Thanks for your thoughtful comments.
> 3.3 is about the right time for increased northern hemisphere glaciation,
> but there are other factors at that time that could cause the glaciation
> without the added help of a bolide. The bolide could have helped, although
> there seems to be uncertainty as to just what effects it would have.
You adopt an exclusively geologists' viewpoint. The question is not whether
a "bolide" is needed. The question is, given the quality and quantity of
astronomical evidence for the Moon-sized, water-bearing body that blew up at
3.2-3.3 Ma, creating all presently existing comets, many small asteroids,
and certain meteorite classes with consistent exposure ages, how could that
explosion avoid dumping massive amounts of water and several good-sized
impactors onto Earth at that epoch? The astronomical evidence says we have a
sufficient cause. Why try to deny its effects?
> I'm still not clear as to why the mid-Pliocene crater needs an exploding
> planet when there are plenty of ordinary asteroids or comets that could
> do the job.
The mid-Pliocene doesn't need an exploding planet. An exploding moon needs
a corresponding mid-Pliocene record. Something important happened to Earth
then that would have involved, as a minimum, several impacts and a
non-negligible increase in global sea level. Otherwise, we'd have an
explosion with debris that completely missed Earth -- not very likely.
> Also, there are other, bigger impacts on Earth besides the K/T.
> Whether there is any evidence for a Permo-Triassic impact is still
debated.
And there are other, bigger, actual planetary explosion events, not just
the Moon-sized event at 3.2 Ma. The largest of these in the last 1 Gyr
created the main asteroid belt and probably corresponded in time with the
P/T boundary.
Check out my book, "Dark Matter, Missing Planets and New Comets" (North
Atlantic Books, 1993; 2nd edition 1999) to decide for yourself whether or
not the evidence quality and quantity is sufficient to justify these
conclusions.
Best wishes. -|Tom|-