Rosa Rubicondior: Refuting Creationism - Why Gaps In The Fossil Record Are To Be Expected, Naturally

Why did some ancient animals fossilize while others vanished?

One of creationism’s favourite fallacies is the claim that gaps in the fossil record disprove evolution, based on a misrepresentation of something Charles Darwin wrote. According to this argument, Darwin supposedly insisted there should be a complete and continuous series of transitional fossils and admitted that any gaps would be fatal to his theory.

As with many creationist arguments, this is a distortion of both Darwin’s actual words and how science works. It reflects a peculiar misunderstanding of science as a belief system centred on the writings of historical figures — Darwin, in this case, cast as the ‘prophet’ of ‘evolutionism’. The implication is that by discrediting Darwin personally, or selectively quoting his early uncertainties, one can somehow bring down an entire field of modern science built on nearly two centuries of accumulating evidence.

The tactic ignores the fact that Darwin's style was to introduce the reader to a seemingly unaswerable problem, or potential difficulty for his theory, before explaining how his theory of evolution by natural selection provided an answer.

But why would any reasonable person expect the fossil record to preserve every generation of every species, in every habitat, across hundreds of millions of years—and for those remains to be conveniently accessible to palaeontologists today? The idea that scientists expect such completeness betrays a deep scientific illiteracy. Fossilisation is an exceptionally rare process requiring very specific conditions. There are many reasons why an organism is unlikely to fossilise, and relatively few that make fossilisation even possible.

Could you outline the main ways in which an animal or plant can become fossilised, please? How Fossils Form: Main Pathways to Fossilisation

Fossilisation is a rare event that requires very specific conditions. Here are the main ways animals or plants can become fossilised:

Permineralisation

Minerals carried by water infiltrate the pores and cavities of buried organisms. Over time, these minerals crystallise, preserving fine details of bone, wood, or shell.

Carbonisation (or Carbon Film)

Soft tissues decay under pressure, leaving behind a thin film of carbon. This often preserves detailed impressions of plants, insects, and soft-bodied organisms.

Casts and Moulds

An organism buried in sediment decays or dissolves, leaving a cavity (mould). This may later be filled with minerals to form a cast—a replica of the original organism.

Amber Preservation

Organisms, typically insects or small vertebrates, become trapped in tree resin, which hardens into amber and can preserve tissues in remarkable detail.

Freezing

In rare cases, entire organisms become trapped in ice or permafrost shortly after death, preserving flesh, fur, and internal organs nearly intact.

Desiccation (Drying)

Organisms in arid environments may dehydrate before decomposing, preserving skin and soft tissues as natural mummies.

Tar Pits and Bog Preservation

Animals trapped in tar or acidic peat bogs can be preserved when microbial decay is slowed or inhibited. Bones, hair, and sometimes skin can survive.

And now a team of researchers at the Université de Lausanne (UNIL), Switzerland, have shown that, in respect of animal fossilisation, even when the conditions are right, the animal's size and chemical makeup can make the difference between becoming a fossil and disappearing without trace.

Why did some ancient animals fossilize while others vanished?
Why do some ancient animals become fossils while others disappear without a trace? A new study from the University of Lausanne, published in Nature Communications, reveals that part of the answer lies in the body itself. The research shows that an animal’s size and chemical makeup can play an important role in determining whether it’s preserved for millions of years—or lost to time.
Fossils are more than just bones; some of the most remarkable finds include traces of soft tissues like muscles, guts, and even brains. These rare fossils offer vivid glimpses into the past, but scientists have long puzzled over why such preservation happens only for certain animals and organs but not others.

To dig into this mystery, a team of scientists from the University of Lausanne (UNIL) in Switzerland turned to the lab. They conducted state-of-the-art decay experiments, allowing a range of animals including shrimps, snails, starfish, and planarians (worms) to decompose under precisely controlled conditions. As the bodies broke down, the researchers used micro-sensors to monitor the surrounding chemical environment, particularly the balance between oxygen-rich (oxidizing) and oxygen-poor (reducing) conditions.

The results were striking and have now been published in Nature Communications . The researchers discovered that larger animals and those with a higher protein content tend to create reducing (oxygen-poor) conditions more rapidly. These conditions are crucial for fossilization because they slow down decay and trigger chemical reactions such as mineralization or tissue replacement by more durable minerals.

