UNDERSTANDING THE DRAMATIC EFFECT OF PALEONTOLOGICAL DATA IN PHYLOGENETIC INFERENCE BY EXPLORING THE DETERMINANTS OF TOPOLOGICAL IMPACT

The majority of organisms that have ever lived are extinct. A significant proportion of this diversity has been preserved in the fossil record, which often provides the most direct evidence of evolution through deep time. Consequently, incorporating fossil evidence has the potential to drastically improve our understanding of evolutionary processes and patterns, as well as expand the scope of questions that can be addressed.

Even though the relevance of paleontological data for comparative paleobiology has long been recognized, its role in inferring the tree of life itself remains controversial. Since the early years of phylogenetic systematics different studies have either dismissed the impact of fossils due to their incompleteness, championed their ability to bridge morphological gaps and overturn phylogenetic hypotheses or concluded that their behavior is indistinguishable from that of extant taxa.

We have developed a series of taxon inclusion-exclusion experiments to quantify the impact that the addition of fossil terminals has on phylogenies built from extant taxa. Applying this approach to a set of large empirical morphological matrices, we find that the topological impact of fossils is generally stronger than that induced by increasing sampling among extant lineages. Fossils also induce the exploration of significantly different regions of treespace, which might underlie their ability to increase congruence with molecular topologies. These results are common to inference under both parsimony and Bayesian approaches.

We further attempt to explain this unique effect of fossils by exploring the determinants of a taxon’s topological impact. Our quantitative analysis largely confirms previous hypotheses, showing significant effects of morphological distinctiveness, missing data, ‘primitiveness’ and distance to cladogenetic events, among other variables. Our results have implications for designing morphological matrices, providing intuitive guidelines for sampling of potentially impactful fossil taxa.