Ah, thanks.
Patterns of Vascular Plant Diversification in the Fossil Record: Proof and Conjecture.
There is a lot of potential bias due to increasing morphological complexity (allowing separation into more morphospecies), founder morphologies (leading to diversity among earlier forms being described in higher ranks) and novel adaptations (angiosperms sometimes speciate soley in order to better adapt to a new pollinator). Of course, these changes themselves might also restructure global ecosystems and background evolution, extinction and radiation patterns.
I don't see how that would be connected to "genus lifespans".
I personally suspect that increasing invasibility of ecosystems (globally) due to the Angiosperm radiation, may have been a major driving
factor in faunal turnover (but I have yet to think of a good way to test this).