This page discusses how the duration of stem-to-crown transitions can be approximately estimated with known fossils of the stem group in question.
Duration of stem groups
Each new crown group begins when a lineage splits into two new lines of descent, each represented by a stem group. Eventually, each stem line also splits into two new lineages that comprise a new crown group. The time between the formation of the parent crown group and each of the two descendent crown groups is termed here the “stem-to-crown transition”, as illustrated in the phylogenetic tree below (Figure 1):
Figure 1. Time tree of the Cyclostomata and Gnathostomata clades
This tree illustrates, as an example, the phylogeny of the formation of the crown groups of the Cyclostomata and Gnathostomata clades. The tree, as for all phylogenetic trees with attached stratigraphic columns presented in this website, were constructed using the R package “strap” (Bell and Lloyd, 2015).
Now the crown node marks the point at which the stem lineage splits into two. The closest approximation to this event is the oldest fossil in the two descendent stem lines. Theoretically, the oldest fossil should be of the same age in both stem lines, but this is commonly not the case, presumably because appropriate fossils have not yet been discovered from one of the stem lines. The difference in ages is represented by a ghost lineage on the stem line with a later appearance of stem fossils (shown as green bars in Figure 1 above).
The duration of the stem-to-crown transition, derived from fossil ages, thus comprises the difference in age between the oldest known member of the parent stem group and the oldest member of the two descendent stem groups, as illustrated in Fig. 1 above. However, the true length of a stem line is actually the age difference between two successive crown nodes of the phylogenetic tree, and the stem-to-crown transition derived from fossils is only a rough approximation to this (note in Figure 1 the differences between the two stem-to-crown transitions and the length of the branches between the vertebrate crown node and the crown nodes of the cyclostomes and the gnathostomes). The lengths of the branches of the tree are not uniquely determined, as they are a function of the tree-construction algorithm used in the R package “strap” to apportion between all the branches of the tree the total geological time represented by the tree (see discussion on pages 380 and 381 of Bell and Lloyd, 2015). Nevertheless, the fossils do provide an objective, even if very approximate, estimate of stem branch lengths.
The two plots below (Figures 2 and 3) show the durations of the stem-to-crown transitions calculated for all vertebrate and tracheophyte (vascular plants) clades for which fossils are known from the stem group of the clade in question.
Now the crown node marks the point at which the stem lineage splits into two. The closest approximation to this event is the oldest fossil in the two descendent stem lines. Theoretically, the oldest fossil should be of the same age in both stem lines, but this is commonly not the case, presumably because appropriate fossils have not yet been discovered from one of the stem lines. The difference in ages is represented by a ghost lineage on the stem line with a later appearance of stem fossils (shown as green bars in Figure 1 above).
The duration of the stem-to-crown transition, derived from fossil ages, thus comprises the difference in age between the oldest known member of the parent stem group and the oldest member of the two descendent stem groups, as illustrated in Fig. 1 above. However, the true length of a stem line is actually the age difference between two successive crown nodes of the phylogenetic tree, and the stem-to-crown transition derived from fossils is only a rough approximation to this (note in Figure 1 the differences between the two stem-to-crown transitions and the length of the branches between the vertebrate crown node and the crown nodes of the cyclostomes and the gnathostomes). The lengths of the branches of the tree are not uniquely determined, as they are a function of the tree-construction algorithm used in the R package “strap” to apportion between all the branches of the tree the total geological time represented by the tree (see discussion on pages 380 and 381 of Bell and Lloyd, 2015). Nevertheless, the fossils do provide an objective, even if very approximate, estimate of stem branch lengths.
The two plots below (Figures 2 and 3) show the durations of the stem-to-crown transitions calculated for all vertebrate and tracheophyte (vascular plants) clades for which fossils are known from the stem group of the clade in question.
Figure 2. Duration of fossil-based stem-to-crown transitions for vertebrates
Figure 3. Duration of fossil-based stem-to-crown transitions for tracheophytes
For the vertebrates, only one stem-to-crown transition (that of the stem-Archosauria, which lasted 7.8 ± 4.8 million years) has a duration less than 10 million years (Fig. 2). Similarly, there is only one tracheophyte transition (the stem-Lycophyta, 5.1 ± 3.2 million years) that lasted less than 10 million years (Fig. 3). The average durations for the vertebrates and tracheophytes are 123 and 120 million years respectively.
Page last updated July 28th, 2026.
Reference
Bell, M. A., & Lloyd, G. T. (2015). strap: an R package for plotting phylogenies against stratigraphy and assessing their stratigraphic congruence. Palaeontology, Vol. 58, No. 2, pp. 379-389.