Ancient relatives of harvestmen, otherwise known as opiliones or daddy longlegs, had two pairs of eyes, according to new research.
The discovery of a 305 million-year-old fossil of an arthropod reveals the creature evolved into having a single pair of eyes, which now differentiates the creature from other species of arachnid.
The fossil, named Hastocularis argus, was found in eastern France. Although harvestmen are classified as arachnids, the latest discovery reveals they are more closely related to scorpions, which are ancestors of ticks and mites.
H. argus shows the arachnid had lateral eyes located either side of its body, as well as median eyes found near the centre of the insect. The finding gives an insight into evolutionary history of arthropods, which inhabit every continent in the world with the exception of Antarctica.
Researchers from the American Museum of Natural History and the University of Manchester used X-ray imaging techniques to distinguish features of the well-preserved fossil.
According to Wired, the technique uses microscopic waves to reconstruct tiny body structures that had been invisible to generations of anatomists.
Dr Russell Garwood, a paleontologist at the University of Manchester, said: "Although they have eight legs, harvestmen are not spiders; they are more closely related to another arachnid, the scorpion."
"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."
Speaking to Eureka Science news, Prashant Sharma, a postdoctoral researcher at the Museum of Natural History, said: "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 unresolved. This exceptional fossils has given us a rare and detailed look at the anatomy of harvestmen that lived hundreds of millions of years ago."
Garwood added that signs of the now-extinct lateral eye exist in embryos of modern harvestmen, yet they do not form on the arachnid.
He said: "Fossils preserved in three dimenstions are quite rare. This is especially true of harvestmen. Our X-ray techniques have allowed us to reveal this fossil in more detail than we would have dreamed possible two decades ago."
The study was supported by the National Science Foundation.