Most of these went extinct as a result of background extinction.
Extinctions can also occur at intermediate levels, either in a taxonomic or a geographic sense.
What concerns us most is the other end-member extreme, mass extinction . Note that plot shows number of families . On a geologic time scale , mass extinctions appear to be spread through time on the order of 100 m.y. Other subevents can also be seen. The mean lifespan of genera is about 20 m.y., although genera that have life spans shorter than this vastly outnumber those having longer life spans.