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Re: The age of the Daohugou Bed (and the rest of China)
<<Insects and conchostracans don't seem to give very precise dates. Remember
that both were used to put the Yixian Fm into the Jurassic.>>
I can think of some vertebrates that could also be unreliable, such as the
"feathered maniraptorans" mentioned. Feathery maniraptorans have a known
record stretching at least back deep into the Upper Jurassic, and an unknown
one (quite posssibly only to me) that must've gone still deeper.
Thinking in terms of mammals, a Lower Cretaceous shuotheriid would be kind
of ever so astonishing. The previously available fossil record wasn't
exactly extensive, eleven teeth and a partial jaw, but it was all Middle to
perhaps Upper Jurassic. Shuotheriids may have managed to sneak beyond the
Jurassic-Cretaceous transition but, if so, then they must have done so with
silent paw-steps. None have been identified in any clearly Cretaceous
faunas; eg (and especially) the two at Yixian. Both Yixian faunas are also
presently docodont-free-zones despite, if I remember correctly, providing
plentiful swimming facilities for those with semi-aquatic tastes, such as
doco-/Castorocauda/. Lower Cretaceous Siberia does provide evidence of rare
docodonts from roughly that time, but Yixian doesn't. (I think rare
doco-dent fossils have also turned up in the US Lower Cretaceous.)
If Daohugou: "probably represents the earliest evolutionary stage of the
Jehol Biota based on both vertebrate biostratigraphy and the
sedimentological and volcanic features which suggest the Daohugou deposit
belongs to the same cycle of
volcanism and sedimentation as the Yixian Formation of the Jehol Group..."
... then the discontinuity of the mammals is starting to get odd. Perhaps
further sampling in both cases might reveal something approaching overlaps,
but I can't say I'm presently expectant (in either sense of the word).
Happily, sampling will carry on and on in both places.
Thanks for the link to the pdf.