Cladistics Illuminate other Sciences
I. Phylogenies and Biostratigraphy: Geologists who use the fossil record to date sedimentary rock units tend to be literal-minded souls who think that either fossils of an organism or group are known to be present at a particular time, or they aren't. Consideration of the cladogram tells us when we ought to be cautious about such literal interpretations.
A. Ghost Lineages: When we know that two taxa are sister taxa, we in essence know that they originated at the same point in geologic time - the time of the speciation event that gave rise to them. Say we know one of these taxa from 100 million years ago, and the other from 90 million years ago. Even without seeing a fossil, we know that the second group must have representatives dating back at least to 100 million years, simply from its sister-taxon relationship with the other. A lineage like this, whose existence can be inferred from the cladogram, but which is not known from actual fossils is called a ghost lineage. The examination of ghost lineages should allow biostratigraphers to refine their models of the stratigraphic ages of organisms.
B. Stratigraphic congruence:. How do we identify ghost lineages and measure their prevalence in a cladogram. All other things being equal, we expect the terminal taxa that branch off of a cladogram first to appear first in the fossil record. When this is true, the cladogram is said to be stratigraphically congruent. Often, cladograms are not stratigraphically congruent. This happens when there are long ghost lineages.
C. Measuring Stratigraphic congruence:. A simple measure of stratigraphic congruence is the Stratigraphic Consistency Index (SCI) of John Huelsenbeck. To calculate it, count the number of stratigraphically consistent nodes in a cladogram. A node is stratigraphically consistent if both of the lineages emerging from it occur later than the node's sister taxon. Divide the number of consistent nodes with the total number of nodes in the cladogram to get the SCI.
D. Examples: I provided two examples of what ghost taxa and stratigraphic inconsistency can tell us about the fossil record:
- Choristoderes. In 1984, when Gauthier performed the first major cladistic analysis of Diapsida, he got a surprise. Choristodera, whose members were known from the Late Cretaceous and Paleocene, appeared to have branched off of the archosauromorph tree by the late Permian. Thus, they sat at the end of a ghost lineage that persisted for over 150 million years. This seemed highly improbable. Was the cladogram wrong or our understanding of the fossil record? As paleontologists began reexamining museum collections, it became apparent that some neglected partially preserved creatures were, in fact, choristoderes. Today, the choristodere record starts in the late Triassic. My dissertation research, for what it's worth, suggests that Choristoderes are sister taxon to rhynchosaurs. Thus, the choristodere lineage may only date to the Early Triassic. Their ghost lineage has been confined to no more than 45 million years. In this case, the ghost lineage both encouraged paleontologists to refine their search image and offered an independent criterion for chosing between hypotheses.
- Placodonts. Most placodonts are known from the Triassic Alpine and Germanic basins of central Europe. In my dissertation research, their phylogeny is pretty consistent with one conspicuous exception. The weirdo Henodus sits at the end of a long ghost lineage. Does this mean that the cladogram is wrong? Far from it. Henodus lived in brackish water, but the rocks of the Germanic and Alpine basins preserve exclusively marine environments except at the very top of the sequence, in their youngest rocks. These show brackish environments and contain Henodus. The ghost lineage tips us off to the fact that the rock record has an environmental sampling bias.