Gloopy goo that fatally snared spiders more than 22 million years ago may have also helped preserve them in exquisite detail.
Researchers from the US and UK suspect sticky sulphurous secretions from algae hardened the brittle exoskeletons of the spiders and staved off decay, allowing them to fossilise.
The study, published today in Communications Earth & Environment, might also help direct palaeontologists to uncover more of these rare, delicate fossils, and get a better picture of ancient environments.
For Alison Olcott, a chemical palaeontologist at the University of Kansas and lead author of the study, the first hint that algae might be involved came when she and her colleague Matthew Downen discovered the fossils glowed.
In daylight, the spider fossils have a recognisable outline, but don't look too dissimilar from the rock they're embedded in.
But under a microscope that threw UV light on the fossils, the spiders lit up in crisp detail.
"It was really exciting just how much more we could see, and we got very interested in what the chemistry of these fossils was that made them glow."
'A race against decay'
The spiders were originally found sandwiched between layers of sedimentary rock from Aix-en-Provence in the south of France, at a site discovered in the late 1700s.
Some 22.5 million years ago, the area hosted a lake or brackish lagoon, and it has yielded a wealth of fossils of organisms that lived in or near water, including insects, shrimp and, of course, spiders.
And it's these remains of softer, squishier animals that put Aix-en-Provence on the palaeontological map.
Spider exoskeletons — their crispy outer layer — decay much faster than, say, bones, shells and teeth, which are composed of hard, mineralised material.
But no-one had teased out exactly why the Aix-en-Provence site managed to preserve so many ancient soft-bodied animals so well.
To find out, Dr Olcott and her colleagues borrowed eight fossil spiders from the French National Museum of Natural History.
They made "elemental maps" showing the chemical make-up of different parts of the fossils.
Then they matched the chemical elements to different colours thrown off by the fossils when illuminated with UV light.
The dark brown remains of the spiders' abdomens glowed orange under UV light, and were made mostly of carbon and sulphur.
Carbon was expected. Spider exoskeletons are made from chitin, and chitin has lots of carbon, but no sulphur.
So where did the sulphur come from?
To fossilise a spider, just add sulphur
A clue to the sulphur source came from the thousands of tiny, needle-like fossils embedded in the rock surrounding the spiders.
These, the researchers suspected, were diatoms: single-celled algae that live in capsules made of glass.
Some diatoms exude gluey sulphur-rich substances, which help them clump together to create "diatom mats" that bloom on the water's surface.
So what Dr Olcott thought happened was an unfortunate spider wandered onto a diatom mat, got stuck, and was quickly encased by the sticky goo.
This formed a barrier that stopped oxygen from getting through to the corpse, and thwarted many microbes that would usually decompose it.
Sulphur in the diatom gloop also reacted with the chitin exoskeleton.
Chitin is made up of long chains that contain carbon. Should sulphur find its way in the mix, it links carbon atoms in neighbouring chitin chains.
This "bridging" would have bestowed extra strength to the chitin, Dr Olcott said.
(Sulphur-bridging is commonly used today to harden rubbers, to make car tyres and the like, in the process known as vulcanisation.)
Eventually, the diatom mat sank to the bottom of the lake, where sediments covered the algae, along with the unfortunate creatures stuck on it.
And over the aeons, those sediments hardened and locked the fossils inside until they were brought to light once again in the 18th century.
Outside of Aix-en-Provence
Australian Museum and UNSW palaeontologist Matthew McCurry, who was not involved in the study, says the new research gets us closer to understanding what conditions are needed to fossilise more delicate species.
"The [fossil] record of spiders is really quite dismal across the whole world.
"They're small, fragile organisms, they don't turn into fossils very easily, so you need these really specific geochemical conditions to aid in their formation."
Knowing how spiders could be preserved might help palaeontologists uncover more similarly fragile fossils by narrowing their search to fossilised diatom mats, called diatomites, Dr Olcott said.
"So rather than just looking through all the rocks [at a site], you might first see what's in the diatomites."
Dr McCurry was wary of extrapolating the results of the new study to sites elsewhere.
"The work they've done on the fossils from that one province is great — they've associated the [diatom] microfossils and the specific [fossilisation] pathway for those fossil spiders.
"But they haven't yet done any chemical analyses on fossils from around the world.
"And so that's really the next step — to take that hypothesis that they've put forward in this paper, and see if that really does hold true across the rest of the world."
