How long did the evolutionary transformation take?Of major relevance to the problem of defining land plants and pinpointing the exact
timing of their origin is the controversial question of how rapidly they evolved. Was the
Figure 7.1 Evolutionary tree for basal land plants. The stratigraphical column is from Harland et al.
(1990). Note the traditional use of the Llanvirn stage, as utilized throughout this contribution.
Phylogenetic relationships based on Kenrick (2000, figure 1a). Nodes calibrated to the
stratigraphical column using: (1) the earliest dispersed spores, believed to derive from liverworts;
(2) the time when trilete spores first become abundant in the fossil record (true trilete spores possibly
represent a synapomorphy of polysporangiophytes—i.e. the clade nested within Horneophyton)
(Gray 1985; Wellman and Gray 2000); (3) the earliest rhyniophytoid megafossils (these most likely
belong with the Rhyniopsida, but possibly represent protracheophytes [PT]); (4) the earliest
lycophyte megafossils (evidence summarized in Edwards and Wellman 2001). Solid lines represent
fossil record, dashed lines represent range extensions.
126 CHARLES H.WELLMAN