Scientists have discovered a 305-million-year-old fossil of an extinct harvestman species in eastern France, but unlike modern day harvestmen which have only one set of eyes, this specimen had two. The results have been published in the journal Current Biology.

Fossil placement is key in recognizing evolutionary transitions and deciphering the order in which a particular characteristic was acquired in organisms of interest.  Harvestmen are arachnids belonging to the order Opiliones, and over 6,300 species have been discovered in every continent across the globe apart from Antarctica. According to author Dr Russell Garwood of the University of Manchester, although harvestmen may look similar to spiders, they are actually more closely related to scorpions. While modern day harvestmen have only one set of eyes, other modern arachnids such as spiders may have more than one pair.

Scientists from the University of Manchester and the American Museum of Natural History used X-ray imaging to examine this exceptionally preserved fossil. They discovered that the specimen, named Hastocularis argus, had a pair of eyes at the center of the body as well as at the side of the body, whereas modern harvestmen only possess a set at the center. This four-eyed harvestman fossil may therefore help to shed light on the evolution of this diverse order of animals.

The team also examined gene expression in extant harvestmen embryos and found evidence which suggested that developing modern harvestmen embryos possess remnants of this now-lost second set of eyes.  

Since harvestmen do not fossilize well, the finding of this specimen presented exciting opportunity to make inferences on the evolution of these animals. According to Dr Garwood “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.” Fossils such as these are therefore critical in understanding the morphological evolution of groups of animals.