Re: Yixian Dating Again

["Jaime A. Headden" <qilongia@yahoo.com>]

Mickey Mortimer (Mickey_Mortimer111@msn.com) wrote:

<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. 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. One's personal preference to
relationship of forms does not always mean this is evidence.
*Sinosauropteryx* may be a compsognathid, *Protarchaeopteryx* may be more
archie-like than others accept. 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*Dendrorhynchoides* (support for that relationship is unpublished, to my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).

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.

Mickey also wrote:

<Not really, we have no Jurassic segnosaurs, oviraptorosaurs,
pygostylians,>

  Not provable. Non preserved, named, described, or recognized. You think
a segnosaur from the Jurassic would look like a segnosaur from the
Cretaceous? The very fact that phylostratigraphy supports a LK or MK
divergence of most maniraptorans indicates we most likely will find these.
There are "enigmosaur" material from the Morrison, but to know whether
this is basal segnosaur, stem-"enigmosaur", or basal oviraptorosaur, is
not something we can tell. *Koparion* does not look like any known
troodontid tooth, has many unique features. It could even belong to a
similar, eumaniraptoran group. But that's not really the point. descriebd
taxa do not define the multitude or even the extent. Phylostratigraphy is
hardly an exact science, and to use it either way is to add problems to
the results, bvut ti does help in determining 1) first appearances, 2)
timing of descent, 3) speciation patterns and probabilities, including
dispersional.
 
<psittacosaurids or neoceratopsians, named or not.>

  With some possible marginocephalian sister groups in the early and
middle Jurassic, do you think you can doubt that they taxa above would not
be present in the Jurassic? Or because all named and known to you taxa are
Cretaceous, it is only a Cretaceous assmeblage? Sorry, doesn't bite.

<Jurassic basal coelurosaurs, enigmosaurs, troodontids and eumaniraptorans
are known. That accounts for five of fourteen taxa,>

  As if this was the end of continuity?

<and is especially suspect given that all of the "known from Jurassic"
groups besides Troodontidae are large clades/grades that were merely
listed because the exact position of the Yixian represntatives is
controversial.>

  Bah, I deal with taxa regardless of how they are in relation to the
Yixian, and so have most others. Makovicky and Chiappe&Frankfurt's work
does not depend on Yixian taxa, Chure's troodontid does not, even Xu still
supports a probably latest Tithonian age for the Tuchengzi and thus
*Chaoyangsaurus*...

  And to reiterate .,.. no one has dated and sampled rigorously all the
fossiliferous levels in the Jehol OR Xinminbao Groups, where most are
correlated based on date estimates from other sites. There is a 16ma
variance in the Sihetun dating techniques, based on the same procedure
using different sources (Smith et al., 1998a-d), with the tuffs and
lacustrine stuff dating at Ar/Ar laser fusion at 112Ma, and the basal
Yixian andesites/basalts with volcanoclastics at 122-128Ma ... does this
mean the dating is concise? Lo et al. (1999a-b) find a much older dating,
and this was done _after_ Smith et al.. Nonetheless, I use this not to
support a Jurassic age, but to cast a little bit of doubt over making
definate statements like "the Yixian is Barremian" without proviuding the
only real means of proving this: dating the levels. Phylostratigraphy only
offers little in the way to resolution.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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