Between approximately 252 and 66 million years ago, the Earth’s oceans experienced a profound transformation that fundamentally reshaped marine ecosystems. This period, known as the Mesozoic Marine Revolution (MMR), was marked by the widespread colonization of planktonic organisms equipped with calcium carbonate skeletons. These tiny marine architects not only altered sedimentation patterns but also initiated the accumulation of extensive carbonate deposits on the seafloor. This monumental geological and biological shift has left an enduring mark on ocean chemistry and life, setting the stage for the ecosystems we observe today.
The colonization of the open oceans by calcareous plankton signaled a pivotal juncture in marine history. These microorganisms, upon death, contributed their calcium carbonate shells to the ocean floor, effectively transforming the seabed into a vast, dynamic archive of carbonate minerals. This sedimentary buildup played a crucial role in modulating ocean chemistry and biogeochemical cycles over millions of years. Such extensive carbonate deposits also gave rise to unique rock formations that scientists can study to decode past environmental conditions.
Recent research led by a team from The University of Texas at Austin delves deeply into this transformative era, focusing specifically on the evolutionary history of foraminifera—microscopic, single-celled protists that produce shells, or “tests,” made of varied materials. Published in the prestigious Proceedings of the Royal Society B: Biological Sciences, this study illuminates how the MMR’s carbonate dynamics influenced these organisms’ evolutionary pathways over the entire Phanerozoic Eon, which spans 541 million years to the present.
Foraminifera, often referred to as “forams,” are indispensable components of marine ecosystems, particularly in deep-sea environments where they constitute approximately half of all biomass. Their microscopic size belies their ecological significance, as they play an essential role in carbon cycling and serve as key indicators in paleoenvironmental research. These protists secrete shells that can vary in composition—some build organic or sedimentary tests, while others produce calcareous shells by precipitating calcium carbonate from seawater.
The study reveals that prior to the MMR, calcareous forams exhibited high sensitivity to environmental fluctuations, with both their origination and extinction rates reflecting rapid oceanic changes. This volatility aligned closely with contemporaneous shifts in seawater chemistry, confirming that these organisms were finely attuned to their marine environment. However, the onset of the MMR brought a dramatic shift in their evolutionary dynamics.
Following the Mesozoic Marine Revolution, calcareous foraminifera began to thrive, diversifying steadily while experiencing a notable decline in extinction rates. This pattern suggests a stabilization of their populations and an increased resilience to environmental perturbations. Remarkably, even during periods of pronounced ocean acidification and other radical chemical shifts in the Cenozoic Era, calcareous foram diversity demonstrated a robust capacity for rapid recovery, highlighting a buffering effect linked to the increased deposition of calcium carbonate on the seafloor.
The buffering phenomenon can be understood in the context of ocean carbonate chemistry. As calcareous organisms proliferated and contributed more carbonate sediments, the ocean’s alkalinity and pH levels stabilized, mitigating the impacts of acidification episodes. These sedimentary deposits essentially acted as a chemical reservoir, dampening fluctuations that might otherwise have caused widespread extinction events. Consequently, calcareous forams emerged as more stable and enduring constituents of marine ecosystems after the MMR.
Co-author Rowan Martindale from The University of Texas emphasized the striking transformation in foram responses to environmental change following the MMR. Despite the numerous and significant climatic and oceanographic upheavals of the Cenozoic, including the Paleocene-Eocene Thermal Maximum and the K/Pg mass extinction boundary, the evolutionary trajectory of calcareous forams remained notably robust. Their stabilized diversity over tens of millions of years attests to the profound influence of the MMR on their evolutionary ecology.
Complementing this view, Chris Lowery, another contributing researcher and assistant professor at the Jackson School’s Institute for Geophysics, points out the remarkable resistance of foraminiferal species to dramatic shifts in ocean pH and chemistry. According to Lowery, despite experiencing pronounced environmental stress markers, forams show no substantial extinction events tied to shell composition changes, underlining their adaptive resilience on geological timescales.
The implications of these findings extend beyond foraminifera themselves. Given that many marine organisms—including corals, mollusks, and calcifying plankton—also rely on calcium carbonate for their skeletal structures, understanding foram responses offers a valuable proxy for investigating the broader biological impact of ocean chemistry fluctuations throughout Earth’s history. The evolutionary stability of calcareous forams following the MMR may thus reflect a more general pattern of ecological adaptation within carbonate-dependent marine communities.
By drawing upon an extensive dataset of foram diversity spanning the entire Phanerozoic, the researchers were able to correlate key evolutionary events with shifts in ocean chemistry, including five major mass extinction events and multiple episodes of ocean acidification. This synthesis of paleontological and geochemical evidence provides a comprehensive picture of how biotic and abiotic factors have interplayed in shaping marine biodiversity over deep time.
Perhaps most intriguing is the revelation that the Mesozoic Marine Revolution not only triggered a geological accumulation of carbonate sediment but also marked a turning point in the evolutionary dynamics of one of the ocean’s most prolific organisms. Through their sustained diversification and resilience, calcareous foraminifera exemplify the intricate feedbacks between life and Earth’s chemical environment—feedbacks that continue to influence modern marine ecosystems and their responses to ongoing environmental change.
As current global oceans face increasing acidification due to anthropogenic CO2 emissions, insights gleaned from fossil records of foraminifera may offer crucial perspectives on potential future trajectories of marine calcifiers. The history of foram evolution, shaped by long-term carbonate chemistry shifts and abrupt environmental upheavals, underscores the importance of geologic context in interpreting biological resilience and vulnerability in a changing ocean.
Overall, this pioneering study enriches our understanding of the evolutionary ecology of foraminifera while illuminating the broader ramifications of the Mesozoic Marine Revolution for Earth’s ocean chemistry and marine biodiversity. It highlights the power of interdisciplinary research—merging paleontology, geochemistry, and evolutionary biology—in unraveling the complex narratives embedded in the fossil record, narratives that continue to echo in today’s oceans.
Subject of Research: Evolutionary dynamics and test composition of foraminifera throughout the Phanerozoic Eon in relation to ocean chemistry changes and the Mesozoic Marine Revolution.
Article Title: Record of Foraminifera test composition throughout the Phanerozoic
News Publication Date: 9-Apr-2025
Web References: https://royalsocietypublishing.org/doi/full/10.1098/rspb.2025.0221
References: DOI 10.1098/rspb.2025.0221
Image Credits: Credit: Chris Lowery / The University of Texas at Austin Jackson School of Geosciences.
Keywords: Evolutionary ecology, Ecological adaptation, Ecological speciation, Extinction, Paleontology, Micropaleontology, Paleoecology, Mass extinctions, Population ecology, Marine ecology, Ocean chemistry
