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Re: Segregated vs age-mixed sauropod herds

T. Michael Keesey <keesey@gmail.com> wrote:

> Isn't it quite possible that Monotremata and Placentalia
> (crown clades) are around the same age?

That seems to be the conclusion of several recent phylogenetic studies.  The 
respective crown clades of Monotremata and Placentalia, as well as Marsupialia, 
were all found to have emerged in the Cenozoic.  

In fact, one study has the crown Monotremata (the platypus + echidna clade) 
first appearing only 20 million years ago.  (This same study proposes that the 
terrestrial echidnas evolved from aquatic platypus-like ancestors.)