If one set of eyes is good, maybe two sets is better -- at least that's what a 305-million-year-old distant relative of modern spiders appears to have decided.

Researchers at the University of Manchester, along with colleagues from the American Museum of Natural History, used advanced X-ray techniques they say provided the ability to discern features on a well-preserved example of a fossil of a creature known as a harvestman at a level of detail never before possible.

Their examination of the ancient fossil of the harvestman, Hastocularis argus, discovered in the east of France, found an expected pair of median eyes -- so called because they are located near the center of the creature's body -- but they also determined it had eyes on the sides of its body, knows as lateral eyes.

"Arachnids can have both median and lateral eyes, but modern harvestmen only possess a single set of median eyes - and no lateral ones," says Manchester researcher Russel Garwood. "These findings represent a significant leap in our understanding of the evolution of this group."

There are differences between a harvestman and a true spider, he says.

"Although they have eight legs, harvestmen are not spiders; they are more closely related to another arachnid, the scorpion."

In today's world spiders are encountered on all continents with the exception of Antarctica.

 A clue to evolutionary change from four eyes to two was confirmed by a gene for an "eye stalk" in living examples of harvestmen; in the embryo of a modern example the gene shows evidence of a the long-lost lateral vision organ.

Studies of ancient terrestrial arthropods, including the harvestman, have always proved difficult because the exoskeletons of the creatures don't preserve as well as bone, the researchers say.

"As a result, some fundamental questions in the evolutionary history of these organisms remain unsolved," museum researcher and study co-author Prashant Sharma says.

"This exceptional fossil has given us a rare and detailed look at the anatomy of harvestmen that lived hundreds of millions of years ago," Sharma says.

The close study of the harvestman wouldn't have been possible until recently, the researchers said.

"Harvestmen fossils preserved in three dimensions are quite rare and our X-ray techniques have allowed us to reveal this exceptional fossil in more detail than we would have dreamed possible just a couple of decades ago," Garwood says.