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.

Nora Corthésy, lead author
Institute of Earth Sciences University of Lausanne, Lausanne, Switzerland.

One might vanish entirely, while the other could be immortalized in stone.

Farid Saleh, senior author
Institute of Earth Sciences University of Lausanne, Lausanne, Switzerland.
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.

Nora Corthésy

These findings not only help explain the patchy nature of the fossil record but also offer valuable insight into the chemical processes that shape what ancient life we can reconstruct today. Pinpointing the factors that drive soft-tissue fossilization, brings us closer to understanding how exceptional fossils form—and why we only see fragments of the past.

Publication:

N. Corthésy, J. B. Antcliffe, and F. Saleh (2025)
Taxon-specific redox conditions control fossilisation pathways Nature Communications 16 3993 DOI: 10.1038/s41467-025-59372-3

Questions to Nora Corthésy, principal author of the study at UNIL:

Q. Why did you choose shrimps, snails and starfish to conduct your study?

A. These present-day animals were the best representatives of extinct animals we had in the lab. From a phylogenetic (relationship between species) and compositional point of view, they are close to certain animals of the past. The composition of the cuticles and appendages of modern shrimps, for example, is more or less similar to that of ancient arthropods.
Q. How can we know that animals lived, then disappeared without a trace, if we have no evidence of this?
A. When studying preservation in the laboratory, it becomes possible to distinguish between ecological and preservational absences in the fossil record. If an animal decays rapidly, its absence is likely due to poor preservation. If it decays slowly, its absence is more likely to be ecological, that is, a true absence from the original ecosystem. Our study shows that larger, protein-rich organisms are more likely to be preserved and turned into fossils. We can therefore hypothesize that smaller, less protein-rich organisms, which have very little chance of dropping their redox potential, may not have been fossilized due to preservational reasons. It is therefore possible that some organisms could never have been preserved, and that we may never, or only with great difficulty, be able to observe them. Nevertheless, all of this remains hypothetical, as we are unable to travel back in time millions of years to confirm exactly what lived in these ancient ecosystems.
Q. What about the external conditions in which fossils are formed, such as climate?
A. The effect of these conditions is very complicated to understand since it is nearly impossible to replicate ancient climatic conditions in the laboratory. Nevertheless, we know that certain sediments can facilitate the preservation of organic matter, giving clues as to which deposits are the most favorable for finding fossils. We also know that factors such as salinity and temperature, also play a role in preservation. For example, high salinity can increase an organism's preservation potential, as large amounts of salt slow down decay in a similar way to low temperatures. Our study here focuses solely on the effect of organic matter and organism size on redox conditions around a carcass. It is therefore one indicator among others, and there is still a lot that needs to be done to understand the impact of various natural conditions on fossil preservation.

Abstract
The preservation of fossils in the rock record depends on complex redox processes. Redox conditions around different decaying organisms have rarely been monitored in the context of experimental taphonomy. Here, microsensors were used to measure redox changes around decomposing carcasses of various taxa, including shrimp, snail, starfish, and planarian. Our results show that different decaying taxa lead to various post-mortem environmental redox conditions. Large carcasses tend to reach reducing conditions more rapidly than smaller ones. However, size does not explain all observed patterns, as environmental redox conditions are also influenced by the nature of the organic material. For instance, taxa with higher proteins-to-lipids and (proteins + carbohydrates)-to-lipids ratios tend to achieve reducing conditions more rapidly than others. The generation of distinct redox environments around different taxa originally put under the same original environmental conditions suggests that various fossilisation patterns of macrofossils and molecules can co-occur within a single sedimentary layer.

Introduction
Numerous lines of evidence enable us to reconstruct past life on Earth using data from the fossil record. These include body fossils, trace fossils such as burrows and footprints, and chemical fossils, which consist of molecules and biomarkers left behind by organisms in the rock record. Both body and chemical fossils undergo similar taphonomic processes, including bacterial decay during early diagenesis, maturation during late diagenesis and metamorphism, and modern weathering^1,2,3. These processes can lead to either information loss due to the degradation of specific molecules and morphologies, or information retention when environmental conditions stabilise certain chemistries or anatomies^3,4.

