Leedsichthys

Leedsichthys was a giant pachycormid from the middle Jurassic period. It was probably the largest fish ever to live.

Behaviour
Like modern whale sharks and baleen whales, Leedsichthys was a gentle giant that lived on the tiny shrimps, jellyfish and small fish that make up plankton. It would have swum slowly through the upper waters of the ocean, taking mouthfuls of plankton-rich water and sieving them through the giant mesh-plates at the back of its mouth. Its feeding habits were similar to the modern baleen whales, which also survives on nothing but plankton, small fish and jellyfish. It was also a ray-finned fish, making it a relative of most fish on Earth.

They travelled large distances to find parts of the world where seasonal conditions caused plankton to form itself into a dense concentrated organic soup. Occasionally, after plankton feasts, Leedsichthys will shed the giant filter plates from the back of its mouth, meaning it was unable to feed itself for several weeks.

Threats
Just because of Leedsichthys' monstrous size didn't mean that the only predator that could have attacked it was Liopleurodon. Hybodus sharks, Metriorhynchus, smaller marine reptiles and other marine reptiles might chance an attack if it is sightly away from the school, because the slow Leedsichthys wouldn't attack back.

Episode 3
A shoal of Leedsichthys appeared in the third episode living in the second deadliest sea of all time. Nigel Marven swam with the shoal but then noticed a old and ill individual. The fish was frequently attacked by creatures like Metriorhynchus and Hybodus whilst the rest of the shoal swam ahead of it. The Leedsichthys later succumbed to its wounds and died. It was later found by a Liopleurodon. During the night, the carcass was scavenged on by a pod of Liopleurodon.

Trivia

 * Leedsichthys was probably the largest fish ever to live.

Errors

 * Leedsichthys was depicted having a sloped head and visible dermal head bosses.  This is inaccurate.