prokaryotes - bacteria see Domain Bacteria Woese et al. 1990
   Domain "Archaea" Woese et al. 1990
     Phylum "Euryarchaeota" Garrity and Holt 2001
       Class Methanococci Boone 2002 [monotypic]
         Order Methanococcales Balch and Wolfe 1981
           Family Methanocaldococcaceae Whitman et al. 2002
             Genus Methanocaldococcus Whitman 2002