Redox conditions are a crucial factor that determines what is lost or retained in the rock record. Redox condition refers to the overall oxidation-reduction state of an environment, which is determined by the balance between oxidising and reducing processes. It is influenced by the presence and activity of electron donors (reducing agents) and electron acceptors (oxidising agents). The redox condition of an environment can be deciphered by investigating the redox potential or oxidation-reduction potential (ORP) of this environment. The redox potential is a measure of the tendency of a chemical species to gain or lose electrons in a redox reaction⁵. It is typically expressed in millivolts (mV) and indicates whether a system is more oxidising (positive ORP) or reducing (negative ORP)⁶. Generally, when organic matter is exposed to oxidative conditions, it decays more rapidly than it would under reducing conditions⁷. However, this is not always the case, especially since numerous chemical and biological processes are interconnected. For instance, some bacteria recycle organic material under reducing conditions more efficiently than under oxidative conditions⁸.

Authigenic mineralisation of organic material is a process by which labile morphological details get replicated by minerals and is a major process for soft tissue preservation or exceptional fossil preservation in the rock record^4,9,10. Authigenic mineralisation also depends on the redox potential¹¹, since it often occurs under reducing conditions^4,10,12, albeit with some rare exceptions¹³. For instance, pyritisation happens under sulphate-reducing conditions and has been shown to replicate in FeS₂ numerous types of organisms, including arthropod bodies with neural anatomies^14,15, echinoderms with water vascular systems^16,17, and prokaryotic cells¹⁸. Phosphatisation happens under reducing phosphorus release conditions, and can replicate soft anatomy in detail in calcium phosphate [Ca₃(PO₄)₂], as observed in muscle tissues¹⁹, trilobite guts²⁰, and ray-finned fish embryo fossils⁹.

Decay experiments conducted under controlled laboratory conditions are particularly useful for understanding processes such as the decay and mineralisation of organic matter^{8,21,22,23,24,25,26,27,28,29,30,31}. Although redox potential plays an important role in organic matter decay and mineralisation, little work has been done to experimentally investigate what changes in redox conditions occur during organic matter degradation^32,33,34,35, and no work has ever investigated how different phyla and organic matter compositions impact redox conditions after death in a single study with directly comparable experimental set ups. This study monitored changes in redox conditions surrounding carcasses of four different animals (shrimp, snail, starfish, and planarian) for a week using microsensors. The results show that redox conditions vary significantly between taxa, depending on the mass and nature of the decaying organic material.

N. Corthésy, J. B. Antcliffe, and F. Saleh (2025)
Taxon-specific redox conditions control fossilisation pathways Nature Communications 16 3993 DOI: 10.1038/s41467-025-59372-3

Copyright: © 2025 The authors.
Published by Springer Nature Ltd. Open access.
a Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

This study provides compelling evidence that challenges a fundamental creationist argument: that gaps in the fossil record undermine the theory of evolution. This argument often hinges on a misrepresentation of Charles Darwin's concerns about the incompleteness of the fossil record.

Darwin acknowledged that the fossil record was incomplete and expressed concern that this might be thought to pose a challenge to his theory of evolution by natural selection. However, he attributed these gaps to the rarity of fossilisation and the limited geological exploration of his time. He anticipated that future discoveries would fill many of these gaps, a prediction that has been borne out by subsequent paleontological findings, which invariably fit neatly into the existing fossil record.

The Lausanne study reinforces Darwin's perspective by demonstrating that the fossil record, despite its gaps, is sufficiently comprehensive to reconstruct evolutionary history. The researchers found that the distribution of gaps is not random but follows a consistent pattern, allowing scientists to infer evolutionary transitions with a high degree of confidence .

This research undermines the creationist claim that gaps in the fossil record invalidate evolutionary theory. Instead, it highlights the robustness of evolutionary science, which can accommodate and explain these gaps through an understanding of the fossilisation process and the geological record. The study affirms that the absence of certain transitional fossils does not constitute evidence against evolution but reflects the natural limitations of the fossilisation process.

In summary, the University of Lausanne's findings provide strong support for the theory of evolution and clarify misconceptions about the significance of gaps in the fossil record, aligning with Darwin's original insights and countering creationist assertions.

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