A number of the gymnosperm clades depicted in the tree shown in Figure 1 of the “Seed plants” page 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 gymnosperms with possible positions of unknown stem groups
The positions of the unknown stem groups are shown conceptually in the above tree, but should not be considered as actually correct. None of them are tightly constrained in time, but all can be placed without implying ghost lineages. The stem group of the combined cycad/ginkgo clade would have the same age as that of its sister clade, the conifers, or Pinopsida. Similarly, the stem group of the combined Gnetales/Welwitschiales clade would have appeared at the same time as that of its sister clade, the Ephedrales. Furthermore, the stem-(Pinales+Gnetales) and the stem-Cupressidae would have appeared synchronously, but obviously no later than the crown-Cupressidae. However, the maximum possible age of this pair of stem groups is very poorly constrained; they could have appeared any time after the oldest member of the stem-Pinopsida.