You wrote: "...a lack of craters corresponding to most mass extinctions"

Short reply: Earth is ¾ water. We may expect that up to ¾ of impacts may leave no obvious trace.

As no less an authority that the late S. J. Gould has noted (my paraphrase), more than "survival of the fittest" evolution is more likely to have been driven by "survival of the lucky."

Africa has been spared recent impacts and the elephant, rhino, and hippo survive. Elsewhere, 30 some species of pachyderm have expired.

Nevertheless, I think there is some truth to the idea that the survivors of impacts are more likely to be small, agile creatures rather than the megafauna.

Additionally, the sudden introduction of more advanced species into an area previously isolated may result in the extermination of the older species by the new arrivals. The new arrivals might have originated in an area where impacts had accelerated evolution by creating voids in favorable ecological niches whereas the newly opened area may have been spared impacts and deprived of the resulting evolutionary acceleration. Such was the case with the re-opening of the land bridge between N. and S. America in the relatively recent past.

Hurridly composed in one go, so I've probably left myself wide open.

Have at it.

;o)
sp

ps. Mark, I was distressed to read earlier of your son's illness and I wish you all godspeed and a happy outcome.