Leedsichthys prehistoric fish. Artwork of Leedsichthys (Leedsichthys problematicus) chasing a school of fish and a pliosaur swimming beneath. Leedsichthys was a giant bony fish that lived during the middle Jurassic (176-161 million years ago). Some estimates have suggested it grew up to 27 metres in length, which would have made it the largest fish that ever lived. However conservative estimates suggest it was more likely to have grown to lengths of around 10 metres. It had thousands of tiny teeth that formed a mesh plate at the back of its mouth. This was used to filter plankton and small animals from the water.