110 Million-year-old Spiders With Eyes That Still Glow in the Dark Discovered

Two 110 million-year-old spiders with eyes that still glow in the dark have been discovered trapped in shale at a fossil site in South Korea. The two specimens found belong to an extinct spider family called Lagonomegopidae, and these specific creatures had huge reflective eyes that allowed them to hunt at night.

The spiders were found in the Lower Cretaceous Jinju Formation, a site known for fossils dating to the Mesozoic Era—the geological period between 252 and 66 million years ago. They were uncovered during a construction project and then studied by Paul Selden, from the University of Kansas, and colleagues from South Korea.

Spiders are soft-bodied, so it is very rare to find them preserved in rock. Normally, scientists have to study specimens that have become trapped in amber—which can lead to features being overlooked. "Amber fossils are beautiful, they look wonderful, but they preserve things in a different way," Selden said in a statement.

"Because these spiders were preserved in strange slivery flecks on dark rock, what was immediately obvious was their rather large eyes brightly marked with crescentic features. I realized this must have been the tapetum—that's a reflective structure in an inverted eye where light comes in and is reverted back into retina cells. This is unlike a straightforward eye where light goes through and doesn't have a reflective characteristic."

The glow in the dark eyes could be seen when the spiders were placed under light, as an image released by the team reveals. Findings are published in the Journal of Systematic Palaeontology.

The spiders lived between 110 and 113 million years ago. Their discovery allows scientists to better understand how other species of spider evolved. "In [modern] spiders, the ones you see with really big eyes are jumping spiders, but their eyes are regular eyes—whereas wolf spiders at nighttime, you see their eyes reflected in light like cats," Selden said. "So, night-hunting predators tend to use this different kind of eye.

"This is an extinct family of spiders that were clearly very common in the Cretaceous and were occupying niches now occupied by jumping spiders that didn't evolve until later. But these spiders were doing things differently. Their eye structure is different from jumping spiders."

These spiders are the first Lagonomegopidae that have not been preserved in amber to be discovered. How they ended up in the shale formation is unclear, but Selden says it could have been that they were washed into a body of water where they were protected from decaying bacteria.

"These rocks also are covered in little crustaceans and fish, so there maybe was some catastrophic event like an algal bloom that trapped them in a mucus mat and sunk them— but that's conjecture," he said.

The team now plans to go back to other amber-preserved Lagonomegopidae to see if they can find other examples of tapetum—now they know to look for it. It's nice to have exceptionally well-preserved features of internal anatomy like eye structure. It's really not often you get something like that preserved in a fossil."

It is thought spiders first appeared around 380 million years ago. There are thought to be around 40,000 and 50,000 species in existence today and they have been found on every continent except Antarctica. Selden said studying these ancient spider eyes will help researchers better understand arachnid evolution: "[It] will help us place this group of spiders among other families," he explained.