How plesiosaurs swam under water

Anna Krahl (front) and Ulrich Witzel used a model made of bone copies and material from the hardware store to reconstruct the muscles. This analog model consists of casts of the front and rear fin, wooden slats, chandelier clamps, eyelets and ropes.
Credit: Ruhr University Bochum

The plesiosaurs are characterized by four uniform fins. Whether they rowed or flew under water could be reconstructed thanks to the combination of paleontological and engineering methods.

Plesiosaurs, which lived around 210 million years ago, have adapted in a unique way to life under water: their front and rear legs have developed into four uniform, wing-like fins in the course of evolution. How they could get on with it in the water, Dr. Anna Krahl worked out in her dissertation supervised at the Ruhr University Bochum and the Rheinische Friedrich-Wilhelms-Universität Bonn. Among other things, by using the finite element method, which is widespread in engineering, it was able to show that the fins had to be twisted in order to advance. Using bones, models and muscle reconstructions, she was able to reconstruct the movement. She reports in the PeerJ journal from 3. June 2022.

Plesiosaurs belong to a group of dinosaurs, the Sauropterygia or paddle lizards, who have adapted to a life in the sea again. They developed in the late Triassic 210 million years ago, lived at the same time as the dinosaurs and died out at the end of the Cretaceous period. Plesiosaurs are characterized by an often extremely elongated neck with a small head - the Elas mosaic animals even have the longest neck of all vertebrates. But there were also large predatory shapes with a rather short neck and huge skulls. In all plesiosaurs, the neck sits on a teardrop-shaped, hydrodynamically well-adapted body with a very shortened tail.

Researchers have been puzzling over the past 120 years about how plesiosaurs swam

The second characteristic that makes the plesiosaurs so unusual is four uniform wing-like fins. “The fact that the front legs are converted into wing-like fins is more common in evolution, for example in sea turtles. However, the hind legs were never converted into an almost equally looking wing-like wing,” explains Anna Krahl, whose doctoral thesis by Prof. Dr. P. Martin Sander (Bonn) and Prof. Dr. Ulrich Witzel (Bochum) was looked after. Sea turtles and penguins, for example, have oar feet. For over 120 years, vertebrate paleontology researchers have been puzzling how plesiosaurs could have swum with these four wings. They rowed like freshwater turtles or ducks? They flew under water like sea turtles and penguins? Or combined underwater flight and rowing like today's sea lions or the Papua soft turtle? It is also unclear whether the front and hind fins were struck in the same or counter clock or shifted in phase.

Anna Krahl has been involved in the physique of the plesiosaurs for several years. She examined the bones of the shoulder and pelvic belt, the front and hind fins, and the shoulder joint surfaces of the plesiosaur Cryptoclidus eurymerus from the Middle Jurassic period (around 160 million years ago) on a complete skeleton exhibited in the Goldfuß Museum of the University of Bonn. Plesiosaurs have stiffened elbow and knee as well as hand and foot joints, but functioning shoulder, hip and finger joints. "The analysis compared to today's sea turtles and based on what you know about their swimming process has shown that plesiosaurs were probably unable to turn their fins as much as would be necessary for rowing," summarizes Krahl one of them Preparatory work together. Rowing is mainly a forward and backward movement, in which the water resistance is used to move forward. The preferred direction of flow movement for plesiosaurs, on the other hand, was a surcharge and tee, as underwater aviators used to generate propulsion.

A twist was possible and necessary

The question remained how plesiosaurs can ultimately twist the fins so that they put them in a hydrodynamically favorable position and produce buoyancy without turning the upper arm and thigh around the longitudinal axis. "This could work by twisting the fins around the longitudinal axis," says Anna Krahl. “Other vertebrates such as the leatherback turtle have also been shown to use this movement to generate propulsion through buoyancy. "When twisting, for example, the first finger is bent far down and the last finger is bent far up. The remaining fingers bridge these extreme positions, so that the tip of the fin is almost vertical without the need for a real rotation in the shoulder or wrist.

A reconstruction of the muscles of the front and hind fins for Cryptoclidus with the help of reptiles living today showed that the dinosaurs could actively enable such fin twisting. In addition to classic models, the researchers also produced computer tomographs of the upper arm and thigh bones Cryptoclidus and created virtual 3D models from it. "These digital models were the basis for calculating forces using a method that we borrowed from engineering: the finite element method, or FE for short," explains Anna Krahl. In a FE computer program, in which natural loads of components, but also prostheses can be simulated, all muscles and their starting angles on the upper arm and thigh bones were virtually reproduced. Based on muscle strength assumptions from a comparable study of sea turtles, the team was able to calculate and visualize the loads on the respective bones.

Twisting of the fins can be proven indirectly

The muscles from the previous examinations were stretched in this model to better understand their geometry. In the model, fin positions could also be changed to measure how much muscles are elongated or shortened.
Credit: Ruhr University Bochum

During a movement cycle, the extremity bones are stressed by pressure, tension, bending stress and torsion. "The FE analysis showed that the upper arm and thigh bones in the fins are mainly stressed by pressure and to a much lesser extent by tension," says Anna Krahl. “This means that the plesiosaur has built up its bones as gently as possible. "This natural state can only be maintained if the muscles for twisting the fins and the muscles that grip the bones are included. "This enables us to indirectly prove that plesiosaurs have twisted their fins in order to swim efficiently," summarizes Anna Krahl.

In addition, the team was able to calculate forces for the individual muscles that generated the up and down stroke. It turned out that the tee of both pairs of fins was more powerful than the serve. This is comparable to today's sea turtles and unlike today's penguins, which advance as far as the tee with the serve as with the tee. "The adaptation of the plesiosaurs to life in the water has been very different from that of the whales, for example," says Anna Krahl, who now works at the Eberhard Karls University in Tübingen. “This unique way of evolution makes it an example of the importance of paleontological research because it is only through them that we can measure the full range of what evolution can produce."

The work was funded by the German Research Foundation, funding code: WI1389 / 8-1.

Source/Credit: Ruhr University Bochum | Meike Drießen

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