An acanthodian is a member of a group of extinct fish which existed from the Silurian to the Permian period.
Acanthodians were the earliest jawed vertebrates. They lived from the Ordovician to the Carboniferous period. While some possessed teeth which allowed them to eat other marine animals, others lacked teeth and were filter feeders.
All four-limbed animals belong to the clade of Tetrapoda. The clade of Tetrapoda is defined as the first basal four-limbed animals and all their descendants, extant and extinct. The clade immediately superior (ancestral) to that is the clade of Teleostomi, which contains all jawed vertebrates, including the tetrapods, bony fish, and the extinct lineage of acanthodian fish.