Plesiosaurs, living about 210 million years ago, adapted to life underwater in a unique way: Their fore and hind legs evolved into four uniform, wing-like fins over the course of evolution. In her thesis, supervised at the Ruhr-Universität Bochum and the University of Bonn, Dr. Anna Krahl examined how they used these to move through the water. Partly by using the finite element method, which is widely used in engineering, she was able to show that it was necessary to rotate the fins to move forward. She was able to reconstruct the movement sequence using bones, models and reconstructions of the muscles.
Plesiosaurs belong to a group of saurs called Sauropterygia, or paddle lizards, that re-adapted to life in the oceans. They evolved 210 million years ago in the late Triassic, lived at the same time as the dinosaurs, and died out at the end of the Cretaceous. Plesiosaurs are characterized by an often extremely elongated neck with a small head – in fact, the elasmosaurs have the longest neck of all vertebrates. But there were also large predatory forms with a rather short neck and huge skulls. In all plesiosaurs, the neck is attached to a teardrop-shaped, hydrodynamically well-adapted body with a distinctly shortened tail.
Researchers have wondered how plesiosaurs swam for 120 years
The second feature that makes plesiosaurs so unusual is their four uniform wing-like fins. “Transforming the front legs into wing-like fins is relatively common in evolution, for example in sea turtles. However, never again have the hind legs evolved into an almost identical-looking wing-like wing,” explains Anna Krahl, whose dissertation was supervised by Prof. Dr. P. Martin Sander (Bonn) and prof.dr. Ulrich Witzel (Bochum). For example, sea turtles and penguins have webbed feet. For more than 120 years, researchers in vertebrate paleontology have wondered how plesiosaurs could swim with these four wings. Did they row like freshwater turtles or ducks? Did they fly underwater like sea turtles and penguins? Or did they combine underwater flight and rowing like modern sea lions or the pig-nosed turtle? It is also unclear whether the front and rear fins flipped simultaneously, in opposition, or out of phase.
Anna Krahl has been studying the body structure of plesiosaurs for several years now. She examined the bones of the shoulder and pelvic girdle, the front and rear fins, and the shoulder joints of the Plesiosaur. Cryptoclidus eurymerus from the Middle Jurassic era (about 160 million years ago) on a complete skeleton on display in the Goldfuß Museum of the University of Bonn. Plesiosaurs have stiffened elbow, knee, hand, and ankle joints, but functioning shoulder, hip, and finger joints. “An analysis comparing them to modern sea turtles, and based on what is known about their swimming process, indicated that plesiosaurs were probably unable to rotate their fins as much as would be necessary for rowing,” concludes Krahl, a researcher. of her in summary. preliminary papers. Rowing is primarily a back-and-forth motion that uses water resistance to move forward. In contrast, the preferred direction of flipper motion in plesiosaurs was up and down, as used by underwater kites to generate propulsion.
The question remained how plesiosaurs could eventually rotate their fins to place them in a hydrodynamically favorable position and produce lift without rotating the upper arm and thigh around the longitudinal axis. “This could work by rotating the flippers on their long axis,” says Anna Krahl. “Other vertebrates, such as the leatherback turtle, have also been shown to use this motion to generate propulsion through lift.” When turning, for example, you bend the first finger way down and the last finger way up. The remaining fingers bridge these extreme positions so that the flipper tip is nearly vertical without requiring any real rotation in the shoulder or wrist.
A reconstruction of the muscles of the fore and hind fins cryptoclidus the use of reptiles alive today showed that plesiosaurs can actively facilitate such flipper turns. In addition to classical models, the researchers also made computed tomographies of the humerus and femur of cryptoclidus and used them to create virtual 3D models. “These digital models formed the basis for calculating the forces using a method we borrowed from engineering: the finite element method, or FE,” explains Anna Krahl. All muscles and their attachment angles on the humerus and femur were virtually reproduced in an FE computer program that can simulate physiological functional loads, for example on structural parts but also on prostheses. Using muscle strength assumptions from a similar study on sea turtles, the team was able to calculate and visualize the load on each bone.
Spinning the flippers can be proved indirectly
During a cycle of movement, the limbs are loaded by compression, tension, bending and torsion. “The FE analyzes showed that functionally the humerus and femur in the flippers are mainly loaded by compression and to a much lesser extent by tensile stress,” explains Anna Krahl. “This means that the Plesiosaurus built its bones by using as little material as needed.” This natural state can only be maintained if the muscles that twist the fins and the muscles that wrap around the bone are included. “So we can prove indirectly that plesiosaurs twisted their fins to swim efficiently,” Anna Krahl summarizes.
The team was also able to calculate forces for the individual muscles that caused the upstroke and downstroke. It turned out that the down stroke of both pairs of flippers was more powerful than the up stroke. This is similar to our sea turtles today and different from today’s penguins, who move the same distance on the upstroke as they do on the downstroke. “Plesiosaurs have adapted to life in water in a very different way than whales, for example,” notes Anna Krahl, who now works at Eberhard Karls University in Tübingen, Germany. “This unique path of evolution illustrates the importance of paleontological research, as it is the only way to appreciate the full range of what evolution can bring about.”
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