Hi!  I recently posed a question about the lack of a supraspinous fossa and received a spectacular answer from John Whitlock.  I now have another question for you!  I have spent many hours trying to find the importance of the lack of fusion of the tibia and fibula in Multituberculates.  In "Mesozoic Mammals The first Two Thirds of Mammalian History" they have included this characteristic along with a slew of others that somehow distinguishes them from other mammals.  So..... What is so important about this lack of fusion?
   Is fusion a primitive trait?  And if so, do cynodonts and morganucodons have unfused tibia/fibula?

It seems odd that this character has been identified as important, given that primates also lack this fusion. The book you are referring to is pretty old (1979) so I think it may be information that is no longer valid with respect to systematic taxonomy.