A plesiosaur is a member of the sea reptile family and the name Plesiosaurs means “close to lizard”. A plesiosaur is not a dinosaur. Instead it’s a sea reptile.
A plesiosaur has 2 front flippers, 2 back flippers, a short tail, sharp pointy teeth and a very long neck.
They were mostly found in Europe and probably lived about 200 million years ago. Plesiosaurs are found in shallow seas and even freshwater lakes.
They are very fast and they had curved teeth useful for catching fish, but not for chewing or crushing. It’s also likely that they caught and ate the squid-like creatures that existed at the time. Some plesiosaur fossils have stones preserved in the stomach area, showing that they swallowed stones (called gastroliths) to grind up food in their stomachs.
They were able to dive down, but they fed near the surface and had no need to go deep. Being reptiles they had to breathe air, so there would have to be good reason for them to leave the surface for a very long time.
Right now there are no more because they are extinct.