An order of actinopterygian fishes in the subclass Chondrostei, which probably originated from palaeonisciform ancestors during the Triassic or Jurassic periods. Acipenseriformes, the only extant order of about 10 orders of chondrosteans, are characterized by a largely cartilaginous skeleton, heterocercal tail, fins with more than one ray per pterygiophore, spiral valve intestine, myodome and preopercle reduced or absent, and gular plate absent. The cartilaginous skeleton of Acipenseriformes, long regarded as primitive, was taken as indicative of an alliance with the Chondrichthyes; however, cartilage is secondary and the true relationship is with bony fishes.