The metamonads are a large group of flagellate amitochondriate excavates. Their composition is not entirely settled, but they include the retortamonads, diplomonads, and possibly the parabasalids and oxymonads as well. These four groups are all anaerobic, occurring mostly as symbiotes of animals.
A number of parabasalids and oxymonads are found in termite guts, and play an important role in breaking down the cellulose found in wood. Some other metamonads are parasites.
These flagellates are unusual in lacking mitochondria. Originally they were considered among the most primitive eukaryotes, diverging from the others before mitochondria appeared. However, they are now known to have lost mitochondria secondarily, and retain both organelles and nuclear genes derived from them. Mitochondrial relics include hydrogenosomes, which produce hydrogen, and small structures called mitosomes.
It now appears the Metamonada are, together with Malawimonas, sister clades of the Podiata.
All of these groups are united by having flagella or basal bodies in characteristic groups of four, which are often associated with the nucleus, forming a structure called a karyomastigont. In addition, the genera Carpediemonas and Trimastix are now known to be close relatives of the retortamonad-diplomonad line and the oxymonads, respectively. Both are free-living and amitochondriate.
The metamonads make up part of the excavates, a eukaryotic supergroup including flagellates with feeding grooves and their close relatives. Their relationships are uncertain, and they do not always appear together on molecular trees. It is possible that the metamonads as defined here do not form a monophyletic subgroup.
The following higher level treatment is based on works of Cavalier-Smith with amendments within Fornicata from Yubukia, Simpson & Leander
Phylum Metamonada (Grassé 1952) Cavalier-Smith 1987 emend. Cavalier-Smith 2003Subphylum Anaeromonada Cavalier-Smith 1997 emend. 2003Class Anaeromonadea Cavalier-Smith 1997 emend. 1999Family Paratrimastigidae Zhang et al. 2015Order Trimastigida Cavalier-Smith 2003Family Trimastigidae Saville Kent 1880Order Oxymonadida Grassé 1952 emend. Cavalier-Smith 2003Family Polymastigidae Bütschli 1884Family Saccinobaculidae Brugerolle & Lee 2002 ex Cavalier-Smith 2013Family Pyrsonymphidae Grassé 1892Family Streblomastigidae Kofoid & Swezy 1919Family Oxymonadidae Kirby 1928Subphylum Trichozoa Cavalier-Smith 1996 emend. Cavalier-Smith 2003 stat. n. 2013Superclass Fornicata Simpson 2003 stat. n. Cavalier-Smith 2013Genus ErgobibamusGenus HicanonectesFamily Kipferliidae Cavalier-Smith 2013Order Chilomastigida Cavalier-Smith 2013Family Chilomastigidae Cavalier-Smith 2013Order Dysnectida Cavalier-Smith 2013Family Dysnectidae Cavalier-Smith 2013Class Carpediemonadea Cavalier-Smith 2013 s.s.Order Carpediemonadida Cavalier-Smith 2003 emend. 2013 s.s.Family Carpediemonadidae Cavalier-Smith 2003Class Eopharyngea Cavalier-Smith 1993 stat. n. Cavalier-Smith 2003Order Retortamonadida Grassé 1952 emend. Cavalier-Smith 2013Family Retortamonadidae Wenrich 1932Order Diplomonadida Wenyon 1926 emend. Brugerolle et al. 1975Family Giardiidae Kulda & Nohy´nkova´ 1978Family Octomitidae Cavalier-Smith 1996Family Spironucleidae Cavalier-Smith 1996Family Hexamitidae Kent 1880 emend. Brugerolle et al. 1975Superclass Parabasalia Honigberg 1973 stat. n. Cavalier-Smith 2003Class ChilomiteaOrder ChilomitidaFamily ChilomitidaeClass Trichonymphea Cavalier-Smith 2003Order Lophomonadida Light 1927Family Lophomonadidae Saville Kent 1880Order Trichonymphida Poche 1913Family Spirotrichosomidae Hollande & Caruette-Valentin 1971Family Staurojoeninidae Grassé 1917Family Trichonymphidae Saville Kent 1880Family Hoplonymphidae Light 1926Family Teranymphidae Koidzumi 1921 [Eucomonymphidae]Class Trichomonadea Kirby 1947 stat. n. Cavalier-Smith 2003Order TrichocovinidaFamily TrichocovinidaeOrder Trichomonadida Kirby 1947Suborder Hypotrichomonadina Čepička et al. 2010 stat. n.Family Hypotrichomonadidae (Honigberg 1963) Čepička, Hampl & Kulda 2010Suborder Trichomonadina (Kirby 1947) Cavalier-Smith 2013Family Trichomonadidae Chalmers & Pekkoloa 1918 sensu Hampl et al. 2006Suborder Honigbergiellina Čepička et al. 2010Family Honigbergiellidae Čepička, Hampl & Kulda 2010Family Hexamastigidae Čepička, Hampl & Kulda 2010Family Tricercomitidae Čepička, Hampl & Kulda 2010Order Tritrichomonadida Čepička et al. 2010Family Dientamoebidae Grassé 1953Family Monocercomonadidae Kirby 1944Family Simplicimonadidae Čepička et al. 2010Family Tritrichomonadidae Honigberg 1963SubclassCristomonadia Cavalier-Smith, 2013Order Spirotrichonymphida Grassé 1952Family Spirotrichonymphidae Grassé 1917Order Cristamonadida Brugerolle & Patterson 2001 emend. Cavalier-Smith 2013Family Calonymphidae Grassé 1911Family Devescovinidae Doflein 1911 [Joeniidae; Rhizonymphidae; Kofoidiidae; Deltotrichonymphidae]
