Two of the clades depicted in the tree shown in Figure 1 of the “Vertebrates (up to tetrapods)” page, the Actinopteri and the Rhipidistia, do not have fossils that have been defined by consensus in the published literature as members of their stem groups. This raises the question as to the age that such fossils would have if they were to be discovered in the future. The following time tree illustrates the time constraints on the age of such stem fossils:
Figure 1. Phylogenetic time tree of the basal Osteichthyes with possible positions of unknown stem groups
The positions of the two stem groups are shown conceptually in the above tree, but should not be considered as actually correct. For the stem-Actinopteri, the constraints are quite wide: the oldest stem group fossil would have had to have appeared no earlier than the oldest stem actinopterygian and no later than the oldest member of the stem-Chondrostei, a time interval of just over 80 million years. However, there is a problem here.
Given that the stem-Actinopteri and the stem-Cladistia are both the product of the division of the actinopterygian stem line at the crown node of the Actinopterygii, they should both have appeared at the same time. But, as can be seen in Figure 1 above, the youngest possible age for the oldest member of the actinopteran stem group is much older – nearly 100 million years - than the oldest member of the stem-Cladistia. 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 cladistian fossils might be found in the future. But if this is not the case, and the stem-Cladistia 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 Actinopterygii.
No such problem occurs with the stem-Rhipidistia. The time constraints are very narrow: no older than the oldest member of the stem-Sarcopterygii and no younger than the oldest stem dipnoan: a period of about 8 million years. Furthermore, there is no inconsistency with the time of appearance of the stem-Coelacanthi, which is the sister group to the stem-Rhipidistia.
Given that the stem-Actinopteri and the stem-Cladistia are both the product of the division of the actinopterygian stem line at the crown node of the Actinopterygii, they should both have appeared at the same time. But, as can be seen in Figure 1 above, the youngest possible age for the oldest member of the actinopteran stem group is much older – nearly 100 million years - than the oldest member of the stem-Cladistia. 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 cladistian fossils might be found in the future. But if this is not the case, and the stem-Cladistia 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 Actinopterygii.
No such problem occurs with the stem-Rhipidistia. The time constraints are very narrow: no older than the oldest member of the stem-Sarcopterygii and no younger than the oldest stem dipnoan: a period of about 8 million years. Furthermore, there is no inconsistency with the time of appearance of the stem-Coelacanthi, which is the sister group to the stem-Rhipidistia.