Microalgae help to keep insect fossils intact

Spider fossils from the Aix-en-Provence Formation in France. Image by Alison Olcott

One study found that 22.5 million-year-old spider fossils unearthed in southern France were unusually well preserved, perhaps thanks to the secretion of microalgae, diatoms. It is rare in the fossil record that small and fragile animals, such as spiders, insects, and amphibians, are preserved intact. The newly described process, assisted by diatoms, contributes to the knowledge hitherto known about the evolution of these organisms. The results were published on April 21 in Communications – Earth and the Environment.

Much of what we know about the history of life on Earth comes from well-preserved fossils. Mineralized body parts— such as shells, bones, and teeth — were preserved relatively straightforwardly, so many of these fossils remain to this day. However, fossils of small, fragile bodies, as well as soft tissues, are rarer because they are less likely to become fossils and we don’t know much about how they are preserved.

Alison Olcott and colleagues at the University of Kansas studied spider fossils found in sediments from an ancient lakebed in Aix-en-Provence, France. Using high-resolution microscopy techniques, they found that the fossils were surrounded by diatom microfoeds. These microscopic algae are known to secrete sulfur-rich substances throughout their lives to form algae pads.

The authors believe that this substance wraps around the spider and promotes vulcanization, a process that fixes and preserves the spider’s fragile body.

For example, the above picture is a spider fossil from the Aix formation in Provence, France, and the chemical images of sulfur (yellow) and silicon (pink) seen in the upper right. Together, this information revealed the presence of a black sulfur-rich polymer and two siliceous microalgae on the fossil: a linear diatom pad on the fossil and a central symmetrical diatom scattered in the surrounding matrix.

Spider fossils from the Aix-en-Provence Formation in France, with overlapping display of hand samples and fluorescence microscope images. Image by Alison Olcott

Under normal illumination, the spider fossil is distinguished from the surrounding rock matrix, but the chemical composition of the fossil after being excited by ultraviolet radiation causes it to emit bright autofluorescence, showing other details of preservation.

They say the process could be very common, and that many of the other fossils in lake sediments that are unusually well preserved in the lake’s sediments may also be credited to 66 million years after diatoms first appeared in continental lakes. (Source: China Science Daily Jinnan)

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