mammalian lineages (was Re: Cretaceous taeniodont)

["Eric Martichuski" <herewiss13@hotmail.com>]

> Yep, you're right.  _Gobiconodon_ and _Repenomamus_ are triconodonts.

I wonder if someone might clarify this area for me.  I've read that there 
was a third (now extinct) lineage of mammals, but I've never been able to 
figure out just _how_ they were different.  The name seems to indicate 
dentition, but that seems too minor to declare such a radical separation.

Do we know if they were placental too or what?  If there a simple clade 
somewhere identifying the branching order?

I guess I'm just having trouble wrapping my brain around the idea of mammals 
that aren't placental _or_ marsupial (or monotreme).  What else is could 
there be? ;-)

Thanks,
Eric
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