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YIXIAN = J OR K?



OK.  I am back from Argentina.  Switch gears and think about China again 
for a minute. 


darren.naish@port.ac.uk wrote:
> 
> Jim Kirkland wrote...
> 
> > I have heard that the Chinese are getting ready to publish a paper 
> > in Science or Nature giving new radiometric Ar/Ar dates from ashes 
> > within the Yixian Formation lucustrine beds giving dates of around 
> > 142-140 mya (pretty basal Cretaceous). Certainly as the 
> > Psittacosaurs have pemaxillary teeth they are more primitive than 
> > any so far described. I think we had all better wait and see.

        Huh, as far as I understand, that paper was submitted to Nature 
and was rejected.  I suspect that everyone knows my views of Ar-Ar dates 
from these units by now, but let's go over it one more time.
        The principal reason that I am using U-Pb as a clock for Sihetun 
is the fact that Ar mobilizes at low temperatures, resulting in 
significant Ar loss from the host rock if said material is exposed to a 
significant source of heat (hmmm...such as the 1200 C  melting temperature 
basalt, perhaps????).  Though I have in the past thought that the 
Canadian Ar-Ar ages were probably ok (though they are certainly not 
concordant ages...), I no longer think this is the case and would caution 
all against toting around Ar-Ar dates from the Yixian as anything 
resembling reality...
        The Psittacosaurs are bloody primitive, but I am also concerned 
about using the terrestrial tetrapods from these beds as biostratigraphic 
indices as the argument is pretty circular.  Like I said at SVP, if these 
new pterosaurs had been found in any other formation at any other time, 
the papers being published could just as easily be proclaiming the first 
occurance of Cretaceous rhamphorinchoids (I know the spelling is off) 
rather then using the taxa as evidence for a Jurassic Sihetun.  Whatever.


> Paul Davis has been covering the respective ages of the Yixian and 
> Jiufuotang Fms in his recent talks on early bird evolution, and in 
> the abstract to his talk given at the Cretaceous Biodiversity 
> Conference (Portsmouth '98) he writes..
> 
> 'The ages of these formations is surrounded in controversy at the 
> moment, basically due to differing dates obtained from different tuff 
> horizons. However all researchers agree that the formations range 
> from Upper Jurassic to the Lower Cretaceous with the Yixian Formation 
> underlying the Jiufuotang Formation.

        Interesting that almost NONE of the published ages represent data 
obtained from tuffs...but are rather igneous rock ages...


> 'The older Yixian Formation has yielded a monospecific avian 
> assemblage (_Confuciusornis sanctus_) and is dated as 141+/- 6 Ma 
> (combined K-Ar, Rb-Sr data). This makes _Confuciusornis_ comparable 
> in age to _Archaeopteryx_.....'
> 
> Ah, but does it? Isn't there is also some controversy over the exact 
> date of Solenhofen: Kimmeridgian or Tithonian? I am aware of some 
> published work that attempts to resolve this issue, but do not have 
> it to hand. It has been discussed on the list before.
> 

        Dating a horizon is almost always a much more complicated affair 
than we as paleontologists generally think it is.  It is also generally 
compounded by the fact that when we go and do get a date we then run around 
publishing on it often having no idea of the structural relationships of 
the units we are talking about...Sihetun is an excellent example of this.


-- 
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Josh Smith
University of Pennsylvania
Department of Earth and Environmental Science
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