Why Some Creatures Fossilize While Others Vanish Without a Trace

A new study explains why the fossil record skips over so much ancient life — and what shrimp can teach us about decay.

One of the first things I learned in fieldwork wasn’t how to swing a rock hammer — it was how much we never find. You spend hours splitting slabs, fingers numb, eyes scanning for signs of ancient life, only to walk away with a single trilobite tail or an outline of something that once squirmed and vanished. For all the incredible fossils we do unearth, the silence in the rock is even louder.

So it’s fair to ask: why do we find some creatures fossilized in exquisite detail, while others leave no trace at all?

A recent study out of the University of Lausanne helps us get closer to an answer. Published in Nature Communications, it shows that fossilization isn’t just about where something dies or how fast it’s buried; it’s also about what the animal’s body is made of and how it interacts with its surroundings as it decays.

The team, led by Dr. Nora Corthésy and Dr. Farid Saleh, discovered that animals like shrimp, with more protein and mass, can actually create the kind of chemical conditions that make fossilization more likely. Meanwhile, smaller, softer creatures might vanish, leaving a gap in the fossil record that no digging can fill.

To figure this out, they ran controlled lab experiments using living analogues of ancient animals: freshwater shrimp, snails, starfish, and planarians (flatworms). They euthanized each organism and placed the carcasses in sealed vials of water. Then, using micro-sensors, they tracked the oxygen and redox (oxidation-reduction) potential around each carcass over the course of a week, basically measuring how the chemical environment changed as the bodies broke down.

The results were surprisingly clear. Larger animals, especially those rich in proteins, lowered the oxygen levels around them more quickly, creating what scientists call “reducing conditions.” These oxygen-poor environments are known to slow decay and help preserve tissues through mineralization.

In the case of the shrimp, the decay process even reached the point of methanogenesis, conditions so depleted in oxygen that microbes start producing methane.

Planarians, on the other hand, didn’t even get close. Despite being nearly pure protein, their small size meant they didn’t significantly affect their chemical surroundings. They decayed in an oxygenated setting, which meant their soft tissues disappeared quickly and without much trace.

As lead author Dr. Nora Corthésy explained, “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.”

That line stuck with me. It explains so much about the fossil record’s gaps — and why sites like the Burgess Shale or Chengjiang Biota, where soft tissue fossils appear, are so rare and valuable. It’s not just that these animals lived in the right sediment or under the right circumstances. Some of them carried the right chemistry inside their bodies to tip the balance.

There’s also a deeper message here about interpreting the past. As paleontologists, we need to make sure we don’t treat absences in the fossil record as evidence: “This species wasn’t here,” or “This trait didn’t evolve yet.” But this study reminds us that absence can just mean we didn’t get lucky. That decay swept the slate clean before the rock could record a thing.

It also challenges the idea that fossilization depends entirely on what’s happening in the environment. While factors like salinity and burial speed matter, this research suggests that each carcass creates its own little chemical bubble, an ecosystem of decay that can shape its chances of being preserved.

I like to think of it like two campers lighting fires in the same forest. One builds a roaring blaze that changes everything around it. The other lights a match that burns out in seconds. Both started in the same place, but only one leaves a mark.

There’s also a practical side to all of this. Knowing which kinds of animals are more likely to fossilize helps us interpret what’s missing. It reminds us not to assume that the creatures we find are the only ones that lived — but rather, the ones whose bodies cooperated with time.

And it adds nuance to those ancient community reconstructions. If most Cambrian fossil sites are full of arthropods, it may not be because they dominated the seas — but because their bodies, packed with protein and sealed in tough exoskeletons, created ideal fossil-making chemistry.

To me, this study highlights something I’ve felt for years but struggled to put into words: the fossil record is less a library and more a memoir. It doesn’t aim for completeness. It’s full of selective memory, biased by the quirks of biology, chemistry, and chance.

But when we get curious about the biases themselves, about what shapes that memory, we get a little closer to hearing all the voices, even the ones that time tried to erase.

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Best,
Sílvia P-M, PhD Climate Ages