prokaryotes - bacteria see Domain Bacteria Woese et al. 1990
   Domain "Archaea" Woese et al. 1990
     Phylum "Euryarchaeota" Garrity and Holt 2001
       Class "Methanomicrobia"
         Order Methanomicrobiales Balch and Wolfe 1981
           1 Family Methanocorpusculaceae Zellner et al. 1989 [monotypic]
           2 Family Methanomicrobiaceae Balch and Wolfe 1981
           3 Family Methanoregulaceae Sakai et al. 2012
           4 Family Methanospirillaceae Boone et al. 2002 [monotypic]
           5 Genus Methanocalculus Ollivier et al. 1998, incertae sedis