Why do some ancient animals become fossils, while others vanish without a trace - A new study from the University of Lausanne reveals that the size and chemical composition of an animal are among the determining factors in its chances of surviving millions of years as a fossil, or vanishing without a trace.that an animal’s size and chemical composition are among the determining factors in its chances of surviving millions of years as a fossil, or vanishing without a trace.
Fossils are not limited to bones; some of the most remarkable finds include traces of soft tissue such as muscles, viscera and even brains. These rare fossils offer a vivid glimpse into the past. However, scientists have long wondered why some animals and organs are better preserved than others.
To unravel this mystery, a team of scientists from the University of Lausanne (UNIL) carried out laboratory experiments under carefully controlled conditions on a series of fossils.controlled conditions on a range of animals including shrimps, snails, starfish and planaria (aquatic worms). Using micro-sensors, they observed the decomposition of the bodies, studying the surrounding chemical environment, and in particular the balance between oxygen-rich (oxidizing) and oxygen-poor (reducing) conditions.
The results, published in Nature Communications , showed that larger animals and those with a higher protein content tended to create oxygen-poor conditions more quickly, which are crucial for fossilization. They slow down decomposition and trigger chemical reactions such as mineralization, or the replacement of tissues with more durable minerals.
"This means that, in nature, two animals buried side by side could have very different fates as fossils, simply because of differences in size or body chemistry.rences in size or body chemistry," explains Nora Corthésy, PhD student at the University of Lausanne and lead author of the study. "One could disappear completely, while the other would be immortalized in stone," adds Farid Saleh, Ambizione SNSF research fellow at the University of Lausanne, and senior author of the paper. According to the study, animals such as large arthropods are more likely to be preserved than small planaria 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", argues Nora Corthésy.
As well as helping to explain the uneven nature of the fossil record, this new analysis offers valuable insight into the chemical processes shaping ancient life, which we can reconstruct today. By highlighting the factors that determine the fossilization of soft tissues, it helps us to better understand the formation of exceptional fossils and why we only see fragments of the past.
Source : N. Corthésy, J. B. Antcliffe, and F. Saleh, Taxon-specific redox conditions control fossilization pathways , Nature Communications, 2025
Research funding: SNF Ambizione Grant (PZ00P2_209102)
Questions to Nora Corthésy, principal author of the study at the University of Lausanne :
Why did you choose shrimps, snails and starfish to conduct your study?
These modern animals are the best representatives of extinct animals that we had in the laboratory. From a phylogenetic point of view (kinship between species and composition), 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.
How can we know that animals lived, then disappeared without a trace, if we have no proof?
Laboratory studies help to determine whether a fossil is missing because the animal was not there, or because it was not well preserved. If an animal decomposes rapidly, its absence is probably due to poor preservation. If it decomposes slowly, its absence is more likely to be ecological, i.e. a genuine 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. 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.
What about the external conditions in which fossils are formed, such as climate?
The effect of these conditions is still very complicated to understand, as it is almost impossible to replicate ancient climatic conditions in the laboratory. We do know, however, that certain sediments can facilitate the preservation of organic matter, giving clues as to which deposits are most favorable for finding fossils. Other factors such as salinity and temperature can also play a role. For example, high salinity can increase the preservation potential of an organism, as large quantities of salt slow down decomposition in the same way as 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 just one indicator among many, and much remains to be done to understand the impact of different natural conditions on fossil preservation.
