The Plesiosaurs (Plesiosauria) were a group of extinct reptiles, belonging to the sauropterigyians. They existed from the middle Triassic to the late Cretaceous, dwelling at the territories of modern Eurasia, Africa, Australia, America, etc, even Antarctica (that, admittedly, was less cold than now).
The peak of the plesiosaur diversity was during the Jurassic, as featured in the 3rd episode of Walking with Dinosaurs, which featured both the long-necked plesiosaur Cryptoclidus and the short-necked pliosaur Liopleurodon.
Being one of the most important group of carnivorous reptiles, the plesiosaurs played an important role in the marine ecosystems of the Mesozoic, similar to that of the modern pinniped mammals. Unlike them, however, and like the ichthyosaurs, the plesiosaurs also gave birth in the water, possibly caring for their offspring as well.
There were three main groups of the plesiosaurs: the pliosaurs, represented by Liopleurodon and Plesiopleurodon (which appeared in a cameo in the 4th episode of Walking with Dinosaurs), the pistosaurids, and the plesiosaurs proper, represented by Cryptoclidus and Elasmosaurus from the last episode of Sea Monsters.
Plesiosaurs had rather small heads, long necks and bulky bodies; the pliosaurs, conversely, had large heads, small necks, and were bigger in size that the plesiosaurs as well. Nevertheless, all of them had died out in the end of the Mesozoic, during the K/T extinction.
Walking with Dinosaurs Pilot
A pliosaur, presumably Liopleurodon, was feutured in the pilot. Most of the shots where early versions of ones from the actual film, such as the stranded pliosaur. Others may been feutured, as only aorund 1/6 of the original footage was released.
This episode featured Cryptoclidus, a cousin of Plesiosaurus. It was a relatively small plesiosaur and it was depicted on WWD as able to come on land to escape Liopleurodon and other pliosaurs, though this was impossible in real life. It fed on fish, squid, and other free-swimming animals, and unlike the ichthyosaurs it used flippers to swim, not its tail. "Cruel Sea" also featured the aforementioned Liopleurodon, probably an apex predator, which was said to be around 20 some meters long on average, with some old males reaching 25m. This also however wasn't based on fact; it was only 7-8m.
This episode featured Plesiopleurodon. A cousin of Liopleurodon from the previous episode, it was based on the same model and was shown briefly stalking the old male Ornithocheirus, while the latter caught fish on the wing.
Sea Monsters, Episode 3
This episode also featured Liopleurodon. Several of them came upon a dead Leedsichthys at night, and when Nigel Marven came to investigate, one of them came too close for comfort but was scared away by the stink suit. Later, this episode also showed a pod of elasmosaurs: like modern dolphins they rode the wave left in the wake of Ancient Mariner, before vanishing into the depths of the sea.