The Sphagnopsida include the Sphagnales, with 150 to 300 living species in the genus Sphagnum, and also the Protosphagnales, which are known only from the Permian.
It is generally agreed that the Sphagnopsida are the sister group to all other mosses, but because of the highly autapomorphic morphology of peat mosses, it is difficult to say this with certainty. The fact that the position of the male structures (antheridia) differs from all other mosses is one of the strongest lines of evidence for a distant relationship, since it more resembles that in some liverworts.
Relationships within the Sphagnopsida have been little studied. Current hypotheses may be found in Daniels & Eddy (1985) and Rice (1994). The former includes only European groups, and does not appear to be a formal analysis; the latter contains a morphological analysis using parsimony, but is based on a representative sampling of sections.
Rice, Steven K. 1994. Form, function and phylogeny in aquatic Sphagnum species. Ph.D. dissertation, Dept. of Botany, Duke Univ., Durham, NC, USA.