Calibrations from the Fossil Record

Molecular clocks can be used to reconstruct evolutionary timescales based on analyses of genetic data, but these clocks need to be calibrated in order to give estimates in absolute time. Calibration is most often carried out using fossil evidence of the timing of evolutionary events, corresponding to internal nodes in phylogenetic trees. Early molecular dating studies treated fossil calibrations as point values, whereas later methods allowed calibrations to be specified as age constraints on nodes. The application of Bayesian methods to phylogenetic analysis opened up opportunities for fossil calibrations to take more complex forms. In this chapter, we trace the development and use of fossil calibrations and describe some a priori and a posteriori methods and criteria for evaluating their quality. We then present two examples of fossil calibrations from modern birds. Our chapter concludes with a discussion of the limitations of fossil calibrations, along with the changing role of the palaeontological record in molecular dating.