The Evolution of the Cactus
Brown University biologists and colleagues have discovered that the rapid speciation of cacti occurred between 5 and 10 million years ago (the late Miocene) and coincided with species explosions by other succulent plant groups around the world.
I haven’t had time to read this through yet, so I’m leaving this here as a marker to get back to later. Here’s another quote in the meantime.
Cactaceae first diverged from its angiosperm relatives roughly 35 million years ago but didn’t engage in rapid speciation for at least another 25 million years.