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RE: Platypuses may be older than we think...

David Marjanovic wrote:

> Fine, but the platypus digs burrows, too, so it's not that far away.

The platypus excavates burrows in the riverbank.  A distinction without a 
difference, I guess, given that your point was that platypodans dig too.

>> _Obdurodon_ (for which a beautiful skull is known) is certainly a bona
>> fide platypus.
> Agreed, but it's only Miocene.

The fossil tachyglossid _Megalibgwilia_ goes back to the Miocene.  

> Then maybe the echidnas are ornithorhynchids and closely allied with
> *Obdurodon*, too. 

That seems like a counsel of despair.  Given how specialized the two lines were 
by the Miocene, the tachyglossans and platypodans must have diverged a long 
time before.   

> With the presently known fossils, we won't find out. We
> still don't know any tachyglossan teeth, and we still don't know anything
> but teeth from *Monotrematum*.

Yes, but the teeth of _Monotrematum_ show a striking resemblance to the teeth 
of the ornithorhynchid _Obdurodon_.  It could be that _Monotrematum_ was a 
non-platypodan with ornithorhynchid-like teeth.  But the most parsimonious 
explanation is that _Monotrematum_ is an ornithorhynchid.



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