You’re likely familiar with the name Charles Darwin. You’ve also likely heard of his ground-breaking work: On the Origin of Species (1859). Something you’ve likely not heard of, however, is his book on earthworms. The Formation of Vegetable Mould Through the Action of Worms (1881) details Darwin’s ideas on how the changes caused by the burrowing of earthworms transformed the land we walk on—a phenomenon of biological reworking of sediments which we now call bioturbation. Unlike On the Origin of Species, which has the ever-growing field of evolutionary biology to answer Darwin’s original questions, modern-day scientists are still asking fundamental questions about bioturbation and its evolutionary history. What were the behaviors of ancient burrowing animals, and when did they appear? When did their activities approach the scale and breadth of modern bioturbation? How did the activities of ancient bioturbators shape the evolution of contemporaneous ocean communities and environments?
In a recent publication, Yale Department of Earth and Planetary Sciences assistant professor Lidya Tarhan, PhD student Kate Pippenger (GSAS ‘26), and their collaborators sought to answer these questions. The team was motivated by the observation that although body fossils (fossilized carcasses of ancient animals commonly seen in museums) have long provided key insights into the evolutionary histories of major animal groups, how behaviors have evolved and “engineered” planetary landscapes and seascapes across geologic time is comparatively poorly constrained. To fill this fundamental knowledge gap, the team turned to trace fossils formed from the biological activities of animals, such as footprints, burrows, nests, or feces—specifically seafloor burrows.
From an extensive literature search, the researchers determined metrics to assess how two major subsets of bioturbation behavior changed through time: churning of the upper seafloor and deep digging into lower layers of the seafloor. The “churning” behavior is responsible for the homogenization and fluidization of the uppermost seafloor, leading to the formation of soupy sediments known as the sedimentary mixed layer. The deepest seafloor burrows, demarcating what’s known as the sedimentary transition layer, are the work of animals constructing more permanent homes or feeding structures. Together, these two bioturbated zones of the seafloor regulate the consistency of, oxygenation of, and nutrient transfer from seafloor sediments—all factors that influence the habitability of the seafloor and the health of marine communities.
Finding ways to track the evolution of the seafloor mixed layer was challenging. “This is often poorly or only indirectly preserved in geologic archives,” Tarhan said. The team combined information about the preservation of different trace fossils, the impact of currents on the seafloor, and the extent to which ancient burrowers overprinted other environmental features. In many cases, this required consolidating data from previous studies.
Fortunately, reconstructing the evolution of the seafloor transition layer was a little more straightforward. The team collected data from previously published studies that reported depths for up to six different kinds of trace fossils. From these papers, they were able to constrain the depth of the deepest continuously occupied burrows and track how these changed through Earth’s history.
In quantifying and comparing the fossil evidence for these burrowing behaviors, the paper revealed new insights into the evolution of keystone or ecosystem engineering species in marine communities. The findings indicate that although deep burrowing that established the transition layer started early in evolutionary history, the escalation of seafloor churning responsible for the mixed layer lagged. Moreover, the team observed that both churning and deep burrowing began earlier in areas of the seafloor closer to shore. This is consistent with other research based on body fossil records that indicates that many organisms experienced an “onshore to offshore” shift in habitat and diversity that took a considerable amount of evolutionary time.
These findings bring a new question into the conversation: why did the expansion of deep and intensive seafloor churning take such a long time? Before this can be answered, Tarhan hopes to fill gaps in these data, as well as better understand the dynamics between bioturbators and their surroundings. For now, Tarhan and her colleagues are eager to share their findings. “This [study] was an opportunity to pause and collate all the available data into a single dataset—and to highlight the critical nature of these data to reconstruct the history of life on the planet, as well as to motivate future work to fill in remaining gaps in these records,” Tarhan said.