A tiny fossil with a 500-million-year story

If you’ve ever picked up a seashell on the beach, you were holding the hard work of a marine organism, an intricate house built out of calcium carbonate, the same chalky mineral found in limestone and even in antacids. For hundreds of millions of years, ocean life has relied on this mineral to build skeletons, shells, and protective coverings. But what happens when the chemistry of the ocean shifts, making it harder for organisms to build and maintain those structures? A study published in [Proceedings of the Royal Society B] by researchers from the University of Oxford, the University of Texas at Austin, the University of Colorado, and the University of Victoria, takes a look at this question by turning to some of the smallest but most revealing fossils on Earth: the so-called foraminifera.

These tiny, single-celled organisms, often called “forams” by paleontologists, build microscopic shells, or “tests”, that accumulate on the seafloor. They’ve been doing this for over half a billion years. Their remains form an unparalleled fossil archive, letting scientists trace not just their own evolutionary story, but also the chemical mood swings of the oceans they lived in.

Why Foraminifera?

Foraminifera are the perfect storytellers for this kind of research. Despite their minuscule size (most are barely the width of a grain of sand), they are incredibly abundant and diverse. More importantly, they build their shells in different ways: calcareous tests made of calcium carbonate, secreted directly from seawater; agglutinated tests cobbled together from surrounding sediment grains, like tiny sandcastles, and organic tests made from softer, carbon-based compounds.

This variety makes forams ideal for testing how different shell-building strategies fared through times of environmental stress, such as mass extinctions or episodes of ocean acidification.

The team did something no one had done at this scale before: they compiled data on 2,282 genera of benthic foraminifera, stretching across the entire Phanerozoic Eon, that’s the last 541 million years of Earth history. Using a classic but comprehensive classification, they tracked which types of forams appeared, thrived, or disappeared across the ages. The central question was: Did long-term changes in ocean chemistry drive the rise and fall of different shell types? And the answer, surprisingly, is: not really. Instead, the study found that foram survival and diversity were more influenced by short-term crises and local conditions than by slow, sweeping chemical trends across the oceans.

The history of life on Earth is punctuated by five catastrophic events, the so-called “Big Five” mass extinctions. Faulkner and colleagues used these benchmarks to see how different foraminifera responded. First, in End-Ordovician (approximately 444 million years ago): despite wiping out about 85% of all marine species, this event barely dented the foraminifera. Agglutinated forms even flourished. Second, in Late Devonian (about 370 million years ago): calcareous forams lost some diversity but quickly rebounded. In fact, this was the period when calcareous shells really took off. Third, in End-Permian (some 252 million years ago): the worst extinction in Earth’s history, driven by massive volcanism, climate chaos, and ocean acidification, was devastating for calcareous forams. Their diversity collapsed by nearly 90%, dropping from dominance to near insignificance. Fourth, in End-Triassic (approximately 201 million years ago): another volcanic episode triggered global stress, but this time forams weathered the storm far better. Calcareous types dipped slightly but survived. And, fifth, in End-Cretaceous (about 66 million years ago), the asteroid that wiped out the dinosaurs also shook ocean ecosystems. Many planktonic forams disappeared, but benthic forms, especially calcareous ones, managed a relatively quick recovery.

Across all five events, only two, the End-Permian and End-Triassic, caused significant drops in calcareous diversity. This suggests that while acidification and ocean chemistry shifts mattered, they weren’t consistently fatal.

The paper highlights a turning point in the history of the Earth: the Mesozoic Marine Revolution (about 250–150 million years ago). During this time, new groups of plankton evolved that pumped calcium carbonate into the oceans on an enormous scale. This “carbonate buffering system” stabilized global ocean chemistry and gave calcareous forams a more secure footing. From the Cretaceous onward, calcareous forms dominated, maintaining around 75–80% of all foram diversity right through the Cenozoic Era (the last 66 million years). Even as climates swung between hothouse and icehouse states, this balance remained surprisingly steady.

So what do 541 million years of foram history teach us? Well, the first takeaway is that resilience is real, but conditional. Foraminifera bounced back from multiple crises, but certain combinations of stressors, like the deadly cocktail of acidification, warming, and deoxygenation at the End-Permian, can still topple even the hardiest groups. Also, local context matters. Instead of smooth, global trends, foram diversity responded in patchwork ways to specific events and habitats. And finally, modern parallels are imperfect. While ancient data show forams can survive big changes, the sheer speed of today’s carbon spike makes the present situation unique. In other words: the past offers hope, but not complacency.

If you want to learn more, read the original article titled "Record of Foraminifera test composition throughout the Phanerozoic" on Proceedings of the Royal Society Biological Sciences at http://dx.doi.org/10.1098/rspb.2025.0221.