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

[Tim Williams <twilliams_alpha@hotmail.com>]

David Marjanovic wrote:

>> The divergence between the platypus and echidna lineages undoubtedly
>> pre-dates the divergence between any two crown-group placental orders.
>
> "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 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.  They also favor a eutherian diversification 
("hotspot") around 100 Ma, when "all four placental superorders (Afrotheria, 
Euarchontoglires, Laurasiatheria and Xenarthra)" emerged.  Thus, if 
Bininda-Emonds &c are correct on the divergence times of the major placental 
groups (and way off on the intra-monotreme divergence times), then no 
crown-group placental "order" could have appeared prior to the platypus-echidna 
split.  Rowe &c place the latter "in or before the Early Cretaceous."  So the 
first monotreme had taken the plunge way back when placentals were just a 
twinkle in the eye.

>> Given just how distinct the two modern monotreme lineages are,
>
> Surely the same must hold for cows and dolphins...?

I meant temporally.  Though I'm not denying they're highly morphological 
distinct - although this is obviously difficult to quantify.

>> the names Platypoda and Tachyglossa might turn out to be useful for future
>> phylogenetic usage.
>
> Agreed.

Yep, Rowe &c refer to _Teinolophos_ as a "basal platypus", so perhaps we could 
substitute "platypodan".  _Teinolophos_, _Steropodon_, and _Kollikodon_ would 
all be "basal platypodans".  Tachyglossans, AFAIK, are known only from the 
crown-group.

Cheers

Tim

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