Plesiosaurus in the ocean being eaten

‘Robotic plesiosaur’ created to study extinct biological propulsion

Image credit: Robert Nicholls/University of Southampton

A PhD student at the University of Southampton created a basic robotic form of the extinct sea monster in order to study its unique four-flippered propulsion technique, which could prove useful in designing stealthy, manoeuvrable submarines.

The carnivorous, sea-dwelling dinosaurs were first unearthed whole by English palaeontologist Mary Anning in 1823. Since then, further fossil discoveries have confirmed that plesiosaurs grew to almost 15m long and were equipped with a set of four powerful flippers, the front and back pairs identical.

This is unique in nature: while other creatures such as turtles and sea lions also have four flippers, they use their front flippers for thrust and their back flippers for steering.

“There has been no consensus […] on how plesiosaurs actually used their flippers,” said Luke Muscutt, the University of Southampton PhD student who led the project. “Many thought they used all four, but exactly how was a mystery. How did the front and rear flippers move in relation to each other? Did they all flap in sync or was it something else?”

Muscutt and his colleagues at the University of Southampton and the University of Bristol brought together palaeontology, robotics, fluid mechanics and biology in order to investigate how plesiosaurs could have propelled themselves through the water 200 million years ago.

Only a limited amount of information about the structure and motion of the flippers could be extracted from the two plesiosaur fossils available to them, so they looked to extant animals for further guidance. The X-rays of penguin, sea lion and turtle flippers provided enough data to estimate the amount of bone and soft tissue in the plesiosaur flippers.

Luke Muscutt with robotic plesiosaur

Luke Muscutt/University of Southampton

Image credit: Luke Muscutt/University of Southampton

The researchers 3D printed two flippers – a front and a back flipper – in rigid plastic and submerged them in a water tank to investigate swimming technique. They found that the alternating upward and downward strokes of the front flipper produces a repeating pattern of vortices swirling in opposite directions: a “vortex street”. This gives a major boost to the performance of the back flipper as it “weaves” between these vortices.

The harmonious flapping of the robotic plesiosaur’s flippers is comparable to that of the motion of a dragonfly’s four wings, which propel the creature through the air with great efficiency and dexterity.

When the back flipper lags behind the front flipper slightly, the coordinated motions increase thrust by 60 per cent and efficiency by 40 per cent in the back flipper. This powerful and energy-efficiency mode of propulsion could, the researchers say, have contributed to plesiosaurs’ evolutionary success: the creatures populated oceans in every corner of the world before their extinction 66 million years ago.

The researchers suggest that this biological propulsion system has potential applications in creating new flipper-driven submarines.

“Ideally, if you want to be stealthy or if you want high manoeuvrability, you want to use flippers over a propeller,” Muscutt told E&T. “One set of flippers isn’t that great really, but if you’ve got another set of flippers behind the first, suddenly you’ve got a system that’s much more efficient than the flippers on their own.”

“It’d be less invasive, so instead of creating a lot of underwater noise it may be able to swim more quietly, disturbing the sediment less […] it wouldn’t necessarily be that quick, but it would have manoeuvrability and stealth.”

These qualities could, Muscutt says, make a plesiosaur-inspired propulsion system ideal for small surveillance or ecological research submarines. They could also replace the static fins which are already attached to deep-sea submarines, in order to help them manoeuvre at the bottom of the ocean.

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