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Ancient spiders had two sets of eyes, according to a new fossil study.

The latest findings debunk the theory that 305-million-year-old harvestman- the ancestors of the modern-day arachnids- had only one set of eyes.

Researchers at the University of Manchester and American Museum of Natural History said the latest findings provide significant detail to the evolutionary story of this diverse and highly successful group of arthropods. Researchers said that spiders are found on every continent except Antarctica.

Researchers said modern x-ray imaging techniques gave them the ability to look at features of usually well-preserved fossils like never before.

The findings revealed that the primitive fossilized harvestman called Hastocularis argus, was discovered in eastern France. Researchers said that the spider had both median eyes, which are found near the centre of the body, and lateral eyes on the side of the body.

"Although they have eight legs, harvestmen are not spiders; they are more closely related to another arachnid, the scorpion," lead author Dr, Russell Garwood, a paleontologist in the University of Manchester's School of Earth, Atmospheric and Environmental Sciences, said in a news release.

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

"Terrestrial arthropods like harvestmen have a sparse fossil record because their exoskeletons don't preserve well. As a result, some fundamental questions in the evolutionary history of these organisms remain unsolved," Co-author Prashant Sharma, a postdoctoral researcher at the American Museum of Natural History, said in a statement.

"This exceptional fossil has given us a rare and detailed look at the anatomy of harvestmen that lived hundreds of millions of years ago. What we were also able to establish is that developing modern harvestmen embryos retain vestiges of eye-growth structures seen only in the fossil," said Sharma.

"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 added.

The study was published in the journal Current Biology.