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

Tiny marine organisms called foraminifera, or forams, build spiral shells that sometimes flip direction — coiling left instead of right — in near-unison across whole ocean basins within a few thousand years. Scientists have puzzled over this "chirality" switch since the 1950s, when Swiss micropaleontologist Hans Bolli first noticed that some coiled foram species strongly favor one direction, with up to 97% of individuals in a species coiling the same way.

Forams are single-celled protists found in every ocean. When they die, their perforated shells pile up on the seafloor, forming a fossil record stretching back some 560 million years. In 1959, marine geologist David Ericson of Columbia University's Lamont Geological Observatory studied the species Neogloboquadrina pachyderma from the North Atlantic and proposed that temperature controlled coiling direction, since shells turned left in cold ice-age waters and right in warmer periods.

That explanation did not hold up. In 2006, Kate Darling, now at the University of Stirling, used genetics to show the two coiling variants of N. pachyderma are actually separate species. In 2013, Yurika Ujiié of Kochi University in Japan found that chirality in other foram species from multiple oceans also did not track temperature.

Bridget Wade, a micropaleontologist at University College London, and colleagues combined five decades of research, studying coiling patterns in several planktonic foram species over the past 56 million years. They found flips recurring across the Atlantic, Indian and Pacific oceans; Paragloborotalia siakensis, for example, switched from a mixed pattern to left-coiling shells about 15 million years ago. Their hypothesis: coiling direction is not itself adaptive, but an accidental marker of some other evolutionary change that starts in a small subpopulation and sweeps across ocean basins.

Julie Meilland, a researcher at Cerege in France who was not involved in the study, said it was refreshing to see researchers who usually study deep-time fossil patterns connect their work with modern foram genetics and biology.