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

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.

How long? Oligocene? Eocene? And the Miocene itself took a long time, too.

Compare any late Eocene artiodactyls, including *Basilosaurus* and *Dorudon*, and then remember they had a common ancestor around the Palaeocene-Eocene boundary.

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_.

This would tell us something if echidnas had different teeth. But we don't know that.

It could be that _Monotrematum_ was a non-platypodan with ornithorhynchid-like teeth. But the most parsimonious explanation is that _Monotrematum_ is an ornithorhynchid.

As I'm saying: maybe, if *M.* is an ornithorhynchid, then Tachyglossa and Platypoda are subsets of Ornithorhynchidae.