The stem group of Opisthobranchia from which probably all other taxa are derived are the "cephalaspids
" (sensu stricto, Architectibranchia and Bullomorpha; Schmekel 1985; Salvini-Plawen 1991a,b).
Early Devonian cephalaspids
(Vertebrata: Osteostraci: Cornuata) from the southern Mackenzie Mountains, N.W.T.
The Early Devonian fish assemblages include, besides the dominating amphiaspidids, also cephalaspids
, different placoderms, acanthodians, sarcopterygians (porolepiforms) and actinopterygians.
The fish which probably made the marks found in the fossils were cephalaspids
, which had a different body shape and fin pattern from the fish which are familiar to us today.
(Pageau 1969a; Belles-Isles 1989), chondrichthyans (Miller et al.