The node calibration procedure used for dating the cycads. (a) Phylogenetic tree of cycads showing the relationships among the 10 genera. Genera are represented by triangles proportional to their species richness. Numbers in parenthesis are the number of species sampled, and the total number of species, within each genus. Pictures illustrate Cycas, Dioon, Zamia, Macrozamia, and Encephalartos species. Black dots indicate the four ‘traditional’ fossil calibrations used for dating, and red dots indicate the two new fossil calibrations evaluated in this study. (b) Information related to the four fossil calibrations (FC, see the text for more details). Ma, million years ago.