Palaeontology: Spider fossils preserved with the help of microscopic algae

The exceptional preservation of 22.5-million-year-old spider fossils from southern France may be due to secretions from microscopic algae, known as diatoms, according to an article published in Communications Earth & Environment. Small and delicate animals — such as spiders, insects and amphibians ⁠— are rarely preserved in detail within the fossil record. The newly described diatom-aided process could be responsible for much of what we know about the evolution of these creatures.

Much of our understanding of the history of life on Earth comes from well fossilized creatures. Mineralised body parts ⁠— such as shells, bones and teeth ⁠— have a relatively straightforward preservation pathway, so there are many of these fossils remaining. However, fossils of small and delicate species and soft tissue fossils are rarer, as they are less likely to fossilize, so their preservation pathways are less well understood.

Alison Olcott and colleagues examined spider fossils, found in the sediments of an ancient lake bed in the Oligocene Aix-en-Provence, France, using high-resolution microscopy techniques and found that they were surrounded by diatom microfossils. These microscopic aquatic algae are known to secrete sulphur-rich substances during life to form algal mats. The authors suggest that this substance coated the spiders and enhanced a process called sulphurization, which stabilised and preserved the spiders’ fragile bodies.

The authors suggest that this process could be widespread and responsible for many more cases of exceptional preservation in lake sediments from the last 66 million years, when diatoms first started to populate continental lakes.