The oldest plesiosaur was a strong swimmer. Plesiosaurs were especially effective swimmer. These long extinct 'paddle saurians' propelled themselves through the World's oceans by employing 'underwater flight' -- similar to sea turtles and penguins. The find comes from the youngest part of the Triassic period and is about 201 million years old. Source 3s
Image from Wikipedia public domain By Dmitry Bogdanov, CC BY-Specific Action: 3.0, Source 5y
Plesiosaurs (Hover)