Scientists have discovered the oldest plesiosaur skeleton, an underwater reptile that swam in Earth’s oceans 201 million years ago.

Described in the journal Science Advances, researchers from the University of Bonn studied a fossil that was found in a clay pit back in 2013 in Germany, which had been acquired by a private collector.

The creature dates back to the Triassic period 251 to 199 million years ago. It is the only plesiosaur skeleton that has ever been found from this period.

They were able to reconstruct the skeleton, finding that this creature was 2.37 meters (7.8 feet) long, although part of the neck was lost in quarrying. This makes it a relatively small plesiosaur, and has been given the name Rhaeticosaurus mertensi.

The creature was found to be a strong swimmer, gliding along with underwater wings rather than pushing water out of the way with paddles. Their small head was on a long streamlined neck, and their bodies contained strong muscles to keep the wings moving.

“Compared to the other marine reptiles, the tail was short because it was only used for steering,” said paleontologist Professor Martin Sander from the University of Bonn in a statement. “This evolutionary design was very successful, but curiously it did not evolve again after the extinction of the plesiosaurs.”

Professor Sander added that when he heard about the discovery, he “could not believe that there was a plesiosaur from the Triassic, given that these animals had been studied by paleontologist for nearly 300 years, and never was there one older than Jurassic.”

Using computed tomography, the researchers were able to look inside the bones of the animals. They also cut thin sections for microscopic study.

Based on growth marks in the bones, they concluded that Rhaeticosaurus was a fast-growing youngster. It’s thought these animals grew extremely fast before reaching sexual maturity.

This is seen as clear evidence that plesiosaurs were warm-blooded, as they would have needed to be able to regulate their body temperature to move into cooler parts of the ocean.

As a result, they became extremely successful and widespread, until they were wiped out along with the dinosaurs at the end of the Cretaceous period 66 million years ago, when a meteorite struck the planet. Together with volcanic eruptions, this likely collapsed their ecosystem.