The tetrapods (Superclass Tetrapoda, Infraphylum Gnathostomata) share a variety of morphological features. These include a pair of bones (the ulna and radius and the tibia and fibula) in the forelimbs and hind limbs, digits on the end of each limb, an oval window (fenestra ovalis) in the skull opening into the middle ear, a stapes (ear bone), and several other skeletal features (Encyclopaedia Britannica).
A summary of the phylogeny of this clade is shown in the following two-part figure:
A summary of the phylogeny of this clade is shown in the following two-part figure:
Figure 1a. Summarized phylogenetic tree of the tetrapods (part 1)
Figure 1b. Summarized phylogenetic tree of the tetrapods (part 2)
The following pages present phylogenetic trees that illustrate the history of development of each stem group within the crown-Tetrapoda. If we summarize and combine these data, we can construct a phylogenetic time tree for the entire tetrapod tree. The following tree illustrates how the successive stem groups relate to one another through geological time. For simplicity, each stem group is represented by the oldest known member of that stem group:
Figure 2. Summarized phylogenetic time tree of the tetrapods
The above tree illustrates the time of first appearance of each stem group (where known) and their phylogenetic relationships, or lines of descent from ancestors to descendants.
As for the equivalent tree of the basal vertebrates (Vertebrates (up to tetrapods)), a striking aspect of the above tree is the great variation in the time interval between the appearance of the oldest known stem group fossil and the formation of two new clades at the next crown node.
Another way of representing this variation is to plot the number of new clades (represented by stem group fossils) appearing in successive intervals of geological time, as shown below:
As for the equivalent tree of the basal vertebrates (Vertebrates (up to tetrapods)), a striking aspect of the above tree is the great variation in the time interval between the appearance of the oldest known stem group fossil and the formation of two new clades at the next crown node.
Another way of representing this variation is to plot the number of new clades (represented by stem group fossils) appearing in successive intervals of geological time, as shown below:
Figure 3. Appearance of crown tetrapod clades over geological time
This plot indicates that the appearance of new clades was fastest over the interval from around 70 to 50 million years (Maastrichtian to Early Eocene). This implies that many clades of the crown-Eutheria all appeared within an interval of about 17 million years, corresponding to a new clade roughly every 0.8 million years on average. The appearance of a new clade every million years or so is markedly faster than the rates seen elsewhere in Figure 3.
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.