|“||This is Haikouichthys. He's the size of your thumb nail, but he's an evolutionary giant. He's the first every fish, our earliest known ancestor.||„|
|— Allen, on Haikouichthys|
Haikouichthys ("Haikou Fish") was was a small, prehistoric fish, notable for being not only the very first fish, but also the oldest, most primitive known backboned animal (vertebrate).
Haikouichthys was the smallest fish that every lived in the sea and the smallest fish that ever existed in general. Although only the size of a human's thumbnail, they were evolutionary giants. As the first every fish and the earliest known ansector of humans, they were unique because instead of having armor on the outside, they were tough inside. They evolved a primitive back-bone, making them the very first vertabrate; forerunners of all future backbone animals from the dinosaurs, to the elephant, to humans.
Their flexible back-bone made them more maneuverable than other animals, like the spineless Anomalocaris. Haikouichthys could scavenge flesh and then dart away unharmed.
Like many fish today, Haikouichthys lived in large schools.