meeting of the Northeast Section of the Geological Society of America.
The Delaware Geological Survey has been conducting the dig since last summer with help from the Smithsonian and the Virginia museum.
The Smyrna deposit, buried 50 feet below the surface, was once a sandy shoal, less than a mile from shore in Miocene times -- 5 million to 26 million years ago, scientists believe.
"Based on microfossils, we've dated the site to about 17.5 million years before the present," Dr. Ramsey said.
The Delaware fossils are similar to, but a bit older than, Miocene marine deposits at Calvert Cliffs State Park, which are about 15 million years old. Few land-mammal remains have been found at Calvert Cliffs.
Most of the Smyrna fossils are also marine -- crocodiles, porpoises, whales, sharks, fish, shore birds, sea turtles, rays, a sea cow and mollusks. How the remains of land mammals got there is the subject of some debate.
"The best explanation is that they died near a stream or near a swamp, and that their bodies . . . floated and eventually found their way out to the ocean," Dr. Ramsey said. "The other possibility is that they died, and . . . as the shore eroded, their bones mixed in with the other marine forms."
From fossil pollens found in the Smyrna pit, Dr. Johan Groot of the Delaware Geological Survey has determined these mammals lived in a forest of oaks and pines that grew to the shoreline.
"There is no indication of broad, expansive marshes like we see today," said Dr. Ramsey. There were palm trees, too, suggesting a climate like that in southern Georgia today.
The Miocene deposits of shells, sharks' teeth and other marine fossils at Calvert Cliffs and at a similar site near Shiloh, N.J., are known to geologists around the world as the Calvert Formation.
Scientists knew from drilling in parts of Delaware that the New Jersey and Maryland sites were likely linked. But not much had been exposed on the surface.
Because land-mammal fossils were so rare at the Maryland and New Jersey sites, little was known of the animals that lived on shore at the time the Calvert marine deposits were being laid down, Dr. Ramsey said.
Mr. Bohaska said the Smyrna dig is significant because the presence of marine and land animals together allows scientists to link them in time and with similar species found at distant locations.
The site was first uncovered last summer by highway construction crews digging for fill.
As the hole got deeper, Delaware geologist Scott Andres spotted Miocene shell beds and alerted paleontologists. Today, the pit covers several acres.
For safety reasons, there is no public access to the site, nor is its exact location disclosed.
Dr. Ramsey said some of the fossils may eventually be displayed at the museum in Virginia and at the Delaware Geological Survey at the University of Delaware in Newark.