Some studies also include the second species L. pachydeirus, but this latter is considered as a probable junior synonym of L. ferox due to its lack of viable diagnosis.

Liopleurodon is a representative of the Thalassophonea clade, a derived group of pliosaurids characterized by a short neck and a large elongated skull.

In 1999, the size of Liopleurodon was greatly exaggerated in the BBC documentary series Walking with Dinosaurs, depicted as reaching 25 m (82 ft) in length.

[1] In 1841, Hermann von Meyer named the species Thaumatosaurus oolithicus based on a fragmentary specimen consisting of partial teeth, skull elements, vertebrae, and ribs from deposits in Württemberg, Germany, possibly dating to the Oxfordian.

[2][3][4][1]: 31  Johann Andreas Wagner published a description of a large plesiosaur tooth from Bavaria, Germany, in 1852, assigning it to a new species that he named Pliosaurus giganteus.

[4] In 1838, Hermann von Meyer applied the name Ischyrodon meriani to a large tooth from Oxfordian-aged rocks in Fricktal, Switzerland.

[8] This tooth lacks identifying characteristics, and therefore it is not clear what it belonged to, although Lambert Beverly Tarlo noted the possibility of it pertaining to Liopleurodon in 1960.

[9][4] A 2022 study by Daniel Madzia and colleagues noted that while the tooth likely came from Liopleurodon or something similar, there was too little information available to make a confident assignment, so they treated Ischyrodon as a nomen dubium.

[11]: 377–380  One tooth, its crown[1]: 133  measuring 7.5 centimetres (3.0 in) long, was found near Boulogne-sur-Mer, France, in layers dating from the Callovian,[1]: 32  and was named Liopleurodon ferox.

[13] In 1888, Richard Lydekker, after studying some teeth attributable to Liopleurodon ferox in the Leeds Collection, concluded that they were so similar to those of Pliosaurus that they should be placed in that genus.

[14][4] W. Kiprijanoff named Thaumatosaurus mosquensis in 1883 based on remains including teeth, vertebrae, and limb bones from Oxfordian-aged rocks in the Moscow Basin of Russia.

Charles William Andrews described the anatomy of the marine reptile specimens of the Leeds Collection acquired by the British Museum of Natural History in two volumes, the first published in 1910 and the second in 1913.

One of these was a poorly preserved partial skeleton excavated from the Oxford Clay of Fletton, England, housed in Institut für Geowissenschaften, University of Tübingen.

Additionally, Linder described some skulls from Fletton housed at both the University of Tübingen and the State Museum of Natural History Stuttgart as specimens of P. grandis.

[21] All of these specimens have since been assigned to Liopleurodon with varying degrees of confidence, though the skull Linder attributed to P. grandis that was housed in Stuttgart was destroyed during World War II.

[4] In 1971, Halstead published another paper about Jurassic pliosaurids, this time focusing on Pliosaurus rossicus, a species he was formerly unwilling to consider valid, due to a lack of information.

The teeth of mounted skeleton in Tübingen, which Tarlo had attributed to L. pachydeirus, showed distinctive characteristics of L. ferox, indicating that cervical vertebrae are not useful for differentiating species, as argued by David S. Brown in 1981.

These fossils were extracted from Kimmeridgian deposits in the Sabinal Formation, and one of them, the partial front end of a snout, was attributable to Liopleurodon, though the researchers considered the remains too fragmentary to provide a species-level identification.

[29] Madzia and colleagues in 2022 noted that the fact that Liopleurodon was named based on a single tooth of dubious distinctiveness is problematic, and that a more complete neotype may need to be designated to preserve the stability of L. ferox.

[34] In 2024, Ruizhe Jackevan Zhao estimates that the largest known specimen of Liopleurodon, NHMUK PV R3536, would have reached a length of approximately 8 m (26 ft) with a body mass of 7.8 t (17,000 lb).

A study involving a swimming robot has demonstrated that although this form of propulsion is not especially efficient, it provides very good acceleration—a desirable trait in an ambush predator.

[42] A fragmentary specimen possibly belonging to a young individual, PETCM R.296, contained numerous hooklets of teuthoid cephalopods, fish bones and a single reptilian tooth in its stomach.