RE: Fossil suggests platypus lived in dinosaur times

[Tim Williams <twilliams_alpha@hotmail.com>]

Steve Walsh wrote:

> Being from the Antipodes I wanted to alert on this one...
>
> http://www.abc.net.au/news/stories/2008/01/22/2143811.htm


As a fellow antipodean, I'm happy to provide the reference for the article 
(which I think is open access)...


Rowe, T., Rich, T.H., Vickers-Rich, P., Springer, M., and Woodburne, M.O. 
(2008).  The oldest platypus and its bearing on divergence timing of the 
platypus and echidna clades.  Proc. Nat. Acad. Sci. U.S.A. 105(4): 1238-1242.


ABSTRACT. "Monotremes have left a poor fossil record, and paleontology has been 
virtually mute during two decades of discussion about molecular clock estimates 
of the timing of divergence between the platypus and echidna clades.  We 
describe evidence from high-resolution x-ray computed tomography indicating 
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.  Strict molecular clock estimates of the divergence between platypus 
and echidnas range from 17 to 80 Ma, but _Teinolophos_ suggests that the two 
monotreme clades were already distinct in the Early Cretaceous, and that their 
divergence may predate even the oldest strict molecular estimates by at least 
50%.  We generated relaxed molecular clock models using three different data 
sets, but only one yielded a date overlapping with the age of _Teinolophos_.  
Morphology suggests that _Teinolophos_ is a platypus in both
 phylogenetic and ecological aspects, and tends to contradict the popular view 
of rapid Cenozoic monotreme diversification.  Whereas the monotreme fossil 
record is still sparse and open to interpretation, the new data are consistent 
with much slower ecological, morphological, and taxonomic diversification rates 
for monotremes than in their sister taxon, the therian mammals.  This 
alternative view of a deep geological history for monotremes suggests that rate 
heterogeneities may have affected mammalian evolution in such a way as to 
defeat strict molecular clock models, and to challenge even relaxed molecular 
clock models when applied to mammalian history at a deep temporal scale." 

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