A reader writes:
"Neither the Cenozoic nor any of its formally recognized epochs are named after a species, a geological force or an event. They are all named after faunal composition."
Precisely. There are logical and epistemological problems with such classifications, as geologists (with whom I've spoken) have observed. These have to do with the (false) conception of time as a linear series of now-points.
The fault the reader observes is in fact a virtue. It would be better to name periods along the lines of the Anthropaocene, as I've argued at Chicago (talk mp3 posted here). This is because geological/ecological time is a series of concentric temporalities whose boundaries are catastrophes, such as oxygen (the "Bacteriocene").