Researchers noticed that a 305-million-year-old Daddy Longleg ancestor had two sets of eyes instead of just one.
The finding could help scientists gain insight into the evolution of these arthropods (Daddy Longlegs are not spiders as is commonly believed), a University of Manchester news release reported.
"Although they have eight legs, harvestmen are not spiders; they are more closely related to another arachnid, the scorpion," author Doctor Russell Garwood, a paleontologist at the University of Manchester's School of Earth, Atmospheric and Environmental Sciences, said in the news release.
The ancient fossil, dubbed Hastocularis argu, had eyes both in the center of the body and on the sides.
"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," Garwood said.
The researchers used X-ray imaging techniques to discover the eyes on the ancient but well-preserved fossil. They also looked at an "eye stalk gene" in modern-day Daddy Longlegs; they found the embryo the gene showed evidence of a past lateral eye.
"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 the news release. "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."
The study findings were published in the journal Current Biology.
"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 said.
