Some animals survive millions of years in stone, but most simply vanish, now we know why

Most of what we learn about ancient animals comes from hard parts – shells, bones, and sometimes soft tissues that have been transformed into fossils over eons. How does this happen? The key lies in the chemistry around a carcass.

As microbes move in, they use up oxygen and flip the local conditions from oxidizing to reducing within minutes to days. Those shifts decide whether tissues vanish or get copied by minerals and locked into rock.

A recent experiment asked a clear question with big consequences: if different animals decay in the same water, will they create the same chemistry above their bodies, or does each one build its own microenvironment?

The answers derived from a study led by the University of Lausanne (UNIL) points straight to why some animals transform into fossils when they die, but most simply return to dust.

How animals become fossils

Researchers placed four aquatic animals – a shrimp, a snail, a starfish, and a planarian flatworm – into sealed jars of gently oxygenated water. For seven days, they measured oxidation-reduction potential (ORP) just above each carcass.

ORP, recorded in millivolts, indicates whether conditions favor oxidation (positive values) or reduction (negative values).

As microbes burn oxygen and switch to other electron acceptors, ORP drops. Each animal drove a different chemical path.

Shrimp pulled ORP down the fastest and deepest, sometimes reaching values linked to methanogenesis. Snails also pushed conditions into strongly reducing ranges, but not as rapidly or as far.

Starfish moved more slowly and ended near the snails after a week. Planarians barely shifted from the starting conditions, much like control jars without an animal.

Minerals in the ground

Different redox windows favor different preservation routes. In oxygen-rich water, soft tissues usually break down quickly.

Under reducing conditions, minerals can grow on or within tissues and capture detail before it’s lost. When sulfate reduces to sulfide and iron is present, iron sulfide (pyrite) can form, producing pyritization.

In other reducing ranges, phosphorus released during decay can bind into calcium phosphate, producing phosphatization.

Because shrimp and snails spent time in reducing “sweet spots,” their remains would be more likely to acquire pyrite or calcium phosphate coatings. Planarians, which stayed closer to oxygenated conditions, were less likely to mineralize that way.

A single sediment bed can therefore hold different preservation styles side by side, driven by the microenvironments that different carcasses create.

Fossils and animal size

Size helps. Larger bodies feed microbes more and draw down oxygen faster, so they reach reducing conditions sooner. But size wasn’t the whole story.

The team normalized results by body mass and subtracted the weight of mineralized parts, such as shells, that don’t decay. Even after those corrections, the taxa did not line up the same way.

Composition mattered. Bodies richer in protein relative to lipid produced stronger ORP drops on a per-mass basis.

Planarians are small and protein-heavy with little fat, giving them a larger per-mass chemical impact than size alone would predict.

Shrimp carry more lipid, which degrades more slowly and blunts the per-mass effect, even though a whole shrimp still drove ORP very negative due to overall size, surface area, and varied tissues.

From the lab into the real world

Several jars showed an ORP rebound after about five days. Conditions became less reducing. That pattern likely reflects microbial succession.

Early colonizers use easy fuels and drain oxygen quickly. Later arrivals run different reactions that tilt the chemistry back toward less extreme values. The decay path bends as the microbial roster changes.

These trials used freshwater at room temperature in sealed, gently oxygenated jars. Natural settings vary. Salty seawater alters protein behavior while cold mud slows reactions.

Flowing pore water can deliver new oxidants. Starfish are marine animals, and in seawater their decay chemistry can shift.

Sediments also matter. Many carcasses land in mud that limits oxygen flow and sets up reducing pockets faster than open water.

The specific numbers will change with the setting, but the pattern holds: taxon identity and tissue chemistry steer decay chemistry under the same starting conditions.

What it all means

“This means that, in nature, two animals buried side by side could have vastly different fates as fossils, simply because of differences in size or body chemistry,” affirms Nora Corthésy, Ph.D. student at the University of Lausanne and lead author of the study.

“One might vanish entirely, while the other could be immortalized in stone,” adds Farid Saleh, Swiss National Science Foundation (SNSF) Ambizione Fellow at UNIL, and senior author of the paper.

According to this study, animals such as large arthropods are more likely to be preserved than small planarians or other aquatic worms.

“This could explain why fossil communities dating from the Cambrian and Ordovician periods (around 500 million years ago) are dominated by arthropods,” states Corthésy.

Preservation of animal fossils

Geologists often picture redox conditions stacked vertically in sediment – from oxygenated at the top to more reducing with depth. That vertical model explains shifts in preservation with burial depth. This study adds a lateral view.

Two carcasses lying inches apart can generate different chemical zones and end up preserved by different mineral pathways, even when the surrounding mud looks uniform.

That helps explain mixed beds where pyritized and phosphatized fossils sit together and why some groups appear repeatedly while others fade.

The chemistry above a carcass is not one-size-fits-all. It changes with body size and, crucially, with what the body is made of. Those differences decide which minerals, if any, have time to copy soft parts before they disappear.

That is why a shrimp and a snail, decaying close together, can leave very different signatures in stone – and why the fossil record, with all its gaps and surprises, makes more sense when you follow the chemistry.

The full study was published in the journal Nature Communication.

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