Several living animals had relatives that experienced gigantism during the Age of Dinosaurs. Fishes have been around for quite a long time and have evolved, reaching different sizes and having different adaptations. According to reports, the oldest fish in the world lived about 500 million years ago. However, the largest bony fish did not come to be until 165 million years ago, during the later parts of the Jurassic Period.

The largest bony fish ever is the Leedsichthys problematicus. The giant fish weighed about 45 tons and reached about 55 feet at maturity. This fish was bigger than even the Megalodon. Leedsichthys problematicus, like the whale and the basking shark, was a filter feeder and a docile sea monster.