Compare figure 22.13, page 633. Once question is how much variation one tolerates before saying that extinction rates are not approximately linear. One interesting observation about extinction patterns is that a periodicity has been documented in one data set. The cycle appears to be 26 million years . See figure 23.7, page 652.
Mass extinctions are quite a different type of extinction than the background extinctions.
Extinctions are some of the best studied. However, many groups were relatively unaffected. See figure 23.5, pg. This element is rare in earth's crust, but not uncommon in meteorites. Or maybe it landed near a subduction zone and the evidence has been conveniently tucked under some continental plate.