Why some animals become fossils, while others just disappear

Humans have marveled at fossils for centuries. Yet many people still wonder why some animals leave beautifully preserved remains, while others fade without a hint of their existence.

Scientists have tackled this question head-on at the University of Lausanne (UNIL). Shortly after conducting their experiments, lead researcher Nora Corthésy emerged with fascinating insights.

Why some animals become fossils

Fossils are not just bones. They can include muscle impressions and even traces of internal organs.

Preservation can seem chaotic because not every creature is equally likely to become part of the rock record.

When Farid Saleh, Swiss National Science Foundation Ambizione Fellow at UNIL, and his team looked at why this happens, they found that the chemical properties within a carcass help determine if it will eventually fossilize.

Animals alter their surroundings

During decay, certain remains spark rapid changes in oxygen levels around the body. Bigger carcasses and those packed with more proteins seem to lower local oxygen supply.

“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,” said Corthésy.

“One might vanish entirely, while the other could be immortalized in stone” noted Saleh.

Decaying animals control fossil formation

Some researchers have observed that proteins break down more swiftly than lipids. This quick decay can reduce oxygen around the carcass faster, shielding the body from further breakdown.

When oxygen is scarce, different bacteria thrive by using alternative elements for energy. This sets the stage for a chemical environment that can swap original tissues with durable minerals.

Chemical conditions shape what endures

Scientists have linked extremely low redox potential to the formation of minerals like pyrite or calcium phosphate, which capture fine details of once-living organisms.

A large, protein-rich animal can slip into this state more easily, leading to a better chance of fossilization.

Smaller creatures that do not rapidly reduce oxygen might disappear completely. That difference could explain why many fossil deposits from hundreds of millions of years ago feature arthropods more than worms.

Soft tissue chemistry and fossil survival

Fossilization isn’t just about having hard shells or bones. The chemical composition of soft tissues – especially the amount of protein – plays a big role in what gets preserved.

This shifts the focus from structure to substance. A jelly-like creature with high protein could fossilize more readily than a tougher animal low in protein, depending on how the surrounding chemistry reacts.

Animals leave different fossil traces

Even when animals are buried in the same spot at the same time, their bodies don’t decay the same way. A shrimp and a planarian lying inches apart can leave completely different chemical signatures in the sediment.

This means some fossil beds might look biased toward certain species, not because others weren’t there, but because their remains didn’t alter the environment enough to trigger fossilization.

Why this matters for paleontology

Understanding how size and body chemistry influence fossilization changes how paleontologists interpret ancient ecosystems. It helps separate the absence of fossils due to true extinction from those missing due to poor preservation.

This distinction matters when reconstructing evolutionary history. If some animals never had a chance to fossilize, their role in the ecosystem could be underestimated or completely overlooked.

The animals we’ll never know

Some organisms may have lived and died without leaving any physical trace behind. If their bodies decayed quickly and didn’t create the chemical conditions needed for preservation, they might have vanished entirely from the fossil record.

This means the fossil record isn’t a complete history – it’s more like a selective snapshot. Animals with soft bodies and low protein content, such as some early worms, may have existed in abundance, but we’ll likely never find their remains.

Understanding fossil formation

Different climates and salinity can affect how proteins and lipids degrade. There may be times when tiny animals end up preserved, but scientists believe that size and body composition often tip the odds in favor of bigger, protein-loaded species.

Although modern labs cannot precisely recreate ancient seabeds or swampy shorelines, today’s experiments are bringing us closer to grasping how our planet’s story is written in stone.

The study is published in the journal Nature Communications.

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