Why Do These Fossil Shells Flip Their Spirals Every Few Millennia?
Ada Zejun Shen for Quanta Magazine
Introduction
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.
David King
“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.
Courtesy of Briget Wade
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.
Unlike shelled species most familiar to us, forams do not live fully inside their shell. The perforated shell sits beneath the cell membrane and is engulfed by jelly-like protoplasm. All the specimens here are smaller than 1 millimeter.
“Flipping, in my opinion, means they’ve speciated,” said Darling, who was not involved in Wade’s study.
Whether the flipped forams are a new species or a genetic variant, they would need to have a significant advantage over other forams to sweep all around the world. “It’s a bit like how Covid-19 variants became really abundant in one place,” Wade said, “and then had a slight fitness advantage over another genetic variant” and rapidly spread to become the dominant strain in virtually every country on Earth.
However, Ujiié cautioned against assuming any simple relationship between a species’ genetic identity and its shell chirality. “Coiling direction appears to have a genetic basis,” she said, “but it does not necessarily mean that dextral [right-coiling] and sinistral [left-coiling] individuals represent separate genetic populations.”
It’s still unclear why an entire population would coil the same way in the first place; it happens only in a subset of planktonic foram species with coiled shells. In other species, shells coil in both directions in a roughly 50-50 split. So why do some have a strong chiral bias? The only thing the scientists seem to agree on is that they don’t know yet.
Meilland, who studies reproduction in live cultures of forams, has observed that ratios of coiling directions in her cultures don’t necessarily match what one would expect to find in the wild, based on fossil studies. She thinks that the drivers of coiling direction could be rather complex. “I think there could be something with genes, with the recombination, with them trying to evolve, with the environment, and also with luck and just life,” she said.
While the near synchronicity of the worldwide foram flips might seem puzzling, Darling and Ujiié pointed out that what appears instantaneous in the fossil record might unfold over 1,000 years or more in real time. Over that period, ocean waters circulate around the entire globe. Water masses also change and move, which could speed up the spread of a new variant.
It’s possible that traits or species of other marine organisms can also sweep the globe this way. But it would be difficult to observe without a clear fossil marker, such as the forams’ shell chirality. These coiling flips are therefore a rare window into how evolutionary processes can play out on a global scale.
