The extremely long-necked body shape of plesiosaurs was associated with lower energy costs when swimming than previously thought, a study shows. Apparently, the effect of the large body of the marine iguanas offset the disadvantage of the increased water resistance due to the body shape. That’s why the long necks of some plesiosaurs grew over the course of their evolutionary history in combination with body dimensions, the scientists say.
When T. rex, titanosaurs and Co. trudged across the earth, the seas also had bizarre creatures to offer: Up to the end of the Cretaceous period, reptiles that had developed from land-living ancestors were roaming around there – similar to today’s marine mammals. But while ichthyosaurs evolved streamlined body shapes like fish or modern-day dolphins, plesiosaurs evolved structures that have no equivalents today. The small head of these reptiles, which can be up to 15 meters long, sat at the end of an elongated neck, which was particularly pronounced in the subgroup of elasmosaurs. In some species it was more than twice as long as the trunk. These snake necks probably helped them catch fast-swimming fish while using their four fins to propel themselves through the water in a “flying” manner.
Until now, it was assumed that the unusual body shape came at a considerable price: it was obvious that hydrodynamic effects when moving in the water slowed down the animals comparatively strongly when swimming. Among other things, the long neck could have caused resistance due to its large surface and turbulence in the water. It was therefore assumed that the plesiosaurs, in contrast to the hydrodynamically optimized ichthyosaurs, were less energy-efficient in the water. However, the extent to which shape and size actually influence the energy requirements of plesiosaurs when they swim has remained unclear.
Paleontological flow simulations
An interdisciplinary research team from the University of Bristol has now devoted itself to this topic. “In order to clarify possible effects, we created various 3D models and performed flow simulations on various representatives of plesiosaurs, ichthyosaurs and whales,” says lead author Susana Gutarra Díaz. “Although these experiments were carried out on the computer, they correspond to experiments in a water tank,” explains the paleobiologist.
As the team reports, their flow simulations initially confirmed that the unusual shape of the long-necked plesiosaurs actually caused a comparatively high braking effect when swimming. But then it became clear that this factor plays an increasingly smaller role with increasing body size: “It was shown that the plesiosaurs had slightly more water resistance than ichthyosaurs or whales of the same mass, but ultimately these differences were relatively small,” says co-author Colin Palmer. “Because if you include the size, the shape differences become less important,” says the researcher.
Through their simulations, the researchers were also able to specifically show the role of the neck: “We were particularly interested in the very long necks of elasmosaurs and therefore created hypothetical 3D models of plesiosaurs with necks of different lengths,” says Gutarra Díaz. “The simulations of these models showed that at a certain point, the neck creates additional drag that makes swimming particularly costly. The limit of the still comparatively favorable neck length is about twice the length of the animal’s trunk,” reports the researcher.
Hardly slowed down long necks
The researchers then compared this result with information from the fossil record: “When we examined the sizes and proportions of many different representatives of plesiosaurs, it turned out that most of them had necks below this threshold for high resistance – i.e. in the range within which neck length does not unduly affect resistance,” says co-author Benjamin Moon. The exception, however, were the elasmosaurs. “It was particularly interesting that we were able to show that these particularly long-necked plesiosaurs had also developed very large bodies. Apparently, they compensated for the increased water resistance,” says the marine reptile expert.
The paleobiological information also revealed that the proportions of elasmosaur representatives changed very rapidly over the course of their evolutionary history. “This confirms that long necks were advantageous for elasmosaurs when hunting, but they were only able to take advantage of this adaptation by also becoming large enough to offset the cost of the increased drag on their bodies,” says co-author Tom Stubbs. His colleague Mike Benton concludes, “The maximum neck lengths appear to weigh the benefits of hunting against the cost of growing and maintaining such a long neck. In other words, the necks of these extraordinary creatures have evolved in balance with their overall body size to minimize the effect of water drag.”
Source: University of Bristol, professional article: Commun Biol, doi: 10.1038/s42003-022-03322-y