The animal, a new member of a large species group called ostracode, was buried under volcanic ash which mineralized and retained an image of its soft body parts. That unique preservation enabled researchers to construct a highly detailed three-dimensional picture of the animal after digging the fossil from a rock bed in Herefordshire.
Details revealed include gills, eyes, limbs designed for swimming and the oldest known male organ in the fossil record. It was this last that led researchers to name the new species, Colymbosathon ecplecticos, which is Greek for "amazing swimmer with large penis."
"The whole animal is amazing," David Siveter, a researcher at the University of Leicester, said in a statement. He is first author of a study appearing this week in the journal Science.
Thomas M. Cronin, a U.S. Geological Survey geologist, said in an interview that the discovery by Siveter and his team "is significant for a number of reasons."
Cronin said the animal is the oldest male in the fossil record and the specimen contains images of soft parts that could only be guessed at by analyzing other 425-million-year old fossils.
"Any fossil with soft parts from the Silurian (430 million to 400 million years ago) is unusual," he said. "They've been able to basically dissect and describe features we never see in fossils."
Siveter and his team used a new technique that combined thin-film cutting and data storage in a computer.
They removed the rock containing the fossil and then cut extremely thin shavings through the remains. Each shaving was then photographed and stored in a computer. Later, the researchers were able to electronically reassemble the slices and create a three-dimensional image of the animal, along with its soft parts.
That, Cronin said, has never been done before for such an ancient animal.
Modern relatives of C. ecplecticos are found in virtually every aquatic environment on Earth, from deep oceans to shallow streams. They are bivalves, but are more closely related to crabs and lobsters than to clams or oysters. Most animals in this group graze on dead organic matter.
C. ecplecticos was about 0.20 inch at its widest dimension and its soft body parts closely resemble those of its modern relatives.
Cronin said this close similarity shows that the ostracode group of animals has changed remarkably little over hundreds of millions of years, even though most other species underwent significant evolutionary redesign or disappeared altogether.
"This specimen shows what nobody has been able to show before - 425 million years of unbelievable stability in an organism," he said. "Most things that lived in the Silurian are long extinct and don't have many living relatives."
By Paul Recer