Fossilized spider

Image by Fabio M. Dalla Vecchia and Paul Selden/Wikimedia Commons (CC BY 2.5)

Ancient spiders have always been difficult to investigate since their soft and squishy bodies don’t fossilize as easily as bones or exoskeletons. Researchers were really excited to find 10 new spider fossils in an unexplored area called the Jinju Formation. This may not seem like a big deal considering that there are more individual spiders and more spider species than any other group of predators. However, a new paper in the Journal of Systematic Paleontology discusses these unique fossilized spiders that were discovered by a team of researchers including one from the University of Kansas. The other two collaborators were Tae-Yoon Park of the Korea Polar Research Institute and amateur fossil hunter Kye-Soo Nam of the Daejeon Science High School for the Gifted.

The Jinju Formation and the tapetum

The Jinju Formation is a Mesozoic geologic formation in South Korea. This area has been a known location for dinosaur bones in the past. This new group of fossils, which researchers from the Korea Polar Research Institute along with the University of Kansas found in shale, has increased the number of spiders in the formation to 11. Two of these spiders stood out among the rest because their eyes still reflected light 110 million years after death.

Paul Selden is a geologist at the University of Kansas and he has stated that “because these spiders were preserved in strange silver on a dark rock, what was immediately obvious was that their rather large eyes were brightly marked with crescentic features.” He realized after further research that this area of the eye must have been the tapetum which is a reflective structure in an inverted eye where the light comes in and is reverted into retina cells. The tapetum is often cone-shaped similar to a Canadian canoe. This structure aids their night vision abilities. Night-hunting predators tend to use this type of eye to their advantage. Human eyes don’t contain a tapetum, but a lot of other animals do. For instance, seeing cat eyes in the dark is due to the tapetum.

A first of its kind

Researchers believe this is the first preservation of a spider eye tapetum discovered. According to Selden, it is possible to distinguish between each type of spider by looking at their eyes, “the ones you see with really big eyes are jumping spiders, but their eyes are regular eyes while wolf spiders have eyes that reflect at night like cats.” Selden has also stated that this discovery gives them a better look at the internal anatomy, like the eye structure. Ancient spiders are often discovered in amber since the sticky substance helps preserve a spider’s soft body.

However, the researchers believe that if these spiders, named Koreamegops samsiki and Jinjumegops dalingwateri, were found in amber, the tapetum might have been missed. Selden and his team stated in their paper that since they don’t have hard shells, they easily decay. It needs to be a unique situation where they are washed into a body of water. Spiders usually float but, in this case, they sunk which was lucky because it kept them away from the decaying bacteria.

A form of biodiversity

These newly discovered spiders may have been occupying the same area as jumping spiders. But these ancient spiders are different because they have an entirely different eye structure. Selden has stated that although some contemporary spiders feature eyes with a tapetum, this new paper is the first to describe the anatomical feature in a fossilized spider. Finding 10 new spiders provides a huge change for the diversity of spiders from the Cretaceous period. The lack of fossils doesn’t give us much information about these ancient creepy crawlies but with further research, future fossils may eventually be discovered.