They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive toothed jaws, commonly known as pliosaurs.

The name Pliosaurus was coined in 1841 by Richard Owen, who believed that it represented a link between plesiosauroids and crocodilians (considered a type of "saurian"), particularly due to their crocodile-like teeth.

The taxonomy presented here is mainly based on the plesiosaur cladistic analysis proposed by Hilary F. Ketchum and Roger B. J. Benson, 2011 unless otherwise noted.

Pliosauridae is defined as "all taxa more closely related to Pliosaurus brachydeirus than to Leptocleidus superstes, Polycotylus latipinnis or Meyerasaurus victor".

Rhomaleosauridae is defined as "all taxa more closely related to Meyerasaurus victor than to Leptocleidus superstes, Pliosaurus brachydeirus or Polycotylus latipinnis".

[4] Anningasaura "Plesiosaurus" macrocephalus Archaeonectrus Macroplata Atychodracon Eurycleidus Rhomaleosaurus Meyerasaurus Maresaurus Thalassiodracon Hauffiosaurus Attenborosaurus

[3] In the summer of 2008, the fossil remains of the huge pliosaur were dug up from the permafrost on Svalbard, a Norwegian island close to the North Pole.

Found in cliffs near Weymouth, Dorset, on Britain's Jurassic Coast, the fossil had a skull length of 6 feet 5 inches (1.96 meters).