Current debates about which are the “best” fossil dates for calibration move to consideration of the most appropriate constraints on the ages of tree nodes.Because fossil-based dates are constraints, and because molecular evolution is not perfectly clock-like, analysts should use more rather than fewer dates, but there has to be a balance between many genes and few dates versus many dates and few genes.The role of fossils in dating the tree of life has been misunderstood.Fossils can provide good “minimum” age estimates for branches in the tree, but “maximum” constraints on those ages are poorer.Good estimates of rate variation are required from the well-calibrated regions of the tree so that the pattern can be extrapolated to other parts of the tree that are poorly calibrated.Furthermore, molecular clock analyses are rarely, if ever, framed around the availability of reliable calibration dates.Fossil data are fundamental to molecular clock methodology, providing the key means of clock calibration, but their commonplace use is far from satisfactory.We consider the utility and qualities of good calibration dates and, on that basis, we propose a number of well-supported dates, and give ages based on the best current information.
However, the widespread congruence between the order of fossils in the rocks and the order of nodes in cladograms (Norell and Novacek 1992; Benton et al.
Note that relaxed-clock methods can often require at least one point calibration or hard maximum constraint in order for the algorithm to converge on a unique solution.
So, debates about the superiority of one “calibration” date or another are irrelevant in the context of a search for the most appropriate distribution of dates and minimum and maximum constraints—the only bad dates are those that predate the evolutionary event upon which they are supposed to provide a minimum constraint.
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