A past predator of the seas may have had a secret weapon: noise-cancelling flippers that helped it sneak up on prey.
Scientists analyzed a fossilized impression of a front flipper ascribed to the large marine reptile Temnodontosaurus. The fossil, which is between 183 million and 181 million years old, reveals that the trailing edge of the flipper was curiously corrugated, rather than smooth, the team reports online July 16 in Nature.
The team then performed computer analyses to simulate how the flipper might have moved through water. Their findings suggest that the strange serrations might have helped manipulate the flow of water around the flipper, dampening the sound of the animal’s swimming — a novel form of stealth attack for an ancient marine reptile, say paleontologist Johan Lindgren, of Lund University in Sweden, and colleagues.
Some modern marine mammals, including killer whales and dolphins, have tiny ridges on their skin that researchers have previously suggested might reduce drag during swimming.
This flipper fossil is not only an exciting find, but also “gives us a new way to start thinking about sensory adaptations in extinct animals,” says Lene Liebe Delsett, a paleontologist at the Norwegian Center for Paleontology in Oslo, who was not involved in the research. And it highlights the importance of finding fossilized soft tissues, which can enrich and broaden our understanding of these ancient creatures, she adds.
The flipper fossil was a chance discovery by a private collector; it was found lying in pieces in the aftermath of road construction work in southern Germany. It eventually made its way to Lindgren, who has previously studied various types of fossilized soft tissues.
The fossil sat in his lab for a few years, until he got around to really looking at it. “And then it occurred to me how weird this flipper really is: very elongated — exceedingly long, almost like an albatross wing. What’s also really weird is that normally, when you have flippers, you have a skeleton that extends all the way to the tip.” But the last quarter or so of the flipper contained no bones, only cartilage, he says. “It’s just more floppy.”
The flipper also had a serrated edge, with each serration reinforced by a needlelike piece of cartilage, a type of reinforcement not seen before. The cartilage supports are reminiscent of osteoderms, the bony deposits that strengthen the skin of many amphibians and reptiles, including some dinosaurs. So the team dubbed these cartilage reinforcements “chondroderms,” after the softer material.
“All of these combined [characteristics] didn’t make any sense at first,” Lindgren says. “Until I started looking around in the literature.”
Serrated trailing edges, it turns out, are commonly used in modern propellors and turbines, as a way to dampen noise. That shed a new light on the matter.
Temnodontosaurus was one of the largest species of ichthyosaur, a predatory marine reptile that evolved from land-based reptiles, much like the evolutionary journey of modern marine mammals including whales and dolphins. The species had a body length of around nine meters, and had the largest eyes of any known creature on Earth, from any era.
“We’re talking plate-sized,” Lindgren says. Those large eyes — a feature found in other denizens of the dark, from owls to giant squid — had already suggested that Temnodontosaurus might be well adapted to hunt under cover of darkness. These flippers might have been an additional stealth adaptation, the team suggests.
“The idea was that if it wanted to produce as little noise as possible, it might move its body as little as possible. We see the same pattern in some sharks today,” he adds. To assess how the flipper would have moved in water, the team used previously estimated swimming speeds for ichthyosaurs, and factored in how serrations and other features of the flipper would affect movement.
Because the fossil was somewhat flattened, making it difficult to accurately determine its actual width, the team substituted the width of a modern minke whale flipper — also relatively elongated — in their calculations. Based on all of these data, the team determined that the ancient flipper did indeed act as a noise dampener during swimming.
“It just shows how little we know about ancient animals,” Lindgren says. “I had no idea this is where we would end, when I started off [with this fossil] three years ago. It kind of blows my mind.”