A recent study published in the journal PeerJ has found that plesiosaurs – a group of saurians that lived about 210 million years ago in the late Triassic and became extinct at the end of the Cretaceous – adapted to life underwater in a unique way: their front and hind limbs evolved to form four uniform, wing-like flippers which they twisted in order to swim efficiently.
Plesiosaurs belonged to group of saurians called Sauropterygia (“paddle lizards”) that, over the course of evolution, re-adapted to living in the oceans. While most of them had extremely long necks and small heads, evidence of some predatory specimens with short necks and huge skulls has also emerged. However, what made plesiosaurs so unusual were their four uniform wing-like flippers.
“Having the front legs transformed into wing-like flippers is relatively common in evolution, for instance in sea turtles. Never again, however, did the hind legs evolve into an almost identical-looking airfoil-like wing,” said study lead author Anna Krahl, a paleontologist at the University of Tübingen.
For over 120 years, experts in vertebrate paleontology have wondered how plesiosaurs might have swum with these four flippers. While some scientists argued that they rowed like freshwater turtles or ducks, and others that they flew underwater like sea turtles or penguins, some researchers believed that they actually combined underwater flight and rowing as modern-day sea lions or pig-nosed turtles do.
By analyzing the body structure of several plesiosaur fossils, Dr. Krahl has found that these creatures had stiffened elbows, knees, hands, and ankle joints, but functioning hip, shoulder, and finger joints. “Analysis comparing them to modern-day sea turtles, and based on what is known about their swimming process, indicated that plesiosaurs were probably not able to rotate their flippers as much as would be necessary for rowing,” she explained.
However, plesiosaurs could ultimately twist their flippers to place them in a hydrodynamically favorable position and produce lift without rotating their upper arms and thighs. “This could work by means of twisting the flippers around their long axis,” Dr. Krahl said. “Other vertebrates, such as the leatherback turtle, have also been shown to use this movement to generate propulsion through lift.” Twisting most probably involved bending their first finger far downward and their last finger far upward, with the remaining fingers bridging these extreme positions, so that the flipper tip was almost vertical without requiring any real rotation in the shoulders or wrists.
By calculating the forces for the individual muscles that generated the upstroke and downstroke, Dr. Krahl and her colleagues discovered that the downstroke of both pairs of flippers was more powerful than the upstroke – similarly with today’s sea turtles, and differently from other aquatic creatures such as penguins or whales.
“Plesiosaurs adapted to life in water in a very different way than whales, for example. This unique path of evolution exemplifies the importance of paleontological research because it’s the only way we can appreciate the full range of what evolution can bring about,” Dr. Krahl concluded.