Material of six osteolepiforrn genera is described, including Askerichthys n. gen., a new Late Carboniferous genus from Norway, MegaIichthys Agassiz, 1835 from the Carboniferous of Great Britain, Latvius Jarvik, 1948 from the Late Devonian of Germany and Latvia, and Osteolepis Agassiz, 1835 and Gyroptychius McCoy, 1848 from the Middle Devonian of Great Britain and northeast Greenland. New information on Eusthenopteron foordi Whiteaves, 1881 from the Late Devonian of Canada is presented in the morphologic discussions. On the basis of the descriptions and previous studies morphologic variation in osteolepiforms is recorded, and it is discussed whether these variations are taxonomic or intraspecific. Morphologic clines are described and it is discussed whether they are trends. When possible, functional implications of the morphologic variations are suggested.
In the phylogenetic and taxonomic section different types of characters as well as use of these characters when reconstructing phylogeny and taxonomy, is discussed. Consideration has been given as to whether diagnoses can be constructed in a more informative way by stating whether characters are necessary, sufficient or indicative.
A tentative phylogenetic model based on the morphologic information in this and other works is presented. This phylogeny leads to a taxonomic model that is expressed as a review of osteolepiform taxa with diagnoses formulated as stated above. The order Osteolepiforrnes Woodward, 1932 is divided into two suborders, Osteolepidoidei Moy-Thomas & Miles, 1971 and Cyclolepidoidei n. suborder. Osteolepidoidei includes the families Osteolepididae Cope, 1889, Thursiidae n. fam. and Megalichthyidae Hay, 1902. Osteolepididae is divided into Glyptopominae Goodrich, 1909 and Osteolepidinae Cope, 1889. Megalichthyidae is divided into the subfamilies Ectosteorhachinae n. subfam., Megalichthyinae n. subfam. and Askerichthyinae n. subfam. Cyclolepidoidei includes Eopodoidea n. superfamily, Parapodoidea n. superfamily, and Rhizodontoidea. Eopodoidea includes the families Gyroptychiidae n. fam., Panderichthyidae Vorobyeva, 1968. Chrysolepididae n. fam. and Eusthenopteridae Berg, 1955. Parapodoidea includes the families Canowindridae Young, Long & Ritchie, 1992 and Medoevididae n. fam. Panderichthyidae is divided into Panderichthyinae n. subfam. and Elpistosteginae n. subfam. New species erected in this paper are the megalichthyids Askerichthys heintzi and Megalichthys syndentolaminaris.