Calibration of avian molecular clocks

Molecular clocks can be calibrated using fossils within the group under stu dy (internal calibration) or outside of the group (external calibration). B oth types of calibration have their advantages and disadvantages. An intern al calibration may reduce extrapolation error but may not be from the best fossil record, raising the issue of nonindependence. An external calibratio n may be more independent but also may have a greater extrapolation error. Here, we used the advantages of both methods by applying a sequential calib ration to avian molecular clocks. We estimated a basal divergence within bi rds, the split between fowl (Galliformes) and ducks (Anseriformes), to be 8 9.8 +/- 6.97 MYA using an external calibration and 12 rate-constant nuclear genes. In turn, this time estimate was used as an internal calibration for three species-rich avian molecular data sets: mtDNA, DNA-DNA hybridization , and transferrin immunological distances. The resulting time estimates ind icate that many major clades of modern birds had their origins within the C retaceous. This supports earlier studies that identified large gaps in the avian fossil record and suggests that modern birds may have coexisted with other avian lineages for an extended period during the Cretaceous. The new time estimates are concordant with a continental breakup model for the orig in of ratites.