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
           Family Methanocaldococcaceae nom. inval. see Family Methanocaldococcaceae Whitman et al. 2002
             Genus Methanocaldococcus Whitman 2001 nom. inval. see Genus Methanocaldococcus Whitman 2002