Large bodies helped extinct long-necked marine reptiles swim, new study finds

Scientists at the University of Bristol have found that body size is more important than body shape in determining aquatic animals’ energy savings when swimming.

This study, published today in biology of communications, shows that large bodies help overcome excess resistance caused by extreme physiques, debunking the long-standing idea that there is an optimal body shape for low resistance.

An important finding from this study is that the large necks of extinct elasmosaurs added additional resistance, but this was offset by the evolution of large bodies.

Tetrapods, or “four-legged vertebrates,” have returned to the oceans repeatedly over the past 250 million years, and they come in all shapes and sizes, from streamlined modern whales over 25 meters long to extinct plesiosaurs with four fins and unusually long necks. and even extinct fish-like ichthyosaurs.

Dolphins and ichthyosaurs have similar body shapes adapted to move quickly through water with little drag or resistance. On the other hand, the plesiosaurs that lived side by side with the ichthyosaurs in the Mesozoic era had completely different bodies. Their four huge flippers, with which they flew under water, and the different length of the neck are unparalleled among living animals. Some elasmosaurs were truly extreme proportions, with necks up to 20 feet (6 meters) long. These necks probably helped them catch fast moving fish, but were also thought to slow them down.

Until now, it has not been clear how shape and size affect the energy requirement for swimming in these diverse marine animals. Paleobiologist Dr Susana Gutarra Diaz of the Bristol School of Geosciences and the National Historic Site London, who led the study, explained: “To test our hypotheses, we created various 3D models and performed computer simulations of the flow of plesiosaurs, ichthyosaurs and cetaceans. . These experiments are carried out on a computer, but they look like experiments with water tanks. »

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

Dr. Gutarra Diaz said: “We were also particularly interested in the necks of elasmosaurs, so we created hypothetical 3D models of plesiosaurs with different neck lengths. Simulations of these patterns show that after a certain point, the neck adds additional drag, which can make swimming potentially expensive. This “optimal” neck limit is about twice the length of the animal’s torso.

Dr. Benjamin Moon, another collaborator and marine reptile expert, continued: “When we examined a large sample of plesiosaurs modeled from very well-preserved fossils in their actual size, most plesiosaurs had necks below this high resistance threshold, in which neck can lengthen or shorten without increasing resistance. But more interestingly, we showed that plesiosaurs with extremely long necks also had very large torsos, and this compensated for the extra drag! »

Dr. Tom Stubbs, another co-author, concluded: “This study shows that, contrary to popular belief, very long-necked plesiosaurs were not necessarily slower swimmers than ichthyosaurs and whales, due in part to their large bodies. We found that in Elasmosaurs, the proportions of the neck changed very quickly. This confirms that long necks were beneficial to Elasmosaurus when hunting, but they could not use this adaptation until they had grown enough to offset the cost of strong body resistance.

Professor Mike Benton, who is also involved in the study, commented: “Our research shows that large aquatic animals can afford to have crazy shapes like elasmosaurs. But there are limits: the size of the body cannot become indefinitely large, because there are certain restrictions on very large sizes. The maximum neck length we see seems to balance the benefits of hunting against the cost of growing and maintaining such a long neck. In other words, the necks of these unusual creatures have evolved according to overall body size to keep friction to a minimum. »

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