Selection of Fossils for Calibration of Molecular Dating Models1

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Abstract
In the past decade, there has been a rise in interest in the plant fossil record. Fossils potentially provide information for assessing homology and evolutionary change (e.g., the popular missing link phenomenon), character evidence that affects phylogenetic conclusions and, thus, our understanding of modern relationships, evidence of past distributions that can aid in understanding biogeographic histories, and estimates of minimum ages of the clades to which they belong. Recently, many molecular biologists have used fossils in their analyses as a way of providing a calibration point for evolutionary models used to approximate ages for the nodes of phylogenetic trees. However, there has been little, if any, discussion of the criteria by which calibration fossils can be selected for these studies. When considering the use of a fossil as a calibration point, it is critical to take into account the quality of preservation, the method and details of identification (reliability of the taxonomic placement), and the accuracy of the published age. Here, we provide basic criteria for the use of fossils to calibrate molecular evolutionary models. These approaches not only provide better primary estimates for ages of clades, but also provide more reliable sources for those molecular biologists wishing to clean up their molecular clocks.