Ichthyosaurs were silent assassins of Jurassic seas

More than 180 million years ago, ichthyosaurs ruled the early Jurassic oceans. The carnivorous marine reptiles ranged from the size of a briefcase to larger than a school bus. The biggest of these whale-like creatures were apex predators, hunting ancient fish, ammonites, and even their smaller reptile relatives. As they searched for prey, some may have swum with surprising stealth. 

A well-preserved ichthyosaur fin fossil (first uncovered at a construction site) reveals a suite of unique adaptations for cutting down on sound, according to a study published July 16 in the journal Nature. The evolutionary quirks unveiled by the new research offer insight into how a subset of ichthyosaurs lived and hunted– and could lead to a better understanding of existing fossils in the record. 

“The soft tissue preservation in this fossil is just one of these amazing discoveries that you stumble across once in a lifetime, perhaps,” Lars Schmitz, a paleontologist and functional morphologist at Claremont McKenna College who wasn’t involved in the study, tells Popular Science. But what makes the work especially exciting isn’t just the “sheer luck,” Schmitz says. It’s also the thoroughness with which the authors approached the find. 

“It’s a really unique and very interdisciplinary analysis, with a whole array of innovative techniques,” he notes. Using advanced imaging tools like electron and X-ray microscopes, coupled with computer modeling, the scientists pieced together both the “what” and “why” of a Jurassic-era jigsaw puzzle.

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Solving the puzzle

In this case, “jigsaw puzzle” isn’t really a metaphor. The fossil was unearthed at a construction site in southwestern Germany in 2009. A controlled blast sprung a layer of limestone loose from shale, and scattered chunks of rock bearing a detailed impression of an ichthyosaur fin across the site. A fossil collector, given a couple of hours to survey the construction zone, gathered all the pieces he could–amassing an array of oddly sized slabs.

From there, the fragments were passed around to various ichthyosaur and fossil experts, who attempted to fit them together. Finally, more than a decade later, Johan Lindgren, a paleontologist at the Lund University in Sweden and the study’s lead author, gave it a shot. 

“One day, around Christmastime one year, I thought, ‘Now, let’s solve this once and for all’,” he tells Popular Science. 

Lindgren sat down at a large table, rearranging the chunks in sand. He was able to combine all of the segments together in a complete and coherent form after he realized that a few of the bits comprised a second, smaller flipper. “That was a big breakthrough. Then I realized: yes, it’s all here it all fits.” 

Solving the physical puzzle turned out to be just the beginning. Once the fin was assembled, it was clear that this distinct fossil was worth a closer look. Long-gone skin and flesh had left rare marks in the rock, alongside the more standard bones. According to Lindgren and his colleagues, the fossil is the first-ever soft tissue impression found from Temnodontosaurs. This  genus of large ichthyosaur (some exceeding 30 feet long) was the first to be discovered in the 1810s by Mary and Joseph Anning.

The exciting fossil first is also full of idiosyncrasies. “I’ve studied these critters for quite some time now, and I’ve never seen anything like this before,” Lindgren says.  

A serrated fin fit for a predator 

For one, the fin’s proportions are unique– it’s especially long and thin. The end of the fin lacks bones, leaving a soft, flexible tip not seen in other known living or extinct animal examples. Then, there are evenly spaced lines running over the entire surface and distinct serrations on the trailing edge. X-ray microscopy showed these serrations were composed entirely of cartilage embedded in skin, another oddity. Some animals, including living reptiles and mammals have bits of bone in their skin for protection, called osteoderms. But no other known animal shows signs of cartilage interwoven into skin the same way, Lindgren notes. He and his co-authors propose a new term for the never-before-seen structures: chondroderms, after the Greek words for cartilage and skin. 

“This paper shows that there are still surprises to find in ichthyosaur evolution,” Nicholas Pyenson, a paleontologist and curator at the Smithsonian Museum of Natural History, tells Popular Science. “And that we do need to look at our fossils much more carefully,” he adds.

So, what were all these stand-out features for? Considering both human inventions and birds brought Lindgren to a hypothesis. Wind turbines, propellers, and aircraft are often built with a serrated trailing edge to reduce noise. Owls manage nearly silent flight with a similar structure: ragged edges along the bottom of their wings. What if the same thing was true of Temnodontosaurus? 

Chicken of the sea, meet owl of the ocean

Individuals in the genus had the largest eyes of any known vertebrate, living or extinct. Prior studies have suggested that their dinner-plate sized peepers helped the giant reptiles navigate and hunt in dimly lit environments, either at night or in deep water, adding fuel to the owl comparison. In these settings, sound would have been key for prey animals trying to avoid getting eaten, and for predators trying to listen for their food. Both factors may have added up to evolutionary pressure on ichthyosaurs, favoring stealthy swimming, says Lindgren. 

“I thought, okay, so what if we basically had a 10-meter underwater owl swimming around,” he says. 

To verify the idea, the team created a partial computer model of the fossil, including the serrations and the parallel, horizontal ridges. Using previous research estimating ichthyosaur swim speeds and educated assumptions about attack angle, they tested the effects of each trait on swim sounds of varying frequency. Both the surface ridges and edge serrations showed promise for minimizing noise, per the simulation– especially for the low frequencies that travel farthest under water. The modeled sound reduction was as high as 10 decibels– about equivalent to the effect of wearing foam earplugs. The flexible fin tip and other features like body shape may have added additional sound-dampening, Lindgren says. However, he and his colleagues haven’t yet created the extra complex models needed to test it.

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Breathing life into the fossil record

“They’ve done the absolute right thing,” says Pyenson. “They have experimental data to show the effect of the structure for the presumed function. That’s about as compelling as you get…That’s breathing life into the fossil record.” 

Yet, certain conclusions are impossible to draw from one fin alone, all sources tell Popular Science. The rest of Temnodontosaurus’ soft tissue features remain a mystery, and the fin fossil doesn’t hold key details, like the appendage’s lateral profile, which would have a big impact on how it moved through the water. Plus, strategic stealth may not explain everything about the fossil fin. Schmitz says It’s possible the unique features were driven by something more direct: speed and maneuverability. 

“My gut feeling is it was probably hydrodynamics first, and then sound dampening was a nice side effect,” he explains. He hopes to see further modeling experiments digging into alternative explanations. 

Schmitz also hopes other scientists take note. Now that these soft tissue features have been identified on one ichthyosaur, paleontologists can revisit other fossils to see what’s potentially been overlooked. It could even spur reevaluation of modern marine animals, Schmitz adds. Perhaps there’s a ridge, bump, or cartilaginous bit with a yet-unknown acoustic purpose, hiding on a porpoise.