The oldest plesiosaur was a strong swimmer
(PHYSorg.com Archaeology & Fossils)

Plesiosaurs were especially effective swimmers. These long extinct `paddle saurians` propelled themselves through the oceans by employing `underwater flight`-similar to sea turtles and penguins. Paleontologist from the University of Bonn, Germany, together with colleagues from Japan and France, now describe the oldest plesiosaur in the journal Science Advances. The fossil comes from the earliest part of the Triassic period and is about 201 million years old.