Leedsichthys

The largest bony fish known, a plankton-straining giant of the Middle Jurassic seas about 165 million years ago. Its skeleton survives poorly, often as isolated fragments, and earlier size claims were dismissed by later researchers as historical arm-waving. A 2013 study read growth rings inside its bones, much like tree rings, and found that an animal could reach 8 to 9 meters in about 20 years and around 16.5 meters after 38, with the species' size range put at 12 to 16 meters. That makes it a rival for the whale shark in length, though it belongs to the bony fishes rather than the sharks.