The imprints, discovered in 330-million-year-old rocks from Pennsylvania, show the webbed feet and bodies of three previously unknown, foot-long salamander-like critters, the researchers said.

"Body impressions like this are wholly unheard of," said paleontologist Spencer Lucas, a curator at the New Mexico Museum of Natural History and Science in Albuquerque.

The imprints, while lacking any bones of the animals, actually contain rare information that bones cannot, said Lucas. Without the imprints of the webbed four-toed feet, for instance, he said it would be virtually impossible to say they were truly amphibians.

Also found in rocks from the same formation and of the same age are footprints of other relatively large animals and fossils of insects and plants. A saucer-sized footprint of an unknown vertebrate that was found suggests larger four-footed beasts lived far earlier than ever before suspected.

Lucas presented the discovery Tuesday in Denver during the annual meeting of the Geological Society of America.