Why Do These Fossil Shells Flip Their Spirals Every Few Millennia?

For thousands and sometimes millions of years, marine plankton all over the world built their spiral shells in one direction. Then, suddenly, the spirals switched direction at the same time, everywhere, only to switch back again later. These microorganisms are types of foraminifera, or forams.

Found in all oceans, from the tropics to high latitudes, they’re among the most abundant eukaryotic organisms on Earth. The single-celled protists secrete a hard shell perforated with many small holes; most species live on the seafloor, while some are planktonic, at the mercy of currents. When they die, their shells blanket the seafloor across the world, forming a natural archive of Earth’s history that goes back some 560 million years.

By studying the composition of species, or measuring isotopes or trace elements, scientists use accumulated foram shells to reconstruct past climates and ocean conditions. Over the years, they have noticed a strange phenomenon. Among the many planktonic forams that have snail-like coiled shells, some species strongly prefer one spiral direction — left or right — over the other, with as many as 97% of the individuals in a species coiling in the same direction.

And sometimes, a new coiling direction dominates seemingly everywhere across the oceans all at once in the fossil record. Ever since the 1950s, when this curiosity was observed, scientists have been trying to figure out what might cause shell direction to change in quadrillions of microorganisms in unison. Recently, micropaleontologists integrated and synthesized data on the shell-coiling direction, or chirality, of several species from case studies of forams dating back as far as 56 million years.

Some researchers speculated that the shell flips were connected to changes in temperature or other climate factors. The new study, however, hypothesizes that shell direction is not itself an adaptive response, but rather is an accidental marker of a significant evolutionary event that starts in a small subpopulation and then sweeps across vast ocean basins.