A team of researchers studying 22.5-million-year-old spider fossils from Aix-en-Provence were taken aback when the petrified pests glowed under a fluorescent microscope. The fluorescence was likely due to the circumstances of the fossilisation, the group said.
The spiders lived during the end of the Oligocene Epoch, in a lake or lagoon environment. The rock layer they were found in is so profuse with fossilized insects that it is known as the Insect Bed and has been studied since the late 1700s.
In this case, the researchers inspecting spider fossils wanted to understand exactly what conditions fostered such good preservation conditions and discovered the fluorescence in the process. Their research is published today in Communications Earth & Environment.
“The autofluorescence we observed here is a result of the chemical composition of the rock matrix and the altered biological remains, but nothing about the autofluorescence is unique to the spiders themselves,” said Alison Olcott, a chemical paleontologist at the University of Kansas and the lead author of the paper, in an email to Gizmodo.
So there’s no ancient Spider-Man narrative here. The spiders were normal-enough arthropods in life, with no glow to their hard exoskeletons. Yet under the fluorescent microscope, details of their anatomy — like an abdomen and claw — were highlighted.
Using a scanning electron microscope, the team found many spherical and needle-like microfossils covering the same rock as the spider. Then, subjecting the fossil to energy-dispersive X-ray spectroscopy (which reveals an elemental map of a target), the team determined that the microfossils were composed of silica.
Most of the microfossils were diatoms, silicified algae that still dominate Earth’s oceans today. The researchers believe the diatoms preserved the soft tissue organisms in this ancient environment; specifically, mats of the microalgae called extracellular polymeric substances stabilised the spiders’ chemistry and protected them from degradation. Different polymers in the fossil cause it to auto-fluoresce under specific lighting.
“If you have ever come across a sticky mat that looks like a colourful raft of goo on top of a lake or pond or on a rock or even in a puddle on the footpath, you have seen EPS,” Olcott said, “as that is what helps the biofilm stick together and adhere to surfaces.” Olcott added that gummy bears use bacterial EPS as a thickener, so you’ve probably eaten it, too.
The researchers theorised the spiders’ pathway to preservation happened like this: The arthropods drifted onto the surface of a lake or lagoon on a mat of diatoms, which sank to the sediment floor. Ensconced in the diatoms, the spiders then experienced the normal compression of sediments that causes fossils to form.
It’s not the first time the University of Kansas has produced research on glowing fossilized spiders. In 2019, Paul Selden — a co-author on the new paper — produced research on the preserved glowing eyes of a 100-million-year-old spider, as reported by Gizmodo. It probably won’t be the last time, either. The team plans to study other deposits besides the site in Aix, to see how much preservation of similar fossils can be linked to diatom mats elsewhere.
To paraphrase Neil Armstrong, that’s one small step for paleontology, eight small steps for the subdiscipline of fossil autofluorescence.