A 304-million-year-old fossil discovered in Eastern France shows primitive living harvestmen -- more commonly called daddy longlegs -- had one more pair of eyes than they do today. 

The ancient harvestmen had a pair of eyes along the middle of the body -- like their modern counterparts -- but they also had a pair of eyes on the side of the body. The findings were reported by researchers from the American Museum of Natural History and the University of Manchester, in the journal Current Biology.

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Scientists studied the fossil using high-resolution X-ray imaging at the Natural History Museum, London.

"Our X-ray techniques have allowed us to reveal this fossil in more detail than we would have dreamed possible two decades ago,” said Russell Garwood, a research fellow at the University of Manchester and a lead author on the study, in a release.

Though Harvestmen have eight legs and are categorized as arachnids, they're not spiders. They're more closely related to scorpions.

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The scientists also examined the expression of an eye-stalk growing gene in harvestmen embryos. The embryos briefly express the gene for the second pair of eyes. But by the time they hatch, the daddy long legs' second pair of eyes are long gone.

Photo: Modern daddy longlegs have just one set of eyes, unlike their primitive ancestors. Wikimedia Commons/Thomas Bresson