In the intricate narrative of life’s ancient tapestry, the fossil record serves as an invaluable archive, capturing fragments of organisms that lived millions of years ago. Yet, this record is notoriously patchy. Why do some creatures leave behind stunningly detailed traces, including soft tissues like muscles or even brains, while others vanish from history with no trace at all? A groundbreaking study by researchers at the University of Lausanne (UNIL), published in Nature Communications, now shines a revealing light on this enduring mystery. The research unravels the crucial influence of an organism’s size and biochemical makeup on its potential to become preserved as a fossil through geological time.
Fossils, often thought merely as mineralized bones or shells, occasionally preserve far more complex remains. Exceptional sites worldwide have unveiled startling examples where soft tissues—normally vanishing rapidly after death—persist. These finds challenge the traditional understanding of fossilization but remain enigmatic because soft tissue preservation is neither universal nor well-understood. To dissect the mechanisms underlying this extraordinary preservation, the UNIL team embarked on an ambitious series of experimental decay studies, applying cutting-edge chemistry and micro-sensing technologies to investigate the processes at play in real time.
Central to their experimental design was the use of a diverse array of modern analogues: shrimp, snails, starfish, and flatworms called planarians. These animals were chosen deliberately to represent a broad spectrum of body architectures and biochemical compositions, resembling extinct species from pivotal epochs in Earth’s history. By allowing these organisms to decompose under rigorously controlled laboratory conditions, the researchers meticulously monitored the microenvironment surrounding decaying carcasses. This approach involved tracking redox conditions—the delicate balance between oxygen-dominated (oxidative) and oxygen-depleted (reducing) chemical milieus—which are known to dramatically influence decay rates and preservation potential.
The findings upend several assumptions. Larger animals and those rich in proteins rapidly transformed their immediate surroundings into reducing environments. Such oxygen-poor conditions are critically important because they inhibit typical decay pathways driven by oxidative reactions carried out by microorganisms. In contrast, smaller or less proteinaceous organisms were less effective at generating these preserving milieus, leading to quicker degradation and loss. This dynamic provides a compelling explanation for why some animals “disappear” chemically after death while others leave behind fossilized remains. The extent to which tissue mineralization occurs—where soft parts are replaced with stable minerals—appears directly tied to this redox modulation.
Beyond size and chemistry, the study contextualizes its significance within the broader fossil record. For instance, it helps explain why arthropods, with their relatively large bodies and protein-rich cuticles, dominate the preserved Cambrian and Ordovician fauna approximately half a billion years ago. Conversely, small, soft-bodied creatures such as planarians are archaeologically underrepresented, not necessarily due to their ecological absence in ancient environments, but more likely due to their low preservation potential. The researchers stress that such biases must be accounted for when reconstructing past ecosystems, as seemingly “missing” organisms might simply have lacked the biochemical and physical traits enabling fossilization.
“We found that the chemical influence of an organism after death plays a pivotal role in the fossilization pathway,” explains lead author Nora Corthésy, a doctoral student at UNIL. “Our investigations reveal that size and body composition create distinct microenvironments that either promote preservation or accelerate decay.” This insight marks a paradigm shift, moving beyond environmental factors commonly blamed for fossil bias, such as sediment type or depositional setting, toward appreciating how the organism itself shapes its post-mortem destiny.
The meticulous experiments employed advanced micro-sensors capable of detecting minute shifts in oxygen concentrations and redox potential surrounding decomposing tissue. This allowed the team to visualize the timeline over which reducing conditions emerged, revealing a rapid onset in the case of large, protein-rich animals—a critical window enabling early mineralization before soft tissues could be obliterated. Such temporal resolution is unprecedented, offering dynamic models for fossil formation that extend beyond static observations of preserved remains or sediment chemistry.
The chemical reactions underpinning fossilization are complex. Reducing conditions favor processes such as mineral precipitation directly onto decaying tissues or gradual replacement by minerals like pyrite or phosphate compounds, which robustly preserve microscopic anatomical details. These pathways effectively “lock in” biological features, imprinting morphology in stone across geological epochs. Conversely, oxidative environments stimulate microbial metabolism that rapidly degrades organic matter, precluding such remarkable preservation. The study rigorously connects these microscale chemical mechanisms with macroscale fossilization patterns.
Though body size and protein content emerge as dominant intrinsic factors, the authors acknowledge that extrinsic environmental variables—including sediment chemistry, temperature, and salinity—also modulate fossilization outcomes. High salinity, for instance, can slow decay similarly to low temperatures by restricting microbial activity. However, replicating ancient climates and aquatic conditions within a laboratory remains a formidable challenge. Thus, the team positions their findings as one crucial piece in a multifaceted puzzle, providing a foundation for further integrative research combining biological, chemical, and geological perspectives on preservation.
This research carries profound implications for paleobiology and evolutionary studies. Many extinct groups and their intricate ecosystems are known only through incomplete fossil archives. Recognizing that body chemistry governs preservation pathways helps refine interpretations of biodiversity through deep time. It cautions against assuming the absence of certain taxa or soft-bodied forms reflects genuine historical scarcity rather than taphonomic bias—a blind spot that may skew reconstructions of early animal life and evolutionary trajectories.
Equally captivating is the philosophical reflection implicit in these findings: fossils are not mere passive remnants but products of intricate interplay between biology and chemistry after death. The “rock immortality” of an organism depends partially on its own internal composition and how vigorously it alters its microenvironment post-mortem. Thus, evolution’s story is as much about what an organism contributes in life as what it bequeaths chemically in death. The fossil record becomes not only a window into ancient life but also a testament to biochemical legacies that survive across deep time.
The study’s revelations open new experimental vistas. Future research may explore a wider diversity of taxa, further dissect biochemical contributions such as lipid or carbohydrate content, and integrate sedimentological factors with organismal biochemistry. Understanding how microscopic redox gradients orchestrate mineral deposition around decaying remains might also inspire novel paleontological methods, refining where and how paleontologists search for the most exquisitely preserved fossils.
In closing, the University of Lausanne’s study breathes fresh vitality into a centuries-old question: why do some organisms fossilize while others fade entirely? By focusing on the subtle chemical dialogues between decaying bodies and their surroundings, this research delivers a compelling answer rooted in the very fabric of life’s chemistry. It reminds us that the past’s preservation hinges not only on the environment’s conditions but on the extraordinary capacity of life itself to influence its afterlife beneath Earth’s surface.
Subject of Research: Fossilization processes influenced by organism size and biochemical composition.
Article Title: Taxon-specific redox conditions control fossilisation pathways
News Publication Date: 29-Apr-2025
References:
N. Corthésy, J. B. Antcliffe, and F. Saleh, "Taxon-specific redox conditions control fossilisation pathways," Nature Communications, 2025.
Image Credits:
© Sinéad Lynch – UNIL
Keywords: Fossilization, redox chemistry, taphonomy, soft tissue preservation, decay experiments, ancient ecosystems, paleobiology, organism size, protein content, mineralization, reducing environment.
