The family is more inclusive than the archetypal short-necked large headed species that are placed in the subclade Thalassophonea, with basal forms resembling other plesiosaurs with long necks.

[1] Pliosauridae is a stem-based taxon defined in 2010 (and in earlier studies in a similar manner) as "all taxa more closely related to Pliosaurus brachydeirus than to Leptocleidus superstes, Polycotylus latipinnis or Meyerasaurus victor".

The name is derived from Greek thalassa (θάλασσα), "sea", and phoneus (φονεύς), "murderer".

It is a stem-based taxon defined as "all taxa more closely related to Pliosaurus brachydeirus than to Marmornectes candrewi".

Thalassophonean pliosaurs represented the largest marine predators during their existence, spanning more than 80 million years.