What is "acanthodian"

a. Pertaining to Acanthodii.(First attested in the mid 19th century.) n. (context zoology English) A member of a group of extinct fish (Acanthodii) that existed from the Silurian to the Permian period.(First attested in the mid 19th century.)Brown, Lesley, ed. ''The Shorter Oxford English Dictionary.'' 5th. Oxford: Oxford University Press, 2003.