Cretoxyrhina

Cretoxyrhina (identified as a 23 foot long shark) is a massive 23 foot long shark from the Cretaceous and the largest fish of the Cretaceous sea. It looks like the great white of today, but only larger.

It was seen in the Cretaceous facing a Prognathodon, a 40 foot mosasaur. Both the shark and the mosasaur are territorial and the Prognathodon goes up to the surface and Cretoxyrhina follows it to keep it away from its territory. The Prognathodon and the Cretoxyrhina both make a showdown to fight. The Cretyoxyrhina then swims to the mosasaur's side and opens its jaws. Fortunately, the Cretoxyrhina's fin was sinking and is revealed that Prognathodon is fine and well and has killed the shark off-screen.

Trivia

 * Cretoxyrhina wasn't identified in Sea Rex.