Plesiosaurs with long necks needed large bodies to minimize friction

Some groups of marine animals have achieved enormous body sizes in the past and, although most of them are now extinct, scientists still puzzle over how their extreme body shapes and sizes allowed them to move efficiently in their aquatic environment. Among the largest, only cetaceans (whales) have survived into the modern era, while the ancient reptiles known as ichthyosaurs and plesiosaurs have become extinct.

Dr. Susana Gutarra Díaz from the University of Bristol and her colleagues have conducted a recent study examining the body shapes of these large marine animals using models and computational fluid dynamics. Some of the animals show more streamlined body shapes, such as modern whales and the dolphin-like ichthyosaurs, while the plesiosaurs had massive paired flippers.and necks that could measure as much as half the total body length. 

“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,” said Dr. Gutarra Díaz. “To test our hypotheses, we created various 3D models and performed computer flow simulations of plesiosaurs, ichthyosaurs and cetaceans. These experiments are performed on the computer, but they are like water tank experiments.”

Tetrapods, or “four-limbed vertebrates,” have repeatedly returned to the oceans over the last 250 million years, and their bodies have become adapted to the marine environment in different ways. For example, dolphins and ichthyosaurs have similar body shapes, adapted for reducing resistance or drag as they swim rapidly through the water. By contrast, plesiosaurs that lived side-by-side with the ichthyosaurs in the Mesozoic Era, propelled themselves underwater using giant flippers, and some had extremely long necks that hardly look helpful for swimming at all. These necks likely helped them to snap up quick-moving fish, but were also believed to make them slower.

The results of the analysis, published in the journal Communications Biology, showed that body size had a major influence on possible shapes for marine reptiles. In fact, body size was more important than body shape in determining the energy economy of swimming for aquatic animals. 

While the large limbs 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 actually reduced.

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

“We showed that although plesiosaurs did experience more drag than ichthyosaurs or whales of equal mass because of their unique body shape, these differences were relatively minor,” explained Dr Colin Palmer, an engineer involved in the project. “We found that, when size is taken into account, the differences between groups became much less than the shape differences. We also show that the ratio of body length to diameter, which is widely used to classify these aquatic animals as more or less efficient, is not a good indicator of low drag.”

In this study, the researchers were particularly interested in one family of plesiosaurs known as the elasmosaurs. These were plesiosaurs with the longest necks of all, measuring up to 20 feet (6 metres) in some species, and having over 70 cervical (neck) vertebrae. In these species, the neck was almost half of the total body length and would have been too long and too heavy to hold above the surface of the water. 

“We were also particularly interested in the necks of elasmosaurs and so we created hypothetical 3D models of plesiosaurs with various lengths of necks,” said Dr. Gutarra Díaz. “Simulations of these models reveal that, past a certain point, the neck adds extra drag, which potentially would make swimming costly. This ‘optimal’ neck limit lies around twice the length of the trunk of the animal.”

“When we examined a large sample of plesiosaurs modelled on really well-preserved fossils at their real sizes, it turns out that most plesiosaurs had necks below this high-drag threshold, within which the neck can get longer or shorter without increasing drag,” said Dr. Benjamin Moon. “But more interestingly, we showed that plesiosaurs with extremely long necks also had evolved very large torsos, and this compensated for the extra drag!”

According to Dr. Tom Stubbs, the study shows that in contrast with prevailing popular knowledge, very long-necked plesiosaurs were not necessarily slower swimmers than ichthyosaurs and whales, and this is in part thanks to their large bodies. “We found that in elasmosaurs, neck proportions changed really fast. This confirms that long necks were advantageous for elasmosaurs in hunting, but they could not exploit this adaptation until they became large enough to offset the cost of high drag on their bodies.”

“Our research suggests that large aquatic animals can afford to have crazy shapes, as in the elasmosaurs,” said Professor Mike Benton. “But there are limits: body sizes cannot get indefinitely large, as there are some constraints to very large sizes as well. The maximum neck lengths we observe, seem to balance benefits in hunting versus the costs of growing and maintaining such a long neck. In other words, the necks of these extraordinary creatures evolved in balance with the overall body size to keep friction to a minimum.”

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By Alison Bosman, Earth.com Staff Writer