In modern evolutionary divergence analysis the role of geological information extends beyond providing a timescale, to informing molecular rate variation across the tree. Continuing research I began during my PhD with David Penny, I will use fossil calibrations to test the accuracy of models of molecular rate evolution for placental mammals, and reveal substantial misspecification associated with life history rate correlates. Adding further calibrations to reduce dating errors at specific nodes unfortunately transfers underlying rate errors to adjacent branches. Thus, to buffer against rate model errors we require tight calibration across the tree, including allowing maximum bounds to be tight when good fossil records permit. Otherwise, divergences deep in the tree tend to be inflated by the interaction of rate errors and asymmetric confidence in minimum and maximum bounds. For placental mammals the potential for transferring calibration and rate model errors across the tree is reduced by focusing on well-supported calibrations with appropriately conservative maximum bounds. The resulting divergence estimates are younger than others published recently, and provide the long-anticipated molecular signature for the placental mammal radiation observed in the fossil record near the 66 Ma Cretaceous-Paleogene extinction event.