By Dr David Whitehouse
BBC News Online science editor
Scientists have identified the oldest male fossil animal yet discovered. It is an ocean-dwelling creature from 425-million-year-old rocks in the UK.
The virtual ostracode (top) with its modern relative (bottom)
Unusually, its soft parts are well preserved as well as its hard shell. It has limbs for swimming and feeding.
It also has what scientists say is the oldest penis seen in the fossil record.
Researchers are puzzled as to why the ancient creature appears so similar to its modern relatives. Their research is to be found in the journal Science.
Three dimensional details
The fossil record is packed with shells thought to be from a group of arthropods called ostracodes. They are so numerous and varied that geologists use them to date rock layers.
Their soft tissues are rarely seen but David Siveter, of the University of Leicester, and colleagues found an ancient ostracode that had been buried in volcanic ash during the Silurian Period.
The creature quickly mineralised and had its most delicate tissues preserved.
The find was made in the county of Herefordshire.
"It pushes back our knowledge about the palaeo-biology of an important group of animals by more than 200 million years," Professor Siveter told the BBC.
His team cracked open the rock that entombed the creature and used a "shave and photograph" technique that yielded a virtual fossil with carefully preserved three-dimensional details.
"The whole animal is amazing," Professor Siveter added. "We have got something we could only dream about."
The ostracode's appendages suggest that it swam and scavenged for food along the ocean floor. It was also definitely male as it has the oldest known example of a penis.
The rock was subjected to a "shave and photograph" technique
The scientists say that the five-millimetre-long fossil is remarkably similar to some modern ostracodes, suggesting an extremely low rate of evolutionary change over the last 425 million years.
"This is a demonstration of unbelievable stability," said Dr Tom Cronin, of the US Geological Survey.
As the discoverer of a new species, David Siveter and his co-researchers provided the name for the ostracode. They have called it Colymbosathon ecplecticos, which means "amazing swimmer with a large penis".