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RE: Segregated vs age-mixed sauropod herds



Anthony Docimo <keenir@hotmail.com>


> > In fact, one study has the crown Monotremata (the
> > platypus + echidna clade) first appearing only 20 million
> > years ago. 
>  
>  um...you mean 20 million years earlier, right?



No, I mean 20 million years before the present.  The study in question 
(Phillips et al., 2009; PNAS 106: 17089â17094) estimated that the platypus and 
echidna diverged anywhere from 19-48 million years ago.  Whichever date it was, 
it is at this time that crown Monotremata first appeared.


However, because the lower Miocene ornithorhynchid _Obdurodon dicksoni_ has a 
very platypus-like skull (and so likely post-dates the split) the emergence of 
crown Monotremata might have occurred quite some time before the most recent 
end of the spectrum (19 mya).  


Ornithorhynchids have also been reported from the upper Oligicene (_Obdurodon 
insignis_) and lower Paleocene (_Monotrematum sudamericum_).  But because they 
are known from fragmentary material (mostly teeth) it is possible that these 
taxa (especially _Monotrematum_) are stem monotremes - and that their 
platypus-like characters are therefore primitive for the crown Monotremata 
clade.  This is what Phillips et al. (2009) propose.


The same study estimated that the monotreme line (stem-group - would that be 
'Prototheria'??) diverged from the Theria in the Early Jurassic.   


Cheers

Tim