Liopleurodon, a typical Pliosaur (Episode 5.2)

Plesiosaurs were a type of carnivorous sea reptiles. First plesiosaurs appeared in Triassic and last died at the end of Cretaceous. Like pterosaurs and mosasaurs, plesiosaurs lived in same time with dinosaurs, but weren't dinosaurs although. The order Plesiosauria was represented by long-necked Plesiosauroidea and short-neck Pliosauroidea.

In Primeval

The Lost Island

Liopleurodon, a huge Pliosaur came trough an anomaly in the Spaghetti Junction on Guns Island. It's attack trawler in the Atlantic Ocean. It's last encountered by ARC team and died from the cold conditions of the Atlantic winter.
Loch Nees Monster

Loch Ness Monster - a long-necked Plesiosaur?

Primeval Evolved Week 1

In Primeval Evolved Sarah Page showed document featured information about Loch Ness Monster which was pictured as long-necked Plesiosaur.

Episode 5.2

A Nuclear Submarine encounters an anomaly and is damaged by a pod of Liopleurodons.


Wikipedia has a more detailed and comprehensive article on Plesiosaur!