Re: Yixian Dating Again

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

----- Original Message -----
From: "Jaime A. Headden" <qilongia@yahoo.com>

> <Incidentally, the Tuchengzi Formation is Late Berriasian (Swisher et al.,
> 2002).>
>
>   This annoys me ... Swisher says this ... There are Ar/Ar dtaes by Lo
> that suggest Jurassic for Sihetun. Which are younger.

Meanwhile revised AFAIK. Weren't they whole-rock ages while the K dates are
based on individual crystals, or am I mixing this up with something? There
are other radiometric dates (also Ar/Ar IIRC) that suggest Barremian for
Sihetun.

> Though the
> palynology and some Ar/Ar dating, and some biogeography stuff suggests
> Lower Cretaceous, there are also Ar/Ar dates, *Hadrocodium*, ostracods,
> Ur/Pb dating to support a Jurassic dating.

*Hadrocodium* is from Yúnnán, more than 3,000 km to the southwest, and from
the Early Jurassic. *Gobiconodon*, on the other hand, supports an EK age.
Jurassic ostracods and U/Pb? News to me.

> One's personal preference to
> relationship of forms does not always mean this is evidence.
> *Sinosauropteryx* may be a compsognathid,

Would say nothing either way, if *Aristosuchus* and/or that pelvis from
Santana is compsognathid, too.

> [...] As david notes, despite other pterosaurs,
> there is an "anurognathid" or anurognathid like animal from the levels,
> *Dendrorhynchoides* (support for that relationship is unpublished, to my
> knowledge, though I do agree with Peters' assessment).

Well, the tail is from a *Microraptor*, isn't it?
There's a second anurognathid from the Yixian Fm, *Jeholopterus* from Inner
Mongolia. The authors

Wang Xiaolin, Zhou Zhonghe, Zhang Fucheng & Xu Xing: A nearly completely
articulated rhamphorhynchoid pterosaur with exceptionally well-preserved
wing membranes and "hairs" from Inner Mongolia, northeast China, Chinese
Science Bulletin 47(3), 226 -- 232 (February 2002)

cite only

C. C. Swisher, Wang Yuanqing, Wang Xiaolin et al.: Cretaceous age for the
feathered dinosaurs of Liaoning, China, Nature 400, 58 (1999)

as their source for an EK age of the Yixian Fm and don't express any doubt
about it. -- Well, an anurognathid or any non-pterodactyloid pterosaur is
not expected for the K, but bigger surprises have happened, like
*Xenocretosuchus* the Impossible Late-Coming Tritylodontid (late EK) or
*Reigitherium* the Incredible Late-Coming Docodont (Maastrichtian).

> David wrote about using angiosperms to date the levels:
>
>   Angiosperms do not an adequate dating marker make ... we do not know the
> earliest occurence, and to say that because *Archaefructus* is the
> earliest known, and is "LK", then to conclude angiosperms were Cretaceous
> only is bad logic. All you can say is that the earliest angio is in the
> Yixian, and probably is EK.

Yes. I just mean it's suggestive, and it neither contradicts an EK age nor
suggests a LJ one. -- AFAIK no J angiosperm pollen are known, right?

> *Koparion* does not look like any known
> troodontid tooth, has many unique features. It could even belong to a
> similar, eumaniraptoran group.

Would you say the teeth from Guimarota, not to mention the MJ ones from
England, look like known troodontids? (I can't tell.)