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