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

Foraminifera, or forams, are single-celled marine organisms found in all oceans, from tropical to high-latitude regions. They are among the most abundant eukaryotic organisms on Earth. Forams secrete a hard shell with many small holes, often coiled in a spiral like a snail. Most species live on the seafloor, while others float in ocean currents as plankton. When they die, their shells accumulate on the seafloor, creating a natural record of Earth’s history spanning 560 million years. Scientists study these shells to reconstruct past climates and ocean conditions by analyzing species composition, isotopes, or trace elements. Some foram species exhibit a strong preference for coiling their shells either left or right, with up to 97% of individuals in a species showing the same coiling direction.

For decades, scientists have observed that the coiling direction of foram shells occasionally flips from left to right or vice versa across the world’s oceans at the same time. This phenomenon was first described in the 1950s when advances in seafloor coring allowed researchers to analyze layers of shells. Initially, some researchers hypothesized that temperature changes might drive these flips, as early studies suggested that cold climates favored left-coiling shells in certain species, while warmer periods favored right-coiling. However, later genetic and fossil evidence disproved this idea. For example, in 2006, genetic work showed that variants of the species Neogloboquadrina pachyderma were actually two distinct species, each with its own coiling direction. By 2013, further research confirmed that shell chirality did not correspond to temperature.

In 2023, micropaleontologist Bridget Wade and her team synthesized data from five decades of studies to analyze changes in coiling patterns in several planktonic foram species over the past 56 million years. They found that shell flips occurred simultaneously across multiple ocean basins and climate belts. For instance, the species Paragloborotalia siakensis changed from mixed to left-handed coiling 15 million years ago, while Globorotalia scitula flipped twice: to left-handed 15 million years ago and to right-handed 10 million years ago. The species Pulleniatina obliquiloculata showed rapid global flips every few thousand years over the past 860,000 years. These shifts were too sudden and widespread to be explained by gradual evolution, suggesting a different process was at work.

Wade’s team hypothesized that the worldwide flips might be linked to the rise of a cryptic species—a genetically distinct group that appears identical to others based on shell shape. This cryptic species could have developed a broad adaptive advantage, allowing it to spread rapidly across the globe via ocean currents. As it spread, it brought a single coiling direction to dominance, preserving the event in the fossil record. This process resembles how variants of the Covid-19 virus spread globally, becoming the dominant strain in many countries. However, researchers caution that the relationship between genetics and shell chirality is not straightforward. For example, coiling direction may have a genetic basis, but left-coiling and right-coiling individuals do not necessarily represent separate genetic populations.