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

[David Marjanovic <david.marjanovic@gmx.at>]

> > "Undoubtedly"?
>
> You don't think so?  What if Rowe &c are correct?  As it says in the SVP 
> abstract, which you kindly posted 
> (http://dml.cmnh.org/2007Oct/msg00309.html)...
>
> "New evidence from high-resolution X-ray computed tomography indicates 
> that *Teinolophos*, an Early Cretaceous fossil from Australia's Flat Rocks 
> locality (121-112.5 Ma), lies within the crown clade Monotremata, as a 
> basal platypus. Divergence of the two monotreme clades therefore had 
> occurred in or before the Early Cretaceous."

I interpret this to mean that the tomography shows the presence of a nerve 
that ended in a bill with electrosensory organs. If this nerve is 
secondarily reduced in the echidnas (which have a bill and an apparently 
reduced electro-sense), we can't tell where on the monotreme tree 
*Teinolophos* lies.

> I know this contradicts Bininda-Emonds et al. (2007; Nature 446: 507-512), 
> who provide a molecular-clock-based divergence at around the end-K 
> extinction for the echidna and platypus lineages.

Worse. They used the latest possible date of this divergence to _calibrate_ 
their tree.

Still worse: they got that minimal age by means of assuming that the 
Palaeocene *Monotrematum* was a platypodan. Problem is, *M.* is only known 
from two teeth, and not a single tachyglossan tooth is known, so we can say 
absolutely nothing about its phylogenetic position with respect to 
Tachyglossa.

> Thus, if Bininda-Emonds &c are correct on the divergence times of the 
> major placental groups

There are several more mistakes in their calibration points; several times 
they used the appearance of total groups as the appearance of crown-groups.

Advice to peer-reviewers: Read the whole supplementary information, tables 
included.