The spider fossil is hard to differentiate from the rock. But when the fossil is studied under a fluorescent microscope, its chemical composition causes it to glow brightly, revealing additional details.

Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more.

CNN  — 

Spiders are not well represented in the fossil record.

Their soft external skeletons don’t typically preserve well – except at a few exceptional sites around the world. There is one remarkable spot in the south of France, where fossils of spiders that last spun a web 22.5 million years ago have been discovered.

Scientists said they have pinpointed why so many soft-bodied creatures such as spiders, insects and fish are entombed and preserved in such detail at this particular rock formation in Aix-en-Provence. The highly favorable conditions involve a substance produced by microalgae that would have coated the spider and promoted a protective chemical change.

“Most life doesn’t become a fossil,” said Alison Olcott, associate professor of geology and director of the Center for Undergraduate Research at The University of Kansas, in a news statement.

“It’s hard to become a fossil. You have to die under very specific circumstances, and one of the easiest ways to become a fossil is to have hard parts like bones, horns and teeth. So, our record of soft-body life and terrestrial life, like spiders, is spotty,” said Olcott, who was the lead author of the study published in the journal Communications Earth & Environment.

“But we have these periods of exceptional preservation when all circumstances were harmonious for preservation to happen.”

Fluorescence provides clues

The discovery was made thanks to a decision to examine the spider fossil under a fluorescent microscope, Olcott said in the release. This type of observation is not part of standard protocol for examining fossils, but the research team thought it might help them discern more details of the fossilized spiders, which blended into the surrounding rock. Different elements in the rock absorb the energy of the UV light in the microscope and remit light at different wavelengths.

“To our surprise they glowed, and so we got very interested in what the chemistry of these fossils was that made them glow. If you just look at the fossil on the rock, they’re almost indistinguishable from the rock itself, but they glowed a different color under the fluorescent scope.”

Not every geological sample is autofluorescent and glows – but when they are it can be spectacular and yield a lot of information, Olcott said. The microscopic aquatic algae revealed by the fluorescent microscope are known as diatom fossils, and when alive they secrete sulphur-rich substances that form algal mats.

“These microalgae make the sticky, viscous gloop — that’s how they stick together,” she said.

The authors suggested that this substance coated the spiders and enhanced a process called sulphurization, which stabilized and preserved the spiders’ fragile bodies.

“Basically, the chemistry of the microalgae and the chemistry of the spiders work together to have this unique preservation happen,” she said.

Olcott said the discovery could help geologists identify other exceptional fossil sites from this time period in other parts of the globe.

“If diatom mats do help induce this exquisite fossil preservation, then we should be able to further explore diatomite units, the diatom-rich rocks that are found globally at this time, to look for more of these deposits.