procaryotes
   Domain Archaea Woese et al. 1990
     Phylum Euryarchaeota Garrity and Holt 2001
       Class Methanococci Boone 2001 nom. inval. see Class Methanococci Boone 2002
         Order Methanococcales Balch and Wolfe 1981
           1 Family Methanococcaceae Balch and Wolfe 1981
           2 Family Methanocaldococcaceae nom. inval. see Family Methanocaldococcaceae Whitman et al. 2002