Find out how an extinct mollusk got its weird shell


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If you’ve seen one ammonite, you might think you’ve seen them all. Most of the 10,000 extinct cephalopod species sported tightly coiled shells with polished mouthfuls of tentacles.

Enter Nipponites mirabilis, a species of ammonite straight out of a painting by MC Escher. Instead of the classic coiled snake shell design, he substituted something much more ridiculous: a convoluted shell twisting on itself with no obvious beginning or ending.

“It looks like a piece of rope someone threw out a window,” said Kathleen Ritterbush, a paleoecologist at the University of Utah.

“The first time you look at it, it’s just this tangled mess,” said Derek Moulton, mathematician at the University of Oxford. “And then you start to look closely and say, oh, actually, there’s a pattern there.”

Dr Moulton and his colleagues have developed a mathematical model that they believe reveals the forces acting on the puzzling shells of Nipponites and the shells of many other mollusks. The research was published in November in the Proceedings of the National Academy of Sciences.

Their model suggests a mismatch between the growth rates of the mollusk’s soft body and its hard shell, which creates mechanical forces that twist the body, resulting in an asymmetric shell. The model also explains how other snails develop their characteristic spiral shells, the researchers said.

“It’s a beautiful result,” said Katharine Long, an applied mathematician at Texas Tech University, who was not involved in the research. “This is the simplest model that could possibly produce all three shapes,” added Dr Long, referring to the traditional spiral of an ammonite shell, the helical spiral of a snail and the meanders of the Nipponites.

The document is the latest collaboration between Dr. Moulton; Alain Goriely, chair of mathematical modeling at Oxford; and Régis Chirat, researcher at the University of Lyon in France. The three scientists seek to understand the physics behind the formation of seashells. They published on the thorny shells of sea snails and the nested shells of oysters.

At one of the team’s first meetings, Dr Moulton and Dr Goriely visited Dr Chirat in Paris and the trio spent an afternoon admiring the seashells and ammonites in the Grande Galerie de l ‘Evolution of the National Museum of Natural History.

“Like children inside the Willie Wonka factory,” said Dr Goriely.

But the knots of the Nipponites were becoming puzzled.

“The Japanese have become an obsession for me,” Dr Chirat said on a Zoom call from his office, which contains hundreds of fossils and seashells.

Molluscs create their own shells using their mantle, a fleshy external organ. The mantle secretes calcium carbonate in layers, which harden in the shell. The researchers wanted to design a model that captured the interactions between the mollusk’s soft body and the shell as it hardened.

When ammonites went extinct about 66 million years ago, they left little trace of their slimy interior in the fossil record. But the evidence suggests that the ammonites, like their living squid cousins, were bilaterally symmetrical; drawing a line in the middle would result in symmetrical halves. The researchers therefore built their model assuming that the ammonites were bilaterally symmetrical.

So how could a symmetrical body secrete an asymmetric shell? “Suppose there is a mismatch between the way the body grows and the way the shell grows,” Dr Moulton said. “That’s the whole premise of the model.”

If the body is growing faster than the shell, it will be too big for its shell and generate mechanical stresses that will cause the body to twist inside the shell. Dr Moulton offered an analogy: imagine the ammonite shell as a long, hard tube filled with two soft pool noodles that are longer than the tube. To relieve stress, the noodles (the soft body) twist inside the tube (the shell). As the soft body twists, it rotates the edge of the mantle secreting the shell, resulting in an asymmetrical shell.

“If the conditions are right, these anomalous forms like the Nipponites emerge,” Dr Moulton said.

By adjusting the level of mismatch and the stiffness properties of the model’s soft body, the researchers produced the bizarre shells of other unorthodox ammonites, such as Didymoceras.

“First it’s straight, then it’s a paperclip, then it’s an upside-down ice cream cone spool, then it’s shaped like a hook,” Dr. Ritterbush said, describing Didymoceras.

But there are other questions left unanswered by the model, she said, including the biological costs, benefits, and tradeoffs of having such an asymmetric shell.

Recent research suggests that the wild shell of the Nipponites helped the ammonite slowly spin around the water column in search of prey. Kenneth De Baets, a paleobiologist at Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany, who was not involved in the new study, said he was curious to see how the model holds up as paleontologists discover more tissue of fossilized soft ammonite.

“These animals have been dismissed as eccentrics and mistakes,” said Dr. Ritterbush. “But it’s actually a perfectly executed plan, a spiral of balance.”

But even with those questions, Dr Ritterbush said, the new model highlights how seemingly bizarre shapes like Didymoceras and Nipponites look more like ordinary ammonites than they appear.

“It gives credit to the idea that for an animal to produce a shell like this would not require moving the sky and the Earth,” she said. “It wouldn’t require an incredibly weird evolutionary leap.”

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