Ancestors of modern-day arachnids had two sets of eyes rather than one, scientists have now discovered from the images of a 305-million-year-old harvestman fossil.
The primitive fossilised harvestman, named Hastocularis argus, was found in eastern France and had not only median eyes -- those found near the centre of the body -- but lateral eyes on the side of the body as well, said the scientists.
"Arachnids can have both median and lateral eyes, but modern harvestmen only possess a single set of median eyes -- and no lateral ones," said Russell Garwood, a palaeontologist at the University of Manchester in Britain.
"These findings represent a significant leap in our understanding of the evolution of this group," Garwood said.
The scientists used the X-ray imaging techniques to reveal features of the unusually well-preserved fossil like never before.
The team supported their results by examining the expression of an "eye stalk" gene in living harvestmen and found that in a modern harvestman embryo this gene shows hints of a now-lost lateral eye.
The findings of the study appeared in the journal Current Biology.
