mammals morphologically evoluated a lot since the Cenozoïcum, invertebrates did not.Why?

[Hans Henderickx]

Dear colleagues,

Especially based on amber fossils, it can be concluded that invertebrates
did not evoluate significant since the early Cenozoïcum. They were other
species, but very much alike the recent ones and often from the same genus.
(Lourenço, Henderickx & Weitschat 2005 (scorpions), Vitali 2005
(Coleoptera),  Heiß (Heteroptera). Francesco Vitali and I are discussing
this paradox. The evolution of insects, pseudoscorpions and spiders is
practically motionless for millions of years, and the ancient forms are
often even more specialized (f.i. Pseudogarypus.) That is very different
from the evolution of Mammalians. This fact is very strange because mammals
are able to have only few generations each century. Hence, their evolution
should be much slower than that of arthropods. Instead we are observing the
contrary. Is it because mammals are a much more recent group and have not
reached a stabilized status yet? Who can help us finding an explanation to
this fact?

Hans Henderickx