Among long-necked plesiosaurus there were animals with diamond-shaped tail fin, similar to one, which flying pterosaurs rhamphorhynchus had, for example, seeleyosaurus. They lived on Earth in the beginning of Jurassic period of Mesozoic era; animals had large paws-fins, with which were able to perform alternating or simultaneous swings, achieving high acceleration and maneuverability. Seeleyosaurus ate fish, birds and other small marine reptiles, wherein they themselves were vulnerable to their short-necked relatives pliosaurus with larger jaws, teeth and mass; they had to show agility, intelligence and maneuverability to not become their breakfast. Currently there is no precise data, but maybe many plesiosaurus had the same diamond-shaped tail fin, there’ll be ability to say this for sure, when more fossils with tail skin impressions will be found. At these days seeleyosaurus are known only from 1 almost complete skeleton, which was found in Germany, it contains impressions of this steering adaptation of the animals.