It is difficult for spiders to become a fossil, because they have soft external skeletons that cannot be well preserved.However, with the help of a microalgae, they can improve their sulfidation process.(CNN) - Spiders are not well represented in the fossil record.Their soft external skeletons are not normally well preserved except at a few exceptional sites around the world.There is a remarkable place in the south of France where fossils of spiders that last spun a web 22.5 million years ago have been discovered.The scientists said they have identified why so many soft-bodied creatures, such as spiders, insects and fish, are entombed and preserved in such detail in this particular rock formation in Aix-en-Provence.Highly favorable conditions involve a substance produced by microalgae that would have coated the spider and promoted a protective chemical change.Read also: They discover the fossil of a dinosaur that could have died the same day that the large asteroid hit Earth"Most life does not become a fossil," Alison Olcott, an associate professor of geology and director of the University of Kansas Undergraduate Research Center, said in a news release.“It is difficult 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-bodied, terrestrial life, such as spiders, is patchy," said Olcott, lead author of the study published in the journal Communications Earth & Environment."But we have these periods of exceptional preservation when all the circumstances were harmonious for preservation to happen."The discovery was made thanks to the decision to examine the spider fossil under a fluorescent microscope, Olcott said in the statement.This kind of observation isn't part of the standard protocol for examining fossils, but the research team thought it might help them discern more detail about the fossilized spiders, which blended into the surrounding rock.Different elements in the rock absorb the energy of ultraviolet light in the microscope and re-emit the light at different wavelengths.Read also: They discover the first fossil record of an ancient diurnal owl“To our surprise, they glowed, so we were very interested in what the chemistry was in these fossils that made them glow.If you just look at the fossil in the rock, they are almost indistinguishable from the rock itself, but they glowed.a different color under the fluorescent microscope.Not all geological samples are autofluorescent and glow, but when they are they can be spectacular and yield a lot of information, Olcott said.Microscopic aquatic algae revealed by fluorescent microscopy are known as diatom fossils, and when alive they secrete sulfur-rich substances that form algal mats."These microalgae form the sticky, slimy mass, that's how they hold themselves together," she said.The authors suggested that this substance coated the spiders and enhanced a process called sulfidation, which stabilized and preserved the spiders' fragile bodies.Read also: The most complete T. rex fossil in the world has a new home"Basically, the chemistry of the microalgae and the chemistry of the spiders work together to make this unique preservation happen," he said.Olcott said the discovery could help geologists identify other exceptional fossil sites from this period in other parts of the world.“If diatom mats help induce this exquisite preservation of fossils, then we should be able to further explore diatom units, the diatom-rich rocks found globally right now, to search for more of these deposits.