In a university swimming pool, scientists and their underwater cameras watch intently as a coiled shell is released from metal tongs. The shell begins to move on its own, giving the researchers a glimpse of what the oceans looked like millions of years ago when they were full of these ubiquitous animals.
This is not a Jurassic Park, but it is an attempt to get to know the old life by recreating it. In this case, the recreations are 3D-printed robots designed to mimic the shape and movement of ammonites, sea creatures that both preceded and were contemporaneous with the dinosaurs.
The robotic ammonites allowed the researchers to explore questions about how shell shapes affected swimming ability. They found trade-offs between water stability and maneuverability, suggesting that the evolution of ammonite shells explored different designs for different benefits rather than converging towards a single best design.
“These results reiterate that there isn’t one optimal shell shape,” said David Peterman, a postdoctoral researcher in the Department of Geology and Geophysics at the University of Utah.
The study is published in Scientific Reports and supported by the National Science Foundation.
Bringing ammonites to “life”
Peterman and Kathleen Ritterbush, assistant professors of geology and geophysics, have been researching the hydrodynamics, or physics of movement through water, of ancient shelled cephalopods, including ammonites, for years. Cephalopods today include octopuses and cuttlefish, with only one group having an external shell – the nautiluses.
Before the present era, there were cephalopods with shells everywhere. Although their rigid coiled shells would have affected their free movement through the water, they were phenomenally successful in evolution, persisting for hundreds of millions of years, surviving any mass extinction.
“These properties make them excellent tools to study evolutionary biomechanics,” says Peterman, “the story of how benthic (bottom-dwelling) molluscs became one of the most complex and mobile group of marine invertebrates. My broader research goal is to provide a to better understand these enigmatic animals, their ecosystem roles, and the evolutionary processes that shaped them.”
Peterman and Ritterbush previously built life-size 3D-weighted models of cone-shaped shells of cephalopods and found, by releasing them into pools, that the ancient animals likely lived vertical lives, bobbing up and down the water column to find food. The movements of these models were determined solely by the buoyancy and hydrodynamics of the shell.
But Peterman has always wanted to build models that are more like living animals.
“I’ve wanted to build robots ever since I first developed techniques to replicate hydrostatic properties in physical models, and Kathleen was also very encouraging,” says Peterman. “On-board propulsion allows us to explore new questions regarding the physical limitations on these animals’ living habits.”
Buoyancy became Peterman’s main challenge. He wanted the models to be neutrally buoyant, not floating or sinking. He also wanted the models to be waterproof, both to protect the electronics inside and to prevent leaking water from altering the delicate balance of buoyancy.
But the extra work is worth it. “These techniques allow exploring new questions,” says Peterman, “including complex beam dynamics, rollout efficiency, and the 3D maneuverability of certain shell shapes.”
Three types of shells
The researchers tested robotic ammonites with three shell shapes. They are based in part on the shell of a modern Nautilus and modified to represent the array of shell shapes of ancient ammonites. The model called a serpenticon had tight whorls and a narrow shell, while the sphaerocone model had few thick whorls and a broad, almost spherical shell. The third model, the oxycone, sat somewhere in the middle: thick wreaths and a narrow, streamlined shell. You may think that they occupy a triangular diagram, which represents “end members” with different scale characteristics.
“Every planispiral cephalopod that ever existed is shown somewhere in this diagram,” Peterman says, helping to estimate the properties for intermediate forms.
After the 3D-printed models were built, rigged and weighed, it was time to go to the pool. Peterman and Ritterbush first worked in the pool of geology and geophysics professor Brenda Bowen and later in the United States’ Crimson Lagoon. They set up cameras and lights underwater and released the robotic ammonites, tracking their position in 3D space for about a dozen “runs” for each shell type.
Not a perfect shell shape
By analyzing the data from the pool experiments, the researchers set out to determine the pros and cons of each shell characteristic.
“We expected that there would be different benefits and consequences for certain shapes,” says Peterman. “Evolution gave them a very unique way of locomotion after being liberated from the seabed with a chamber-shaped, gas-filled shell. These animals are essentially rigid submarines propelled by jets of water.” That shell isn’t great for speed or maneuverability, he says, but coiled-shell cephalopods still managed to achieve remarkable diversity with each mass extinction.
“During their evolution, externally-shelled cephalopods have circumvented their physical limitations by endlessly experimenting with variations on the shape of their coiled shells,” says Peterman.
So, which shell shape was the best?
“The idea that one form is better than another is meaningless without asking the question, ‘better at what?'” Peterman says. Narrower shells enjoyed less drag and more stability when traveling in one direction, improving their blasting efficiency. But wider, more spherical shells could change direction more easily, spinning on an axis. This maneuverability may have helped them capture prey or avoid slow predators (like other shelled cephalopods).
Peterman notes that some interpretations view many ammonite shells as hydrodynamically “inferior” to others, limiting their movement too much.
“Our experiments, along with the work of colleagues in our lab, show that shell designs traditionally interpreted as hydrodynamically ‘inferior’ had some drawbacks, but are not immobile drifters,” says Peterman. “For externally shelled cephalopods, speed is certainly not the only measure of performance.” Nearly every variation in shell design occurs iteratively at some point in the fossil record, he says, showing that different shapes yielded different benefits.
“Natural selection is a dynamic process, which changes over time and involves numerous functional tradeoffs and other limitations,” he says. high evolutionary rates.”
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