The most phylogenetically ancient (least derived) phylum contains the genus
Aquifex and relatives, all of which are hyperthermophilic H
2-oxidizing chemolithotrophs.
Other nearby phyla such as
Thermodesulfobacterium,
Thermotoga, and the
green non-sulfur bacteria (the
Chloroflexus group), also contain thermophilic species.
Many other phyla contain thermophilic extremophiles, many of which are included in this taxonomy browser.
Taken from the text Brock Biology of Microorganisms (10th ed.). Madigan, M.T., Martinko, J.M., and Parker, J. 2003. Prentice Hall. 353p.