Leedsichthys are the biggest fish that appeared in the Jurassic Period - and the biggest ever known. It grew to 27 meters long (18 metres in truth, in more modern estimates, 27 metres in the old show estimates - still the largest fish ever even at 18 instead of 27 metres), and sometimes larger, making it the largest bony fish to ever live.
A Leedsichthys plays a minor role in Sea Monsters: A Walking with Dinosaurs Trilogy. While visiting the second most dangerous sea, Nigel Marven swims with some Leedsichthys, and watches a Liopleurodon eat one of them. Hybodus and Metriorhynchus are also seen attacking and eating bits off the Leedsichthys.