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Viva Marsupialia!



> It also may be a mistake to assume that placentals have a superior
> reproductive strategy just because they are more succesful than
> marsupials nowadays.  Marsupials held thier own against placentals
> through most of the Cenozoic.

In Australia, marsupials not only held their own in the Cenozoic, but 
may even have triumphed over the placentals.  A tooth from the early 
Tertiary of northeastern Australia (named _Tingamurra_) seems to 
come from a condylarth.  (The Condylarthra is a paraphyletic group of 
placentals that is believed to have given rise to the whales and all 
ungulate groups.)  Placentals (least of all condylarths) weren't 
supposed to be in Australia until the arrival of rats and bats in the 
Quaternary.
 
> However, although you do see some
> big (Mesozoically speaking) mammals toward the end of the Cretaceous,
> saying they could have been in direct competition with any but the
> smallest dinosaurs is absurd.  

I agree, but I'd go one step further and say there was no competition 
at all between Mesozoic mammals and dinosaurs.
There were pretty big (Mesozoically speaking) mammals from Australia 
of mid-Cretaceous age.  A pair of monotremes (_Steropodon_ and 
_Kollikodon_) were at least wombat-sized (or beaver-sized to you 
New Worlders).  There's evidence that both these Mesozoic mammals were 
aquatic.  
Which finally brings me to something that is relevant to the topic of 
dinosaurs.  There were a huge number of niches in the Mesozoic 
(including terrestrial ones) that the dinosaurs didn't occupy and 
mammals exploited.  With the extinction of the dinosaurs, many more 
(many, many, many more) opened up.