Leedsichthys was a giant fish that would have dwarfed every other animal in the sea, but it 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 blue whale, which also survives on nothing but plankton.
Below is a size chart of its size compared to other large sea animals that were very large back then too.
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