"Pliosaurus" andrewsi is an extinct species of pliosaurid plesiosaurs that lived during the Callovian stage of the Middle Jurassic, in what is now England.

The taxonomic history of this animal is quite complex, because several of its fossils were attributed to different genera of pliosaurids, before being concretely named and described in 1960 by Lambert Beverly Tarlo as a species of Pliosaurus.

At least five other pliosaurids are known from the Peterborough Member, but they were quite varied in anatomy, indicating that they would have eaten different food sources, thereby avoiding competition.

The first possible mention of "Pliosaurus" andrewsi in scientific literature dates back to 1871, in which John Phillips catalogued pliosaur fossils having been discovered by Charles Leeds in the Oxford Clay Formation, England.

[2]: 316–318  This formation, well known because of its significant preservation of plesiosaurians, is dated to the Callovian stage of the Middle Jurassic,[3][4] a period ranging from 166 to 164 million years.

[6]: 163–164  However, in the official 2022 description of Eardasaurus, another pliosaurid from the Oxford Clay Formation, Phillips's mandible (cataloged as NHMUK R2443) is considered as a specimen of undetermined affinities and cannot be clearly attributed to "P." andrewsi.

[8]: 439–441  Two years later, in 1960, he named the taxon Pliosaurus andrewsi and designated the partial skeleton (cataloged as NHMUK R3891) as the holotype of this species.

[6]: 163–164  The holotype specimen is a skeleton consisting of a mandible, teeth, a complete vertebral column as well as parts of the fore and hind limbs.

[6]: 163–164  The specific epithet andrewsi is named in honor of Andrews,[10] who was the first to propose that the fossil remains of this taxon belong to a different genus from Peloneustes.

[7]: 72 The taxonomic identity of this species remained undisputed for decades,[11][12] but in the early 2010s phylogenetic and anatomical revisions showed that it did not belong to the genus Pliosaurus.

[13] After this discovery, the taxon was renamed as "Pliosaurus" andrewsi in studies published since, the quotation marks indicating its non-belonging within this genus.

[19]: 367 [a] Despite the fact that the vertebral column of "P." andrewsi is fully known, Tarlo did not analyze the caudal and dorsal vertebrae, because they do not have enough notable features to be described in detail.

[6]: 164 Since the first descriptions made on the fossils now referred to "Pliosaurus" andrewsi, they were classified in different genera of the Pliosauridae, a family to which the taxon has always been assigned since.

[24] The Peterborough Member is primarily composed of grey bituminous (asphalt-containing)[23] shale and clay rich in organic matter.

These include the chondrichthyans Asteracanthus, Brachymylus, Heterodontus (or Paracestracion),[11] Hybodus, Ischyodus, Palaeobrachaelurus, Pachymylus, Protospinax, Leptacanthus, Notidanus, Orectoloboides, Spathobathis, and Sphenodus.

Actinopterygians were also present, represented by Aspidorhynchus, Asthenocormus, Caturus, Coccolepis, Heterostrophus, Hypsocormus, Leedsichthys, Lepidotes, Leptolepis, Mesturus, Osteorachis, Pachycormus, Pholidophorus, and Sauropsis.

They filled a variety of niches, including invertebrate eaters, piscivores, and, in the case of Leedsichthys, giant filter feeders.

[11] Plesiosaurs are common in the Peterborough Member, and besides pliosaurids, are represented by cryptoclidids, including Cryptoclidus, Muraenosaurus, Tricleidus, and Picrocleidus.

[20] They were smaller plesiosaurs with thin teeth and long necks, and, unlike pliosaurids such as Peloneustes, would have mainly eaten small animals.

These include the gavial-like teleosauroids Charitomenosuchus, Lemmysuchus, Mycterosuchus, and Neosteneosaurus[29] and the mosasaur-like[11] metriorhynchids Gracilineustes, Suchodus, Thalattosuchus,[30] and Tyrannoneustes.

[20] Besides "P." andrewsi, these pliosaurids include Liopleurodon ferox, Simolestes vorax, Peloneustes philarchus, Marmornectes candrewi,[32] Eardasaurus powelli, and, potentially, Pachycostasaurus dawni.

[33][4] However, there is considerable variation in the anatomy of these species, indicating that they fed on different prey, thereby avoiding competition (niche partitioning).