why is ‘cambrain explosion’ such a crucail time

Question: why is 'cambrain explosion' such a crucail time period for fossils.

Asked by 07cross to Duncan on 15 Mar 2010 in Categories: Theories.
- Duncan Murdock answered on 15 Mar 2010:
  
  Before the Cambrian Explosion the only fossils we find are soft-bodied animals a bit like jellyfish, then, suddenly, there appeared a whole load of fossils with shells, bones and teeth. We don’t really know what caused animals to suddenly get hard parts, or if something changed in the oceans to change how fossils get preserved. I’m trying to answer parts of these questions.

07cross commented on 19 Mar 2010:

thank you for answering my question with such detail. I will be voting for next week + asking alot more questions hope you win 🙂
- Duncan commented on 21 Mar 2010:
  
  Thank you, I look forward to getting more questions.

Question: why is 'cambrain explosion' such a crucail time period for fossils.