Glowing Ancient Spider Fossils Offer Rare Glimpse of Extinct Life

"It was just an amazing rainbow of autofluorescence," said the lead author of a study on the spider fossils.
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Fossilized spider from the Aix-en-Provence Formation. Image: 

Olcott et al

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Scientists have solved a paleontological mystery by examining the multihued glow of ancient spider fossils that are exposed to ultraviolet light, reports a new study. 

The discovery opens a new window into a 23-million-year-old lake ecosystem that is preserved in exceptional detail in rocks found near Aix-en-Provence, France. Researchers have been studying these fossils in the so-called “Insect Bed” since the 1700s because of their spectacular quality. But now a team led by Alison Olcott, associate professor of geology at the University of Kansas, has pinpointed the secret to their longevity over millennia, and it comes down to an odd glow. 


A new study describing the work chalks this glow up to the role of diatoms, single-celled organisms that form microalgae, which have preserved the exquisite remains of soft-bodied creatures such as spiders, which normally decompose without leaving fossils.

Olcott and her colleagues present “the first description of diatoms from the Aix-en-Provence Formation, despite its long history of investigation,” according to a study published on Thursday in the journal Communications Earth & Environment. The study reports that the diatoms played a “hitherto unknown” role in preserving soft-bodied species that may be “responsible for much of our understanding of insect, arachnid, amphibian, and plant life” in these lake settings.

“As far as we know, nobody has ever reported diatoms from the site,” said Olcott, who is also director of the Center for Undergraduate Research at KU, in a call. “This Aix-en-Provence fossil deposit is interesting because there's this historical aspect as well, where people have been describing fossils for centuries—looking at all these really cool insects, spiders, fish, shrimp that they pull up.” 

While the site has long been known to researchers, the mechanism that captured these incredible fossils has remained elusive. Now, Olcott and her colleagues have literally shed light on this question by studying the spider fossils under ultraviolet wavelengths.


“I have a fluorescent microscope and I love seeing what rocks and fossils do underneath it, because geological specimens can fluoresce depending on their mineralogy and chemistry,” Olcott said. With that in mind, she invited study co-author Matthew Downen, who was then a doctoral candidate at KU and now serves as the assistant director at the Center for Undergraduate Research, to take a look at the spider fossils with the instrument.  

The results were “really exciting,” she said. “It was just an amazing rainbow of autofluorescence, with all these details.”


Spider fossil with the presence of two kinds of siliceous microalgae: a mat of straight diatoms on the fossil and dispersed centric diatoms in the surrounding matrix. Image: Olcott et al

In addition to revealing incredibly fine features of the spiders, such as the hairs on their legs, the technique also exposed the presence of diatoms in the rock. Subsequent observations using a scanning electron microscope confirmed that the microalgae in this ancient lake created a complex chemical environment that was essential to entombing this rich ecosystem for posterity.

Previous studies have noted that microalgae chokes out oxygen in aquatic environments, creating anaerobic patches that slow the decomposition of dead animals. But Olcott and her colleagues show that a goopy material made by the diatoms, called extracellular polymeric substances (EPS), also plays a major role in the fossilization process. The EPS from the microalgae spurs bacteria in the environment to make sulfide, which in turn reacts with the spider exoskeleton in ways that promote fossilization.  


“It's basically the perfect combination of chemical environments,” Olcott explained. “The sulfide stabilizes the spider exoskeleton compounds, and then lets it get preserved into the rock record. It gives it a fighting chance to be fossilized.”

This overlooked process of fossilization may explain similar formations around the world, and it can reveal new information about inhabitants of bygone habitats that are largely lost to time. There are also clues about the present day hidden in these diatom structures; because lakes are generally more sensitive to environmental changes than marine habitats, scientists can study these deposits to get a sense of how modern ecosystems might respond to our era of human-driven global warming.

“We had all these time periods of climate change, so if we can understand how life responded in those time periods, that would be really useful for figuring out what could happen now,” Olcott said. “But to know how to interpret the fossil record, I think we have to know how we got that fossil record. Are we looking at an accurate representation of what that environment was? The first step to knowing that is knowing how those fossils came to be to start with.”

“Right now, we have great evidence for this is what happened at Aix-en-Provence and really intriguing hints that this could have happened wider,” she concluded. “And I really hope to go and see what else can be seen in these other deposits with their chemistry and fossils.”