Microscopic algae may have helped preserve 22.5-million-year-old spider fossils

Tiny creatures known as diatoms may have helped preserve rare spider fossils over 22 million years old, according to a new paper by US-based researchers.

Fossils are crucial to understanding the history of our planet, but some things get fossilised more often than others. To understand why, let’s briefly recap how fossils form.

Mineralised structures like bones and teeth are most likely to be fossilised; when animals die, their soft tissues are usually eaten or rot away, leaving the more durable mineralised structures behind. If a skeleton is buried in sediment, it may become fossilised as minerals from groundwater gradually seep into the bone, crystallise, and turn to stone in the exact same shape as the skeleton. This is known as “petrification”, which literally means “turned to rock”.  

However, arthropods like insects and spiders don’t have bones. Instead, they have a tough exoskeleton made of a carbonaceous polymer called chitin – which is actually a type of sugar. This sort of non-mineralised structure is less likely to be preserved in the fossil record – and we don’t have such a good understanding of their fossilisation process as we do for skeletons.

One place where rare arthropod fossils can be found is a geological formation near Aix-en-Provence in France. That’s where the scientists who worked on the new paper – led by Alison Olcott of the University of Kansas, and her then-graduate student Matthew Downen – found the spider fossils that led to their new discovery.

“Matt was working on describing these fossils, and we decided – more or less on a whim – to stick them under the fluorescent microscope to see what happened,” recalls Olcott.

“To our surprise, they glowed, and so we got very interested in what the chemistry of these fossils was that made them glow.”

Scientific figure consisting of a photograph of a spider fossil in rock with a white box overlaid on the abdomen. The white box corresponds to an inlaid image showing (top) a chemical map of pink silica molecules and yellow sulphur molecules and also a monochrome scanning electron microscope image of the region
Spider fossil from the Aix-en-Provence Formation with white box indicating location of scanning electron microscopy image and chemical map of sulphur (yellow) and silica (pink) seen in upper left. Together these reveal a black sulphur-rich polymer on the fossil and the presence of two kinds of siliceous microalgae: a mat of straight diatoms on the fossil and dispersed centric diatoms in the surrounding matrix. Credit: Alison Olcott.

Olcott and colleagues found that the spider fossils contained a black polymer made of carbon and sulphur, and were surrounded and covered by microscopic algae called diatoms. Diatoms secrete a sticky substance called extracellular polysaccharide, or EPS.

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

“I hypothesised that the chemistry of those microalgae, and the stuff they were extruding, actually made it possible for this chemical reaction to preserve the spiders.”

The researchers proposed that a chemical reaction between the chitin in the spider exoskeleton and the sulphur in the EPS allowed the fossils to be preserved. The process is similar to vulcanisation, an industrial treatment that uses sulphur and heat to make rubber in car tyres and other products more durable.

“Sulphurisation takes carbon and cross-links it with sulphur and stabilises the carbon, which is why we do it to rubber to make it last longer,” Olcott says.

“What I think happened here chemically is the spider exoskeleton is chitin, which is composed of long polymers with carbon units near each other, and it’s a perfect environment to have the sulphur bridges come in and really stabilise things.”

She wants to see if this hypothesised preservation process is supported by evidence from other fossil sites containing diatoms.

“Of all the other exceptional fossil preservation sites in the world in the Cenozoic Era, something like 80% of them are found in association with these microalgae,” she points out.

A closeup of the inlaid image of the chemical map and scanning electron microscope image of the fossil spider abdomen as shown in the previous image
Scanning electron image of fossilised spider abdomen revealing a black polymer on the fossil and the presence of two kinds of microalgae: a mat of straight diatoms on the fossil and dispersed centric diatoms in the surrounding matrix. This image is overlain by chemical maps of sulphur (yellow) and silica (pink) revealing that while the microalgae are siliceous, the polymer covering the fossil is sulphur-rich. Credit: Alison Olcott.

The intriguing new discovery might be one of the few upsides to the COVID-19 pandemic. Locked down with her family in 2020, Olcott had to change her approach to research.

“I honestly think this study is partially a result of pandemic science,” she says.

“I spent a lot of time with these images and these chemical maps and really explored them in a way that probably wouldn’t have happened if all the labs were open and we could have gone in and done more conventional work,” she says.

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