This page discusses how the duration of stem-to-crown transitions can be approximately estimated with or without 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.
An important point to realize here is that the method outlined above can only be used if fossils are known from the stem line of a crown group. If this is not the case, the only estimate we have of the branch length is that provided by the phylogenetic tree itself, which in this website is always a product of applying the “strap” package. We will now discuss this further as we consider the clades that lack stem-group fossils.
An important point to realize here is that the method outlined above can only be used if fossils are known from the stem line of a crown group. If this is not the case, the only estimate we have of the branch length is that provided by the phylogenetic tree itself, which in this website is always a product of applying the “strap” package. We will now discuss this further as we consider the clades that lack stem-group fossils.
Clades lacking stem-group fossils
This section deals with the clades for which there is no consensus in the literature that any fossils represent the stem group in question. In an attempt to constrain the length of these branches, phylogenetic time trees for the closest related fossils have been constructed using the R package “strap”. These trees are shown below, firstly for the vertebrates (Figures 4 – 9) and then for the tracheophytes (Figures 10 – 11). Note that the topology of each tree is consistent with that of the corresponding trees shown in the pages of this website under the “Animals” and “Land Plants” tabs under the “Evolution of Life” tab in the navigation pane above.
Figure 4. Time tree of fossils closely related to stem-Actinopteri
Figure 5. Time tree of fossils closely related to stem-Rhipidistia
Figure 6. Time tree of fossils closely related to stem-Batrachia
Figure 7. Time tree of fossils closely related to stem-Archelosauria
Figure 8. Time tree of fossils closely related to stem groups within the crown-Aves
Figure 9. Time tree of fossils closely related to stem groups within the crown-Marsupialia
Figure 10. Time tree of fossils closely related to stem groups within the crown-Eutheria
Figure 11. Time tree of fossils closely related to stem groups within the crown-Euphyllophyta
Figure 12. Time tree of fossils closely related to stem groups within the crown-Spermatophyta
In each of the trees shown above, the branches of internal nodes for which no stem-group fossils are known and which have a length of less than 10 million years are colored red. Furthermore, the length of each such branch is shown as a red number near that branch on the tree. This threshold duration was selected because it is close to the absolute minimum duration of the fossil-based stem-to-crown transitions for both the vertebrates and the land plants (see Figures 2 and 3).
Some of the trees (Figures 4, 5, 6, 7 and 11) concern only a single branch lacking stem fossils. However, the others illustrate cases in which a single crown clade contains a number of stem branches that lack fossils (Figures 8, 9, 10 and 12).
The stem branch lengths shown in the above trees are compiled in the following table:
Some of the trees (Figures 4, 5, 6, 7 and 11) concern only a single branch lacking stem fossils. However, the others illustrate cases in which a single crown clade contains a number of stem branches that lack fossils (Figures 8, 9, 10 and 12).
The stem branch lengths shown in the above trees are compiled in the following table:
Table 1. Stem-branch lengths in millions of years (my)
The above table indicates that a significant number of stem branches without known fossils have a duration of less than 10 my. This suggests that the rate of evolution leading to the formation of their respective crown groups was faster than for almost all the fossil-based stem-to-crown transitions. Furthermore, the durations shown in the above table are more than an order of magnitude smaller than the average durations of the vertebrate and tracheophyte fossil-based transitions. This indicates the possibility that the stem branches included in the above table represent rates of evolution that might have been 10 times faster than those of many of the fossil-based stem-to-crown transitions.
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.