Three of the clades depicted in the tree shown in Figure 1 of the “Vascular plants (up to the seed plants)” page, the Monilophyta, Ophioglossidae and Polypodiidae do not have fossils that have been defined by consensus in the published literature as members of their stem groups. This raises the question as to the age that such fossils would have if they were to be discovered in the future. The following time tree illustrates the time constraints on the age of such stem fossils:
Figure 1. Phylogenetic time tree of the basal tracheophytes with possible positions of unknown stem groups
The positions of the three stem groups are shown conceptually in the above tree, but should not be considered as actually correct. However, all three are quite tightly constrained. The stem-Monilophyta would have had to appear no earlier than the stem-Euphyllophyta and no later than the stem-Equisetales, a period of around 43 million years. In the case of the stem groups of the other two clades, we can assume that they would have appeared at the same time as the stem-groups of their sister clades: the stem-Equisetales for the stem-Ophioglossidae and the stem-Marattiales for the stem-Polypodiidae. In contrast to the stem groups of the basal vertebrates and the tetrapods, for the basal tracheophytes there is no evidence of ghost lineages.