Psilophyton - Phylogeny

Phylogeny

At first most of the early polysporangiophytes (land plants other than liverworts, mosses and hornworts) were placed in a single class, Psilophyta, established in 1917 by Kidston and Lang. As additional fossils were discovered and described, it became apparent that the Psilophyta were not a homogeneous group of plants. In 1968, Banks proposed splitting it into three groups, one of which was the subdivision Trimerophytina, informally called the trimerophytes. Psilophyton was a prominent member of this group. The distinction between main stems and strongly branched lateral stems evident in species such as P. forbesii has been considered to be one of the key steps towards the evolution of the leaves of euphyllophytes (modern members of which are the ferns and seed plants), based on the theory that such leaves evolved through 'webbing' of flattened lateral branching systems.

In 2004, Crane et al. published a simplified cladogram for the polysporangiophytes, based on a number of figures in Kenrick and Crane (1997). Part of their cladogram is reproduced below (with some branches collapsed into 'basal groups' to reduce the size of the diagram). It shows two of the better-known species of Psilophyton as early-diverging members of the euphyllophytes.