Recommended fossil calibrators for time-scaled molecular phylogenies of Afrotheria

A phylogenetic framework provides the necessary evolutionary context for studies of comparative anatomy, life history, behavior, biogeography, systematics, and conservation. Time-scaled phylogenetic analyses require researchers to include calibration ages which are used to fit a model that transforms tree branch lengths into units of time. The inclusion of multiple calibration ages (if they are available) is a best practice that brings all the available evidence to bear on the temporal model. While selecting the appropriate ages of calibrating fossil taxa is obviously important, perhaps more relevant is where those calibrations are applied in the tree. A misattributed fossil calibrator (e.g., using a stem-taxon to set a minimum age on a tipward crown-node) can severely distort the results. Our aim is to provide a summary of fossil calibrators that can be used in molecular phylogenetic assessments of Afrotheria. The literature that documents these fossils, and discusses their ages and affiliations, is somewhat scattered. Included in this contribution are the oldest extinct species that are (in our opinion) securely attributable to stem lineages of afrotherian clades. Each informs a minimum age for the adjacent rootward tree node. We then assemble a large DNA supermatrix that includes 32 genes and 39 afrotherian genera – including recently extinct elephants and sea-cows. A time-scaled phylogenetic estimate is derived from this dataset with explicit inclusion of fossil taxa in the analysis.