Microscopic, single-celled organisms reproduce rapidly and have only the most fleeting existence as individuals. Wrong – millions of years ago, there were disc-shaped, single-celled organisms called nummulites, some of which were as big as coasters and, according to new research, could have lived for more than a century – certainly the biggest, and arguably the longest-lived, of all known single-celled creatures.

Nummulites are a subgroup of a large and important variety of single-celled organisms, or 'protists', called foraminifera. These otherwise amoeba-like creatures build themselves elaborate shells of calcium carbonate. These shells increase in size in a predictable way as the creatures age, and, being made of mineral, the shells are preserved readily as fossils. Foraminifera are generally microscopic: however, Nummulites millecaput, which lived in the Eocene epoch around 50 million years ago, during the warmest climatic episode since before the demise of the dinosaurs, grew to 160 mm in diameter. After this acme, nummulites declined in size and diversity, and their living relatives seldom exceed 2 mm in size or live longer than a year or two.

Louise M. A. Purton of Trinity College, Dublin, Ireland, and Martin D. Brasier of the University of Oxford, UK, have been studying the environment in which nummulites lived, to try and understand how and why these "dinosaurs of the unicellular world" attained their great size. Did they live in a nutrient-rich environment, growing rapidly and dying young? Or did they live in leaner times, living long and growing much more slowly? As Purton and Brasier show in the latest [August] issue of Geology, the answer is both – and neither. Nummulites had the best of both worlds, living in nutrient-rich environments, and managing to do so for a long time. The cost was an ability to weather the inevitable changes in the environment that long-lived creatures encounter during their lifetimes.

The researchers were not able to study examples of Nummulites millecaput, the behemoths of the protist world. Instead, they studied fossils of a relative, N. laevigatus, which, although somewhat smaller – around 10 mm across – is still gigantic by protist standards. Extremely precise studies of the composition of various parts of the carbonate shells of nummulites show that these creatures withstood dramatic, seasonal changes in their environment. This seasonal record allows the researchers to measure how old the creatures were when they died. An individual of N. laevigatus could expect to live for five or six years, reaching its final size by slow and steady growth – a complete contrast to the small body size and rapid growth rates of other protists. Extrapolating the results to the much larger N. millecaput gave age estimates for these creatures of more than a century.