ALROY, J.; University of California: Reconstructing sequences of speciation and extinction events in the fossil record

Paleontology has long offered three main products to the larger scientific community: the reconstruction of ancient environments; the morphology and relationships of primitive organisms; and the age of biotic events, which can constrain the ages of sedimentary rocks. Traditionally, biotic events were put in time order by expert opinion. The last several decades have witnessed more and more application of quantitive methods to this problem of “biochronology.” There are two kinds of events – first (F) and last (L) appearances – and four possible relations among them: F before (<) F, L < L, L < F, and F < L. The goal of biochronology is therefore to use these relations to define sequences of events. Collection of more fossils always can invalidate all of the relations save the last, which are irrefutable. A method called appearance event ordination concentrates on these F < L relations, minimizing the number that are implied by an ordering but are not demonstrated yet by fossils. The method has been generalized to incorporate maximum likelihood statistics, and has been used to reconstruct the time scale for all North American fossil mammals. Event sequences have many applications. First, they constrain the relative timing of divergences among living clades. Second, they constrain phylogenetic reconstruction by showing whether hypothesized sister groups originated at similar times. Third, they define rates of biotic turnover and levels of species diversity, making it possible to better study mass extinctions and diversity dynamics. The AEO-based event sequence has been used to show, for example, that climate change and bolide impacts have had very little impact on mammalian evolution; that diversity tracks a rough equilibrium; and that there is a trend toward increasing body size in mammals.