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

[Ken Kinman]

Dear Hans,
      Adding to what Brian said, I think you need to take a careful look at how the K-T extinction (at the beginning of the Cenozoic) affected different groups of organisms.  Especially on land, it was far more devastating to vertebrate taxa (birds in particular, but mammals didn't do much better).  Being an endotherm is energetically expensive and thus there is a very high price to pay during a mass extinction.  The relatively few surviving lines of birds and mammals then rapidly diversified during the Paleocene and Eocene, but the tempo began to slow once empty niches were reoccupied (for example, even Eocene bats look much like today's bats).  By comparison, spiders and scorpions didn't suffer a catastrophic reduction in their biodiversity during the K-T extinction.  A lot more of them nest underground, they can endure long periods without food, and they recovered quickly because their insect food supply recovered quickly.  Not surprisingly, most mammals which survived were small, burrowing, insectivores.  So I think the answer to your question ultimately lies in just how catastrophic the K-T extinction was on different groups.
  --Cheers,
         Ken Kinman