Scientists have discovered a 305-million-year-old fossil of a daddy longlegs, a joint-legged invertebrate, which revealed that the ancient predecessors of this arachnid -- also known as harvestmen -- had four eyes, compared with currently found harvestmen, which have only two eyes.

The scientists at the American Museum of Natural History in New York and the University of Manchester in England found the fossil in eastern France, and they expect it to help researchers better understand the evolutionary story of harvestmen, a diverse group of arachnids commonly known as daddy longlegs, particularly in Britain and North America.

“Terrestrial arthropods like harvestmen have a sparse fossil record because their exoskeletons don’t preserve well,” Prashant Sharma, a researcher at the American Museum of Natural History and one of the lead authors of the study, said in a statement. “As a result, some fundamental questions in the evolutionary history of these organisms remain unresolved. This exceptional fossil has given us a rare and detailed look at the anatomy of harvestmen that lived hundreds of millions of years ago.”

Arachnids can have two kinds of eyes -- lateral (on the side of the body) and median (near the middle of the body). According to the study, published in the journal Current Biology on Thursday, the harvestmen living today lack the lateral eyes. But, Hastocularis argus, the new species described by the researchers based on the fossil, had both median and lateral eyes. The scientists used high-resolution X-ray imaging to study the fossil’s strange anatomy.

Although harvestmen have eight long legs, they are not spiders, but are more closely related to scorpions among the arachnids, a group that also includes mites and ticks. The most obvious difference between harvestmen and spiders is that, in harvestmen, the connection between the cephalothorax -- the front section among arachnids that consists of the head and thorax -- and the abdomen is broad, so that the body appears to be a single oval structure.

“Fossils preserved in three dimensions are quite rare,” Russell Garwood, a research fellow at the University of Manchester and a lead author of the study, said in the statement. “Our X-ray techniques have allowed us to reveal this fossil in more detail than we would have dreamed possible two decades ago.”