Class †Placodermi McCoy, 1848
   Order †Arthrodira Woodward, 1891 see [unranked] Arthrodira Woodward, 1891
     †Aspinothoracida see [unranked] Aspinothoracidi Stensiö, 1959
       Family †Selenosteidae Dean, 1901
         Genus †Gymnotrachelus Dunkle & Bungart, 1939, sedis mutabilis
           1 †Gymnotrachelus hydei Dunkle & Bungart, 1939