Fossils preserve both skin and scales from an ancient sea monster

(Trilobites)

With serpentine necks, flippers and a mouth full of needle-sharp teeth, plesiosaurs have captured imaginations since paleontologists uncovered the first specimen more than two centuries ago. Their skeletal anatomy is well documented, but their external appearance has largely remained a mystery.

Now researchers have conducted the first detailed analysis of plesiosaur soft tissue, offering a more complete look at what these real-life sea monsters might have looked like when they lived from 215 million to 66 million years ago.

Published Feb 6 in Current Biology, the findings suggest that some plesiosaurs had humanlike skin on their tail regions and fishy scales on their flippers, similar to the features of some living sea turtle species. The research highlights an evolutionary detour that runs counter to other ancient marine reptiles like ichthyosaurs and mosasaurs, which evolved away from scales in favor of skin, or much smaller scales, to allow them to move more efficiently through their marine habitats.

"These are iconic animals, and the way we reconstruct them hasn't changed for nearly 200 years, so this is a big update," said Miguel Marx, a doctoral student at Lund University in Sweden and lead author of the paper.

"It changes our perspective on their evolutionary history and how they adapted to life in the ocean."

Marx and colleagues analysed three soft-tissue skin samples, each about the size of a fingernail, from a flipper and the tail of a 183 million-year-old long-necked plesiosaur specimen. The species is to be named in a future peer-reviewed paper. But the samples came from the Posidonia Shale in Germany, where the ocean chemistry preserved soft tissues. That left it frozen in time. Some of the tissue remains were so flawlessly fossilised that researchers could see skin cell nuclei under the microscope.

While it's difficult to know for sure how extinct animals would have maneuvered through their environments, Marx said that the scales observed on the plesiosaur had probably stiffened the trailing edge of the flipper, allowing for enhanced propulsion through the water, another feature shared with today's sea turtles.

Plesiosaurs may also have used scales on their flippers for traction and protection as they sifted through sand and vegetation on the ocean floor for food. Previous research suggests that preserved marine trackways found near Ancona, Italy, came from plesiosaurs, another indication that they might have spent time feeding at the bottom of the ocean.

F. Robin O'Keefe, a vertebrate paleontology at Marshall University in Huntington, West Virginia, who was not involved in the study, said that the findings were "remarkable" in their use of technology to analyse plesiosaur soft tissue, even showing that the skin of a human was similar to that of a plesiosaur.

However, O'Keefe is skeptical that the scales provide enough evidence to show that plesiosaurs walked on the ocean floor, because it's difficult to infer such a behavior from fossils. "I don't see this animal spending a lot of time on the bottom," O'Keefe said.

However, he did agree with Marx that the scales provided plesiosaurs with extra propulsion. He said that instead of bottom-feeding, the animal was a speedy hunter with a high metabolic rate.

"You don't need to go to all the trouble of having a hyper-efficient wing if you're going to be sitting on the bottom all the time," O'Keefe said. "This was an active predator that was really cruising around."