Psilotopsida is a class of fern-like plants.[1] As circumscribed by Smith et al. (2006) it contains two families, Psilotaceae and Ophioglossaceae, placed in orders Psilotales and Ophioglossales, respectively. The affinities of these two groups have long been unclear and a close relationship between them has only recently been confirmed through molecular systematic studies. Psilotopsida is the sister-group to all other ferns (including Marattiaceae and Equisetaceae).
Pryer, Kathleen M., Eric Schuettpelz, Paul G. Wolf, Harald Schneider, Alan R. Smith and Raymond Cranfill. 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany 91:1582-1598 (online abstract here).
