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Four Eyed Harvestman Fossil Offers Insights in Arachnid Evolution

Four Eyed Harvestman

Image credit: Paris NHM/Russell Garwood

In the eastern part of France, scientists have found the fossil of an extinct harvestman species dating back to 305 million years ago. However being different from the today’s harvestmen with one set of eyes, this specimen had two. The results have been released in the article of the Current Biology.

Fossil placement is the important key in identifying evolutionary transitions and decoding the order in which a certain characteristic was obtained in organisms of interest.

As kind of arachnids, harvestmen belong to the order Opiliones, and so far more than 6,300 species have been already found all over the world apart from Antarctica.

Dr Russell Garwood with the University of Manchester notes in his article that though harvestmen might look quite similar to spiders, in fact, they are much closely associated with scorpions. While today’s harvestmen have just one set of eyes, other existing arachnids like spiders may have more than one set of eyes.

Based on examination of such exceptionally preserved fossil by X-ray imaging, the scientists from the American Museum of Natural History and the University of Manchester have found that compared with modern harvestmen with one set eyes at the center, the specimen, called as Hastocularis argus, had two sets of eyes which are respectively located at the center and the each side of their body. The finding of such four-eyed harvestman fossil could give the clues to the evolution of this diverse order of animals.

The scientists were also engaged in examination of gene expression in existing harvestmen embryos, thus finding remarkable evidence that the embryos of modern harvestmen did had remnants of the second set of eyes which are now extinct.

As the harvestmen are incapable of fossilizing well, the finding of this specimen could provide the good chance to make inferences about how on these animals had been evolved.

Dr Garwood said that harvestmen fossils well preserved in three dimensions are not easily acquired. By application of X-ray techniques, the scientist would be able to get more information about this exceptional fossil that they possibly expected just a couple of decades ago. Therefore such fossils are thought to be critical to understand the morphological evolution of these animals.

Source: Current Biology