A simple method for bracketing absolute divergence times on molecular phylogenies using multiple fossil calibration points

A central challenge facing the temporal calibration of molecular phylogenies is finding a quantitative method for estimating maximum age constraints on lineage divergence times. Here, I provide such a method. This method requires an ultrametric tree generated without reference to the fossil record. Exploiting the fact that the relative branch lengths on the ultrametric tree are proportional to time, this method identifies the lineage with the greatest proportion of its true temporal range covered by the fossil record. The oldest fossil of this calibration lineage is used as the minimum age constraint. The maximum age constraint is obtained by adding a confidence interval onto the end point of the calibration lineage, thus making it possible to bracket the true divergence times of all lineages on the tree. The approach can also identify fossils that have been grossly misdated or misassigned to the phylogeny. The method assumes that the relative branch lengths on the ultrametric tree are accurate and that fossilization is random. The effect of violations of these assumptions is assessed. This method is simple to use and is illustrated with a reanalysis of Near et al.'s turtle data.