†Placodermi McCoy, 1848 †Arthrodira Woodward, 1891 †Brachythoraci Gross, 1932 †Eubrachythoraci Miles, 1971 †Pachyosteomorphi Stensiö, 1944 †Aspinothoracidi Stensiö, 1959 Family †Hadrosteidae Gross, 1932 1 Genus †Hadrosteus Gross, 1932 [monotypic]