A robot developed at Southampton University has helped researchers understand and analyse the propulsion method of plesiosaurs, marine reptiles that died out more than 65 million years ago.

Plesiosaurs are unique among vertebrates because they used two near-identical pairs of flippers to propel themselves through the water – whereas other animals, including existing species such as turtles and sea lions, have differently constructed front and back sets, using the front ones mainly for thrust and the back ones for steering. 
However, the propulsion dynamics of the plesiosaur have long been debated, with various theories proposed since the 1950s.

After careful analysis of plesiosaur fossils and X-rays of existing flipper-powered animals, the Southampton group produced 3D printed flippers which were attached to a robotic mechanism able to mimic a range of movement combinations.

A series of water tank experiments, described in Proceedings of the Royal Society B, were then used to shed light on the creature’s swimming style.

The team found that swirling movements in the water created by the front flipper allowed for a major increase in thrust and efficiency by the back flipper (increasing thrust by up to 60 per cent and efficiency up to 40 per cent) strongly suggesting that plesiosaurs would have used all four flippers to propel themselves through the water.

“Short of genetically engineering a plesiosaur, our best available option was to create a robot to show how it might have happened,” said Luke Muscutt, a PhD student in Engineering and the Environment at Southampton. “The results were amazing and indicate why plesiosaurs were such a successful species, retaining four flippers for more than 100 million years.

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Muscutt added that the findings might also have eventual real-world applications in the development of propulsion systems for undersea vehicles.