prokaryotes - bacteria see Domain Bacteria Woese et al. 1990 Domain "Archaea" Woese et al. 1990 Phylum "Euryarchaeota" Garrity and Holt 2001 Class Methanobacteria Boone 2002 [monotypic] Order Methanobacteriales Balch and Wolfe 1981 Family Methanobacteriaceae Barker 1956 (Approved Lists 1980) 1 Genus Methanobacterium Kluyver and van Niel 1936 (Approved Lists 1980) 2 Genus Methanobrevibacter Balch and Wolfe 1981 3 Genus Methanosphaera Miller and Wolin 1985 4 Genus Methanothermobacter Wasserfallen et al. 2000