In a paper published in the Journal of Systematic Palaeontology, an international team of paleontologists from the University of Kansas, the Korea Polar Research Institute and the Daejeon Science High School for the Gifted has described a collection of fossil spiders from an area of Korean shale called the Jinju Formation. What’s the most remarkable: two of the newly-described specimens feature reflective eyes that enabled their nighttime hunting.

The newly-discovered spiders lived in what is now Korea between 110 and 113 million years ago (Cretaceous period).

The Jinju shale preserved the fossils in a manner that highlighted the reflectivity of the tapetum, a feature that may have been missed had the spiders been preserved in amber instead, as is more typical.

“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,” said University of Kansas’ Professor Paul Selden.

“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.”

Some contemporary spiders feature eyes with a tapetum, but this study is the first to describe the anatomical feature in a fossilized spider.

“In 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,” Professor Selden said.

“So, night-hunting predators tend to use this different kind of eye.”

“This was the first time a tapetum had been in found in fossil.”

“This tapetum was canoe-shaped — it looks a bit like a Canadian canoe. That will help us place this group of spiders among other families.”

The description of the fossils increases the number of known spiders from the Jinju Formation from one to 11.

Beyond the novelty of discovering the first fossilized-spider tapetum, they informed the scientific understanding of Cretaceous biodiversity.

“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,” Professor Selden said.

“But these spiders were doing things differently. Their eye structure is different from jumping spiders. 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.”

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Tae-Yoon S. Park et al. A diverse new spider (Araneae) fauna from the Jinju Formation, Cretaceous (Albian) of Korea. Journal of Systematic Palaeontology, published online January 28, 2019; doi: 10.1080/14772019.2018.1525441