The Phylogenetic Placements of Psilophyton and the Psilotaceae

The monophyletic status of the taxa assigned to the iconic fossil genus Psilophyton remains problematic because of the possible extensive convergent evolution among organographically simple and ancient Paleozoic plants as well as their fragmentary nature. A similar issue arises when considering the morphological simplicity of lineages manifesting what appear to be highly reduced features, such as the Psilotaceae (Psilotum and Tmesipteris). A revised character matrix consisting of 53 characters and 146 character states for 41 fossil taxa (including four putative species of Psilophyton) and for the extant genera Equisetum, Psilotum, and Tmesipteris was used to assess the phylogenetic placement of the Psilotaceae and the fragmentary remains of Psilophyton. Analyses placed all four taxa assigned to Psilophyton within a well-defined trimerophyte-euphyllophyte clade but failed to find these taxa sharing a single common node. Analyses also could not displace the Psilotaceae from a poorly resolved zosterophyllophyte-lycophyte clade, regardless of how Psilotum and Tmesipteris character states were scored. The assignment of all four putative species of Psilophyton to a single genus was not supported. Analyses also indicate that the Psilotaceae would likely be classified as a fossil lycophyte lineage if known exclusively from their fossil remains.