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Plesiosaurs had some of the most extreme necks to have ever evolved, with some species, such as Albertonectes vanderveldei, boasting 7-metre-long appendages made up of 76 vertebrae. But an ultra-long neck seems difficult for aquatic creatures like plesiosaurs to evolve, as they could hamper the ability to swim, so how did they arise? Big bodies made all the difference, according to a new analysis.

Susana Gutarra Díaz at the University of Bristol, UK, and her colleagues examines the body shapes of plesiosaurs and other marine reptiles through the lens of computational fluid dynamics. Some appear to have been more streamlined, such as the shark-like ichthyosaurs, while plesiosaurs were much more variable in shape and size. “Until now, it was not very clear how this great diversity of shapes and sizes affected the energy demands of swimming in these marine animals,” says Gutarra Díaz.

The researchers found that body size had a major influence on possible shapes for marine reptiles. While the large plimbs and long necks of many plesiosaurs created a significant amount of drag, bigger bodies and larger torsos lowered the energetic cost of moving through water. This is because drag is created by the friction between water and an animal’s skin, and as bodies get larger, the ratio of surface area to mass is reduced.

“Large animals have a greater drag in absolute terms,” says Gutarra Díaz. “But the power they need to invest to move a unit of body mass is smaller.”

While there may be other factors to take into account, this kind of analysis is the best way to explain constraints and body plan limits among extinct organisms, says José O’Gorman at the La Plata Museum in Argentina.

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