Leedsichthys problematicus ("leeds fish") was a giant fossil fish of the Jurassic period. It was a pachycormid, a group of extinct ray-finned fishes (Actinopterygii). "Leedsichthys" is the largest fish known, with an estimated length of up to 16 meters. The blue whale is twice as long, at 30 metres, but that is a mammal, not a fish.