Why did some animals from ancient eras become fossils, while others simply disappeared without a trace? The answer, at least in part, may lie within their own bodies, according to a study from the University of Lausanne (UNIL) published in Nature Communications.

Researchers found that the size and chemical composition of an organism play a decisive role in its ability to be preserved over millions of years—or, conversely, to be lost to the oblivion of geological time.

It’s not only bones that fossilize; in exceptional cases, soft tissues such as muscles, intestines, and even brains can also be preserved. For a long time, scientists have wondered why only certain animals and organs manage to fossilize under these conditions.

To solve the mystery, a team of researchers from UNIL conducted controlled decomposition experiments, analyzing how organisms like shrimp, snails, starfish, and planarians (flatworms) degraded in carefully monitored environments.

why animal fossilize
Cretaceous fossil shrimp from Jbel Oum Tkout, Morocco registered at the Museum d’histoire naturelle de Marrakech. Credit: Sinéad Lynch / UNIL

Using microsensors, they measured the chemical changes in the animals’ bodies, paying special attention to the fluctuation between oxygen-rich (oxidizing) and oxygen-poor (reducing) conditions. The results showed that larger animals and those with higher protein content rapidly generated reducing environments—crucial for slowing down decomposition and triggering processes such as mineralization or the replacement of tissues with more durable minerals.

In nature, two organisms buried together can have completely different fates as fossils, simply due to differences in their size or internal chemistry, explains Nora Corthésy, a PhD student at UNIL and lead author of the study.

One may disappear completely, while the other becomes immortalized in stone, adds Farid Saleh, principal investigator and co-author of the work. According to the data, large arthropods—like certain crustaceans—are more likely to be preserved than small aquatic worms or planarians, which could explain why Cambrian and Ordovician fossils (around 500 million years ago) are dominated by arthropods.

Misleading Absences in the Fossil Record

The study also helps interpret gaps in the fossil record. By simulating decomposition in the lab, we can distinguish between ecological absences (when an animal never inhabited an ecosystem) and preservational absences (when it existed but didn’t fossilize), Corthésy notes. Small organisms with low protein content, which don’t generate reducing conditions, are less likely to be preserved, meaning some ancient groups may have vanished without a trace for that reason.

Even so, external factors like climate, salinity, or sediment type also influence fossilization, but reproducing those variables in the lab is complex. We know that saline or cold environments slow down degradation, but our study focuses on the role of organic matter and body size, Corthésy clarifies. It’s one more piece of the puzzle, but there’s still much to explore.

The research, funded by the Swiss National Science Foundation, reinforces the idea that the fossil record is a biased archive, where what we see doesn’t always reflect the true diversity of the past. Understanding these biases brings us a bit closer to reconstructing ancient life as it really was, not just as we find it, concludes Saleh.

SOURCES

Université de Lausanne

Corthésy, N., Antcliffe, J.B. & Saleh, F. Taxon-specific redox conditions control fossilisation pathways. Nat Commun 16, 3993 (2025). doi.org/10.1038/s41467-025-59372-3

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