CarolC: Thanks for those links, but I still disagree. This idea of 'primitive' is not accepted in modern palaeontological circles; This article explains it about as well as any and centres around the derived characteristics of Platypi (?). Note that it's thought the marsupials and placentals branched off the mammalian lineage at the same time, so one is not more advanced than the other, they're different. It's worth checking out his other blogs too.

Actually, I am wondering if we're arguing slightly at cross-purposes here because when I was reading this blog on the mammalian family tree I realised we're both right in a sense, although I still dispute the fact modern montremes and marsupials are more primitive than placentals.

Good debate : )