Two new species of deep-bodied Paleozoic actinopterygians, Aesopichthys erinaceus gen. nov., sp. nov. and Proceramala montanensis gen. nov., sp. nov., are described from the Upper Chesterian, Upper Mississippian (Namurian E2b, Lower Carboniferous) Bear Gulch Limestone of Montana, USA, and the new family Aesopichthyidae is erected for them. These two fishes share features usually associated with deep body and deep head specializations, particularly in Palaeoniscimorpha, such as a shortened gape, tall maxilla, vertical suspensorium, deepened flank scales and an elongate dorsal fin. Aesopichthys in particular, for which more complete information is known, also possessed a suite of very specialized adaptations in feeding mechanism, ganoine sculpturing, fin form, and cranial defensive structures that have strong ethological implications. These features include small mobile premaxillae, a spinous and apparently rotatable posteroventral infraorbital, lobed pectoral base, and partially webbed pectoral, dorsal, anal and caudal fins.

Within a general scheme of interrelationships of primitive actinopterygians, these two new taxa pertain to Palaeoniscimorpha, i.e., basal Actinopteri. Then, from a cladistic analysis limited, beside these two new taxa, to Wendyichthys and Cyranorhis (two other already described actinopterygians from Bear Gulch), Platysomus, and Cheirolepis, it appears that the Aesopichthyidae display no close relationship to the Platysomidae but should be the sister group of the Rhadinichthyid-group i.e., [Cyranorhis [Rhadinichthyidae Wendyichthys]].