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. However, it is important to note that while the age of first appearance of each stem group, which is a terminal node of the tree, is known, that of the intermediate nodes is not directly constrained by fossil evidence. The software used to construct the tree (Bell and Lloyd, 2015) follows logical rules to ensure that the tree honors all the fossil ages, but the user can control the appearance of the tree in terms of branch length (time between the division of one branch into two branches and the appearance of fossils on the branches, or between two successive intermediate nodes). The input parameter is minimum branch length, or “MBL”. For the above tree, an MBL of 1 million years was selected because this minimized the occurrence of crown nodes that pre-date the age first appearance of the corresponding stem group. (Such a configuration is impossible because the crown group cannot appear before the stem group.)
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 branch length, from the specified minimum value of 1 million years to more than 170 million years, as is the case with the birds (Aves). These variations represent differences in the time interval between the appearance of stem groups and the formation of two new clades at the next crown node.
Another way of representing this is variation to plot the number of new clades (represented by stem group fossils) appearing in successive intervals of geological time:
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 branch length, from the specified minimum value of 1 million years to more than 170 million years, as is the case with the birds (Aves). These variations represent differences in the time interval between the appearance of stem groups and the formation of two new clades at the next crown node.
Another way of representing this is variation to plot the number of new clades (represented by stem group fossils) appearing in successive intervals of geological time:
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.