A 305-million-year-old fossil found in eastern France reveals that primitive harvestmen had two sets of eyes rather than one.
Living harvestmen (Opiliones) – a group of arachnids more commonly known as daddy longlegs – have a single pair of eyes that help them navigate habitats in every continent except Antarctica.
But the newly discovered Carboniferous fossil of a primitive harvestman named Hastocularis argus shows that wasn’t always the case.
Paleontologists from the United States and United Kingdom used X-ray imaging techniques to reveal features of the unusually well-preserved fossil like never before.
“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,” said Dr Russell Garwood from the University of Manchester, the lead author of the paper appearing in the journal Current Biology.
“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 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,” explained study co-author Dr Prashant Sharma from the American Museum of Natural History.
The Hastocularis argus fossil was unearthed in France and had not only median eyes – those found near the centre of the body – but lateral eyes on the side of the body as well.
“Although they have eight legs, harvestmen are not spiders; they are more closely related to another arachnid, the scorpion. Arachnids can have both median and lateral eyes, but modern harvestmen only possess a single set of median eyes – and no lateral ones,” Dr Garwood said.
“These findings represent a significant leap in our understanding of the evolution of this group.”
The scientists corroborated their results by looking at the expression of a particular ‘eye-stalk-growing’ gene in a living species of harvestmen.
They found that developing embryos briefly express this gene in the lateral area, where the second eye pair is located in the fossil. But by the time they hatch, harvestmen only form the median eyes.
Russell J. Garwood et al. A Paleozoic Stem Group to Mite Harvestmen Revealed through Integration of Phylogenetics and Development. Current Biology, published online April 10, 2014; doi: 10.1016/j.cub.2014.03.039