Re: Hell Creek and stegosaurs

["David Marjanovic" <david.marjanovic@gmx.at>]

> "The little...mass extinction." ??  When is a mass extinction not a mass
> extinction?

Forgot if there's a more official definition... each peak above the
background rate of extinctions is a mass extinction. Said background appears
to be pretty constant, though maybe that's a problem with the scale, to some
extent. Only extinction counts, pseudoextinction -- when a species, ignoring
all problems with species concepts, changes into one or more descendant
species -- doesn't. The Tertiary examples: in the Pliocene (2.3 Ma ago); in
the mid-Miocene (11 or 15 Ma ago); _very_ shortly before the IMHO
ill-defined end of the Eocene; and maybe there's one at the Paleocene/Eocene
boundary.