Fossilized fin shows this Jurassic marine predator used extreme stealth to hunt 183 million years ago

A fossilized fin from southern Germany shows that a giant Early Jurassic ichthyosaur hunted in darkness with surprising stealth. The one meter long flipper came from Temnodontosaurus, a predator over 30 feet long.

Its shape and internal structures suggest it moved through deep, dim water with very little self made noise, a finding supported by advanced imaging and modeling from an international research team.

Temnodontosaurus had giant eyes

Temnodontosaurus was an ichthyosaur, a fast swimming marine reptile with a streamlined body, long snout, and powerful tail.

Adults could reach more than 30 feet in length and sat at the top of Jurassic food webs, feeding on large fish, squid, and even other marine reptiles.

The work was led by Dr. Johan Lindgren, a paleontologist at Lund University in Sweden. His research focuses on ancient marine reptiles and the rare fossils that preserve their soft tissues, which can reveal behavior that bones alone cannot show.

The eyes of Temnodontosaurus were enormous, up to about 10 inches across, and fossil measurements show that ichthyosaurs had the largest eyes of any vertebrate known.

One analysis found a Temnodontosaurus eye ring with a diameter of 264 millimeters, bigger than a modern soccer ball, giving strong evidence that these reptiles were adapted to see in extremely low light.

The work  also suggests that such oversized eyes would have gathered light efficiently at depth, where sunlight quickly fades.

That combination of a huge, light hungry eye and a quiet swimming style points to an animal that often hunted where vision and sound both mattered.

In such shadowy water, even a small splash or pressure wave could warn prey, so any edge in staying undetected would have helped this predator catch fast moving targets.

Once-in-a-lifetime discovery

The story begins at a temporary road cutting near the village of Dotternhausen in southwestern Germany. Fossil collector Georg Göltz spotted an unusual slab in the exposed dark limestone and realized he had found almost an entire front flipper from a large ichthyosaur, preserved with skin and other soft tissues.

The fossil includes both part and counterpart, meaning impressions from each side of the fin were preserved as matching slabs.

That level of detail is rare for any marine reptile and had never been seen before in a giant ichthyosaur, where soft tissue usually rots away long before burial.

The upper portion of the fin is missing, and the team thinks the flipper may have been torn off the body, possibly during an attack by another large reptile.

If so, the limb may have drifted to the seafloor on its own and settled into quiet, low oxygen mud, which helped lock its tissues in place for millions of years.

Stealth swimming revelations

In the new study, the team combined high energy X-ray scans, chemical tests, and computer simulations to analyze the fossilized front flipper in fine detail.

Those methods revealed not only the bones, but also layers of skin, pigment cells, and internal fibers that once shaped how the fin flexed and pushed water.

The flipper is long and narrow, with most of its bones packed along the front edge and a soft tip where the skeleton stops.

Its surface shows thin front to back stripes, and its trailing edge forms small scallops reinforced by rodlike chondroderms made of specialized cartilage.

The flipper’s wing-like shape, soft tip, and serrated rear edge suggest the animal evolved features that kept its swimming nearly silent, much like the quiet flight of owls.

Together, the flexible tip, ribbed surface, and serrated edge supported by chondroderms likely smoothed water flow, reducing the small disturbances that create low frequency noise and alert prey.

Testing the silent fin

To understand how this unusual fin actually behaved in water, the researchers turned to computational fluid dynamics, computer based modeling of how fluids move around complex shapes and how those flows generate forces and sound.

They built a digital cross section of the flipper, based partly on the shape of a modern whale fin, then tested different versions with and without serrations and surface ridges.

The simulations indicated that trailing edge serrations lowered low frequency noise by a few decibels, and adding surface ridges reduced it even further without affecting stability.

Modern engineering research on owl inspired wing designs has reached similar conclusions about serrated trailing edges.

Those studies find that carefully shaped saw tooth patterns at the rear of a wing can lower broadband noise while keeping the wing efficient, which supports the idea that the Jurassic flipper was also tuned for quiet motion rather than brute force.

Together, the fossil and simulations point to a fin that was both precise and quiet. At cruising speed, the animal likely glided with minimal tail movement, keeping pressure shifts and noise low while maintaining lift.

Lessons from Temnodontosaurus

Many marine animals rely heavily on sound, not light, to find food, avoid predators, and communicate. In the last century, growing anthropogenic noise – sound from ships, sonar, seismic surveys, and offshore construction – has filled large parts of the ocean with persistent background noise that can mask natural signals and even damage hearing in some species. 

One global review concludes that such noise can change behavior, harm sensory organs, and reduce the success of fisheries and other marine life.

Reducing noise from ships and underwater machines is a growing engineering priority, and natural designs offer useful clues.

The Temnodontosaurus flipper shows that quiet movement evolved long before modern technology. Its flexible tip, ridged surface, and chondroderms may help inspire quieter underwater foils that create fewer disturbances for marine animals.

The discovery also ties back to early ichthyosaur research. More than two centuries ago, Mary Anning and her brother Joseph found the first scientifically described Temnodontosaurus on the English coast, a landmark now noted by the Geological Society of London.

Lomax said the new find feels like a full-circle moment that continues the line of surprises sparked by Anning’s original work.

The study is published in Nature.

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