Many of the clades depicted in the tree shown in Figure 1 of the Tetrapods page do not have fossils that have been defined by consensus in the published literature as members of their stem groups. Most of these clades are in the placental mammals (Eutheria), but they also occur in the marsupials (Marsupialia), the basal reptiles and birds (Sauria) and the amphibians (Amphibia). This apparent absence of stem group fossils raises the question as to the age that such fossils would have if they were to be discovered in the future. We will consider this question for each of the clades mentioned above.
In the amphibians, there is a single clade, Batrachia, for which no stem group fossils are known. The following time tree illustrates the time constraints on the age of the stem-Batrachia:
In the amphibians, there is a single clade, Batrachia, for which no stem group fossils are known. The following time tree illustrates the time constraints on the age of the stem-Batrachia:
Figure 1. Phylogenetic time tree of the basal amphibians with possible position of unknown batrachian stem group
The positions of the batrachian stem group is shown conceptually in the above tree, but should not be considered as actually correct. The constraints are quite wide: the oldest stem group fossil would have had to have appeared no earlier than the oldest stem amphibian and no later than the oldest member of the stem-Anura, a time interval of just over 80 million years. However, there is a problem here.
Given that the stem-Batrachia and the stem-Gymnophiona are both the product of the division of the amphibian stem line at the crown node of the Amphibia, they should both have appeared at the same time. But, as can be seen in Figure 1, the youngest possible age for the oldest member of the batrachian stem group is older – around 30 million years - than the oldest member of the stem-Gymnophiona. This discrepancy comprises a “ghost lineage” on the cladistian stem line, shown as a blue bar in Figure 1. The supposition would be that much older stem group fossils of the Gymnophiona might be found in the future. But if this is not the case, and the stem-Gymnophiona really are as young as indicated in Figure 1, we would need to acknowledge that the time tree shown in Figure 1 does not properly represent the phylogenetic relationships of the basal amphibians.
The Sauria contain two clades without known stem group fossils – the Lepidosauria and the Archelosauria. The following time tree illustrates the time constraints on the age of their respective stem groups:
Given that the stem-Batrachia and the stem-Gymnophiona are both the product of the division of the amphibian stem line at the crown node of the Amphibia, they should both have appeared at the same time. But, as can be seen in Figure 1, the youngest possible age for the oldest member of the batrachian stem group is older – around 30 million years - than the oldest member of the stem-Gymnophiona. This discrepancy comprises a “ghost lineage” on the cladistian stem line, shown as a blue bar in Figure 1. The supposition would be that much older stem group fossils of the Gymnophiona might be found in the future. But if this is not the case, and the stem-Gymnophiona really are as young as indicated in Figure 1, we would need to acknowledge that the time tree shown in Figure 1 does not properly represent the phylogenetic relationships of the basal amphibians.
The Sauria contain two clades without known stem group fossils – the Lepidosauria and the Archelosauria. The following time tree illustrates the time constraints on the age of their respective stem groups:
Figure 2. Phylogenetic time tree of the basal saurians with possible positions of unknown stem groups
Given that the lepidosaurs and the archelosaurs are sister clades we can make the assumption that their stem groups appeared at the same time. If this is true, both stem groups must have appeared no earlier than the earliest stem saurian and no later than the oldest member of the stem-Archosauria, an interval of about 58 million years. There are no discrepancies of the type discussed above for the amphibians.
Turning now to the marsupials, we find that there are no known stem groups of intermediate nodes within the crown group. The following tree illustrates the time constraints on the age of five such stem groups:
Turning now to the marsupials, we find that there are no known stem groups of intermediate nodes within the crown group. The following tree illustrates the time constraints on the age of five such stem groups:
Figure 3. Phylogenetic time tree of the marsupials with possible positions of unknown stem groups
The two most basal stem groups are not tightly constrained. They must have appeared no earlier than the oldest member of the stem-Marsupialia, and no later than the earliest stem group fossil of the Microbiotheria, a time period of around 60 million years. The next stem group, that of the Agreodontia/Diprodontia clade, should have appeared at the same time as that of its sister clade, the Microbiotheria, while the two remaining stem groups are constrained in age between the ages of the Microbiotheria and the Stem-Peramelemorphia or total-group Dasyuromorphia.
Again, we have a problem with ghost lineages. In the absence of discovery of older fossils than those currently known from the Notoryctemorphia, Didelphimorphia and Paucituberculata, the marsupial phylogeny shown above remains open to question.
The final tetrapod clade with unknown stem groups is the Eutheria. As illustrated below, there are 20 intermediate nodes for which stem groups have either not been described (the majority) or have not been defined by consensus in the published literature.
Again, we have a problem with ghost lineages. In the absence of discovery of older fossils than those currently known from the Notoryctemorphia, Didelphimorphia and Paucituberculata, the marsupial phylogeny shown above remains open to question.
The final tetrapod clade with unknown stem groups is the Eutheria. As illustrated below, there are 20 intermediate nodes for which stem groups have either not been described (the majority) or have not been defined by consensus in the published literature.
Figure 4. Phylogenetic time tree of the Eutheria with possible positions of unknown stem groups
Without going into the details of each unknown stem group, it can be seen from the above tree that several unknown stem groups imply long ghost lineages in their sister clades (e.g. the stem-Tylopoda and the stem-Suina). Clearly, older fossils need to be found from many stem groups of the eutherian crown group before the tree shown above can be considered as beyond question.