“It’s probably one of the first times that people who usually do more biostratigraphy — basically, deep-time research — on foraminifera are approaching such a question,” said Julie Meilland, a researcher at the Cerege, a research institute in France, who was not involved in the study. In addition to data on coiling direction from multiple species going back millions of years, the study incorporates insights from modern foram genetics and biology. “It was very refreshing to see these worlds connect because very often people doing more modern research don’t necessarily connect to people doing deep-time research,” Meilland added.
The work offers a new perspective on a decades-old mystery and a rare glimpse into an obscure process that enables new traits to sweep across large populations.
A Twisted Hypothesis
The phenomenon of the flipping forams was first described in the early 1950s, when advances in seafloor coring techniques allowed researchers to analyze accumulated layers of shells. The Swiss micropaleontologist Hans Bolli first noted that among forams with coiled shells, several species had a directional preference, and that sometimes this preference changed through time.
An explanation for this curious occurrence came from a seminal 1959 study, for which the marine geologist David Ericson, a core specialist at Columbia University’s Lamont Geological Observatory (now the Lamont-Doherty Earth Observatory), sifted through hundreds of coiled shells from the species Neogloboquadrina pachyderma collected from the North Atlantic. Ericson observed that in cold climates during the ice ages, the shells tended to coil left, while during warmer periods they turned right.
He wasn’t sure why coiling direction would relate to climate, but he speculated that temperature was this species’ determining factor. However, as more cores were retrieved from around the globe, and with advances in genetics, the temperature hypothesis didn’t hold up.
In 2006, Kate Darling, now an honorary professor at the University of Stirling, published genetic work showing that the variants of N. pachyderma are, in fact, two distinct species, each with its own coiling direction. Then, in 2013, the evolutionary paleobiologist Yurika Ujiié, now a professor at Kochi University in Japan, found that shell chirality in different foram species, collected from multiple oceans, did not correspond to temperature.
Each study countered Ericson’s hypothesis in a different way. Plus, it was hard for many researchers to imagine what advantage coiling direction would offer a single-celled organism with no obvious handedness. Half a century after Ericson’s initial observation, the driving force behind the flips once again became a mystery.
It wouldn’t take long to pique another researcher’s interest. Bridget Wade, a micropaleontologist at University College London, had been studying sediment cores for decades when her team noticed a curious pattern. Several foram species seemed to flip their shell direction around the same time at different latitudes in the Atlantic, Indian, and Pacific oceans. In one species, the flips seemed almost instantaneous in the tropics as well as in higher latitudes. This evidence that the phenomenon extended far beyond a single ocean basin suggested a global process with more than temperature at work.
To sate their curiosity, Wade’s team synthesized data from five decades of studies and analyzed changes in coiling patterns in several planktonic foraminifera species from the past 56 million years. For each species, they found evidence of flipping across multiple ocean basins and climate belts. Paragloborotalia siakensis changed from mixed to left-handed coiling 15 million years ago. Globorotalia scitula flipped twice: from mixed to left-handed 15 million years ago, and then to right-handed 10 million years ago. “It seems truly puzzling that a species could exist for millions of years coiling one way, and then suddenly reverse, for no apparent reason,” the authors wrote.
Bridget Wade, a micropaleontologist at University College London, points to a species of foram she described at the Smithsonian National Museum of Natural History.
Pulleniatina obliquiloculata was an especially useful example, Wade said: It has an exceptionally detailed fossil record, is still living today, and occurs throughout tropical oceans worldwide. For the past 860,000 years, its shell has coiled almost exclusively to the right. But before that, it went through a sequence of rapid shell-coiling flips that occurred globally every few thousand years.
The shifts were far too sudden and widespread to be explained by gradual evolution. “That was quite a surprise because if it was a local event, it would be easier to think about a local, changing environment,” Wade said. The fact that it was happening everywhere suggested a different process at work.
Cryptic Meaning
What could explain the worldwide sweep of a chirality switch? Wade knew that despite their apparent uniformity, oceans hide many distinct habitats that differ in temperature, currents, ultraviolet light, chemistry, and oxygen. Likewise, an apparently global population of a foram species can hide cryptic species. Genetic studies have revealed that often what was considered a single species, based on shell shapes (including coiling direction), was in fact more than one.
What if, her team hypothesized, one of these cryptic species developed some broad adaptive advantage. This cryptic species might spread across the globe, carried by ocean currents and its own success, in a gigantic population sweep — and bring a single coiling direction to dominance along the way, thereby preserving the event in the fossil record.
