(HEREFORDSHIRE, ENGLAND) -- A tiny fossil of a creature that lived some 425 million years ago has entered the record books as the oldest unequivocally male animal. The organism, which resembles a cross between a shrimp and a clam, sports a large penis which has been perfectly preserved in three dimensions.

David Siveter, professor of palaeontology at Leicester University, said there was no doubt the fossilised creature was a male, making it the oldest unambiguous member of its sex.

Professor Siveter said the fossil was found at a site in Herefordshire where soft body parts of ancient creatures have been perfectly preserved by being buried in volcanic ash before being rapidly mineralised within rocky nodules.

The animal, whose Latin name Colymbosathon Ecplecticos means "swimmer with a large penis", is just 5mm (2/10ths of an inch) long and a member of a group of crustaceans known as the ostracods, crustaceans like crabs and lobsters and the most complex organisms studied within the field of micropalaeontology.

Three-dimensional images of the fossil, in the journal Science, were constructed by the scientists who carefully ground away layers of the rock while photographing polished cross-sections of the fossil as they went. One of the most intriguing aspects of the study was the discovery that the basic body plan of the 425 million-year-old ostracod looks essentially the same as the body plan of present-day ostracods, Professor Siveter said.

"The ground plan of the fossil strikingly resembles living groups of ostracods which shows the pace of evolutionary change has not been particularly fast," he said.

The copulatory organ of the fossilised male is "large and stout" which indicates that "maybe size mattered" for the ancient ostracods, he added.

"This is a male and there must have been females around. The discovery tells us something about the biology of reproduction; it provides unequivocal evidence for sex in an animal that lived in the Silurian age," he added.

In addition to the penis, the scientists were able to trace the animal's entire digestive tract from mouth to anus as well as study anatomical details such as its six pairs of gills and the limbs it used for sensing, feeding and swimming.

"The whole animal is amazing," Professor Siveter added. "We've got something we could only dream about before."