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: