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Acipenseriformes (subclass Actinopterygii, superorder Chondrostei) An order of rather primitive fish characterized by a cartilaginous skeleton, a heterocercal tail fin, and a head with a pointed, protruding rostrum and ventrally located mouth. Extant members of the order either have a naked skin or possess five rows of large, rhomboid, bony scales along the body; a spiral valve is found in the intestinal tract. Apart from the sturgeon and paddlefish the order includes a number of fossil representatives dating back to the Carboniferous Period.