Ancient Daddy Longlegs was a Four-Eyed Critter, While Modern Species Just Have Two
They go by a number of names - harvestmen, opiliones, daddy longlegs - but all living examples of the arachnid have one thing in common: a single pair of eyes. That was not always the case, however.
New research shows that ancient relatives of the polynymous arachnid had two pairs of eyes, a revelation that adds significant detail to the evolutionary history of the creature.
The find is the result of analyzing an exceptionally preserved, 305-million-year-old fossil found in eastern France.
"Terrestrial arthropods like harvestmen have a sparse fossil record because their exoskeletons don't preserve well," said lead study author Prashant Sharma, a postdoctoral researcher at the American Museum of Natural History. "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."
Although harvestmen have eight legs and are classified as arachnids, the arthropods are not spiders, despite their similarities.
The arachnids are more closely related to scorpions, which are related to mites and ticks. These arachnids have one of two types of eyes: lateral (on the side of the body) and median (near the middle of the body).
Extant harvestmen have median eye placement. Lateral eye placement is not seen on species of harvestmen found on any continent today.
But the fossilized specimen has both median and lateral eyes, a surprising find, the researchers said.
After scanning the fossil with high-resolution X-ray imaging equipment and building a 3D model of the ancient creature, the researchers turned towards the present to corroborate their results.
The found that a particular "eye-stalk-growing" gene in living species of harvestmen can been seen briefly in the arachnid's embryos. The gene expresses itself in the lateral area, but by the time the harvestmen hatch, they only have one pair of eyes, in the median position.
Sharma and her colleagues published their work in the journal Current Biology.
An earlier version of this article inaccurately described harvestmen as insects, the error has been corrected. Many thanks to Nature World News readers for pointing out the mistake.