Plesiosaurs, living about 210 million years ago, adapted to underwater life in a unique way: their front and hind legs evolved during evolution, forming four uniform, wing-like flippers. In his dissertation at the Ruhr-University Bochum and the University of Bonn, Dr. Anna Krahl 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 turn the swimming fans to be able to travel forward. She was able to reconstruct the movement sequence with the help of bones, models and reconstructions of the muscles.
Plesiosaurs belong to a group of sauris called Sauropterygia, or paddle lizards, which adapted to live in the oceans. They evolved at the end of the Triassic 210 million years ago, lived at the same time as the dinosaurs and became extinct at the end of the Cretaceous. Plesiosaurs are characterized by an often extremely elongated neck with a small head – the elasmosaurs even have the longest neck of all vertebrates. But there were also large forms of predation 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 markedly shortened tail.
For 120 years, scientists have wondered how plesiosaurs swam
The other characteristic that makes plesiosaurs so unusual is their four uniform wing-like flippers. “Getting the front legs transformed into wing-like flippers is relatively common in evolution, for example in sea turtles. Never again, however, have the hind legs developed into an almost identical-looking bearing plane-like wing,” explains Anna Krahl, whose doctoral dissertation was supervised by Professor P. Martin Sander (Bonn) and Professor Ulrich Witzel (Bochum). Sea turtles and penguins, for example, have flippers. For more than 120 years, researchers in vertebrate paleontology have wondered how plesiosaurs may have swum 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 flying and rowing like modern sea lions or the pig-nosed turtle? It is also unclear whether the front and rear flippers fluttered in unison, in opposition or out of phase.
Anna Krahl has for several years studied the body structure of plesiosaurs. She examined the legs of the shoulder and pelvic girdle, the anterior and posterior swimming feet and the shoulder joint surfaces of the plesiosaur Cryptoclidus eurymerus from the Middle Jurassic period (about 160 million years ago) on a complete skeleton on display in the Goldfuß Museum at the University of Bonn. Plesiosaurs have stiffened elbow, knee, hand and ankle joints, but functioning shoulder, hip and finger joints. “Analysis that compared them to today’s sea turtles, and based on what is known about their swimming process, indicated that plesiosaurs probably could not rotate their swimming feet as much as would be necessary for rowing,” concludes Krahl, summarizing one of her preliminary papers. Rowing is primarily a back and forth movement that uses water resistance to move forward. The preferred direction of pinball motion in plesiosaurs, on the other hand, was up-and-down, which was used by underwater pilots to generate propulsion.
The question remained how plesiosaurs could eventually turn their flippers 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 twisting the flippers around their long shoulder,” says Anna Krahl. “Other vertebrates, such as the leatherback turtle, have also been shown to use this movement to generate propulsion by lifting.” Rotation means, for example, bending the first finger far down and the last finger far up. The remaining fingers bridge these extreme positions so that the pinhole tip is almost vertical without the need for proper rotation of the shoulder or wrist.
A reconstruction of the muscles on the front and back flaps for Cryptoclidus using reptiles that live today showed that plesiosaurs could actively enable such pinball rotation. In addition to classic models, the researchers also performed computed tomography of the humerus and femur Cryptoclidus and used them to create virtual 3D models. “These digital models were the basis for calculating the forces with 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 practically reproduced in a FE computer program that can simulate physiological functional loads, for example on structural components but also on prostheses. Based on muscle power assumptions from a similar study on sea turtles, the team was able to calculate and visualize the load on each leg.
Rotation of flippers can be proven indirectly
During a movement cycle, the leg bones are loaded with compression, tension, bending and twisting. “The FE analyzes showed that the humerus and femur in the flippers are functionally loaded mainly by compression and to a much lesser extent by tensile stresses,” explains Anna Krahl. “This means that the plesiosaur built its legs by using as little material as necessary.” This natural state can only be maintained if the muscles that rotate the flippers and the muscles that wrap around the leg are included. “We can therefore indirectly prove that plesiosaurs twisted their swimming feet to swim efficiently,” concludes Anna Krahl.
The team was also able to calculate forces for the individual muscles that generated the impact. It turned out, for example, that the downward stroke for both pairs of swimming phones was more powerful than the upward stroke. This is comparable to our sea turtles today and differs from today’s penguins, which move forward the same distance with the upward stroke as with the downward stroke. “Plesiosaurs adapted to life in water in a completely different way than whales, for example,” says Anna Krahl, who now works at the Eberhard Karls University in Tübingen, Germany. “This unique path of evolution exemplifies the importance of paleontological research because it is the only way we can appreciate the full spectrum of what evolution can accomplish.”
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