Prashant Sharma, a postdoctoral researcher in the American Museum of Natural History Division of Invertebrate Zoology, and Dr. Russell Garwood, a paleontologist in the University of Manchester's School of Earth, Atmospheric and Environmental Sciences, presented the first evidence that ancient harvestmen had four eyes in the April 10, 2014, edition of the journal Current Biology. Daddy long legs are also known as harvestmen.
The 305-million-year-old fossil of Hastocularis argus was originally discovered in France. Technology at the time of discovery could not detect the presence of two sets of eyes in the fossil. Harvestmen are not spiders. Harvestmen are more closely related to modern scorpions and are classified as an arachnid because the insects have eight legs.
The researchers used high-resolution x-ray microscopy to examine the harvestman fossil and found two sets of eyes. One pair of eyes was on the side of the body and one pair of eyes was near the middle of the body. Modern harvestmen have only one set of eyes near the middle of the body.
The fossil was essentially flat but x-ray techniques were able to produce a three-dimensional representation of the fossil harvestmen. The fossil is approximately 10 millimeters long and 20 millimeters wide including the legs.
The researchers examined the embryonic development of modern daddy longlegs in hopes of determining the evolutionary development that produced a single pair of eyes in modern arachnids that are related to harvestmen. During embryonic development, modern harvestmen briefly demonstrate the expression of a gene that forms a vestigial pair of eyes on the side of the body. The genetic material is available for development of a second pair of eyes but is not fully expressed in modern harvestmen.
The rarity of fossil harvestmen and other ancient arachnids makes an interpretation of the loss of a second pair of eyes difficult. The researchers speculate that environmental factors, a change in predators, or a change in harvestmen prey may have produced the adaptation to one pair of eyes.