ANGIELCZYK, K.D.; California Academy of Sciences: What is the role of stratigraphic data in phylogeny reconstruction?

The inclusion of stratigraphic data in phylogeny reconstruction is a contentious issue in paleontology, but most of this debate has focused on whether stratigraphic data should be included in phylogenetic analyses. The question of why we might want to include stratigraphic data has received less attention, and most workers who support its inclusion contend that it will result in more accurate hypotheses of relationships. This contention is difficult to test because we have no means of knowing the accuracy of our empirical phylogenies, and the simulations presented to date are only rough approximations of biological reality. Here, I approach this question from a different perspective, namely that of historical signal. Ideally, we would expect both stratigraphy and morphology to preserve the same historical signal, and if this is the case, the inclusion of stratigraphic data in phylogenetic analysis should not strongly affect how well the results fit the known fossil record. If a significant increase in fit occurs with the inclusion of stratigraphic data, then there must be conflict in the preserved signals, and both morphology and stratigraphy must be reassessed. A preliminary comparison of results from recent phylogenetic studies that have included stratigraphic data suggests that hypotheses built using stratigraphy usually fit the fossil record better than those based on morphology alone. However, much of this gain is the result of allowing real taxa to occupy ancestral positions on phylogenetic trees. When ancestorization is not allowed, there is usually not a significant difference in the fit of the two classes of hypotheses to the fossil record. These results suggest that a standard cladistic approach may not be sensitive to subtle differences between the signal preserved in stratigraphy and morphology.