procaryotes Domain Archaea Woese et al. 1990 Phylum Euryarchaeota Garrity and Holt 2001 Class Thermococci Zillig and Reysenbach 2001 see Class Thermococci Zillig and Reysenbach 2002 Order Thermococcales Zillig 1988 Family Thermococcaceae Zillig 1988 Genus Pyrococcus Fiala and Stetter 1986