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YIXIAN = J OR K?
OK. I am back from Argentina. Switch gears and think about China again
for a minute.
> 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
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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