TAXA OF LIFE	DOMAIN EXCAVATA
KINGDOM EUEXCAVATAE

Euexcavatae (u-eks-ka-VA-te) is made of one Greek and two Latin roots which mean true (eu -ευ); from (ex); a cavity (cavus).  The reference is to those organisms that have or are related to taxa which have a special feeding groove called an excavate.

INTRODUCTION TO THE KINGDOM EUEXCAVATAE

The Kingdom Euexcavatae is one of two kingdoms within the Supergroup Excavata (defined as a higher level group by Cavalier-Smith 2002).  These organisms are primarily unicellular, motile and range from animal symbiotes to free-living taxa.  I derived this taxonomic system as a reflection of the systems of Simpson (2003) and Cavalier-Smith (2002 and 2003), both of whom group the parabasalids, metamonads, diplomonads, oxymonads, jakobids, and carpediemonids together with the discicristates.  I have done that, too, but I treat both groups as kingdom-level clades that together make up the Supergroup Excavata.    

The synapomorphic characters that define this kingdom include multiple flagella associated with nuclei and a feeding groove, the excavate, in the cell associated with the flagellar insertions.  I derived the name, Euexcavatae, meaning the "true excavates", to describe those organisms with a depression or feeding groove located on the cell surface and associated with one or more flagella.

Formerly, the major taxa in this kingdom (parabasalids, metamonads, diplomonads, and oxymonads) were considered to be among the most basal of the eukaryotes.  They figured prominently in the Archezoa Hypothesis (Cavalier-Smith 1983 and Roger 1999), a view that the basal eukaryotes evolved before the acquisition of particular lines of mitochondria (platy-, disco-, and tubulocristate mitochondrial lines).  Discoveries of related taxa with mitochondria and degenerate mitochondria in amitochondriate taxa helped to put the Archaezoa Hypothesis to rest.  Furthermore, that almost all of the amitochondriate taxa were animal gut parasites or commensals further called into question their apparent primitive natures.  Thus, the deep branching of taxa like Giardia was artifactual.  When multiple tree analyses (called supergroup analyses) were produced (e.g. Baldauf 2003a), the base and crown of the eukaryotic tree disappeared and what remained was a branching pattern that resembled the petals of a flower.  Furthermore, the supergroup analyses helped to resolve orphaned taxa like the free-living Jakobids and the recently-discovered Carpediemonas.  

PHYLA OF THE EUEXCAVATAE JAKOBADA METAMONADA PARABASALA EOPHYRINGA Incertae Sedis: CARPEDIEMONADA

A photomicrograph of Trichonympha from the gut of Zootermopsis.

A more complete taxonomy (to Order) of the Euexcavatae.

LITERATURE CITED

Baldauf, S. L. 2003a. The deep roots of eukaryotes. Science. 300 (5626): 1701-1703.

Cavalier-Smith, T. 1983. A six-kingdom classification and a unified phylogeny. In: Schenk, H. E. A. and W. S. Schwemmler, eds. Endocytobiology II.  de Gruyter, Berlin.  pp. 1027-1034.

Cavalier-Smith, T. 2002a. The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa: International  Journal of Systematic Evolutionary Microbiology. 52:297–354.

Cavalier-Smith, T. 2003a. Protist phylogeny and the high-level classification of Protozoa. European Journal of Protistology. 39:338-348.

Roger, A. J. 1999. Reconstructing early events in eukaryotic evolution: American Naturalist. 154(supplement): S146–S163.

Simpson, A. G. 2003. Cytoskeletal organization, phylogenetic affinities and systematics in the contentious taxon Excavata (Eukaryota). International Journal of Systematic and Evolutionary Microbiology. 53: 1759-1777.