A time-calibrated phylogeny inferring the origin of unisexual Ambystoma , fitted to a geological timescale. Times for nodes [A-F] are estimated by BEAST. The horizontal gray bars through these nodes indicate 95% credibility intervals from the BEAST analyses. Non-ambystomatid specimens are not shown in the tree. Detailed time estimates can be found in Table 4. An early Pliocene origin for unisexual Ambystoma is proposed.