[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: on-line fossil birds list



David Marjanovic wrote-

Scansoriopterygidae, Epidendrosaurus (Early Cretaceous)

Might be Middle or Late Jurassic; this hinges on the age of the Daohugou Bed.

Which is... drum roll... Early Cretaceous. It lies below the Yixian Fm, but above the Tuchengzi Fm. A radiometric date from "the upper part" of the latter is very close to the Berriasian-Valanginian boundary, so the Daohugou Bed could be anything from Valanginian to Barremian in age. I posted on this last summer.

This seems to be wrong. Read Gao and Ren (2006). It argues VERY convincingly for a Middle Jurassic age.


"He et al. (2004) provided no evidence to challenge the
results of Ren et al. (2002) and Liu and Jin (2002), who, as
noted above, independently concluded that the fossiliferous
volcanic shales at Daohugou rest unconformably on pre-
Cambrian rocks. No known geological process could have
completely overturned the ignimbrite+fossiliferous shale
sequence at Daohugou while leaving no evidence of such a
disruptive event on the pre-Cambrian rocks beneath. He et
al. (2004) also claim that, contrary to Ren et al. (2002) and
Liu and Jin (2002), ¡°at the Daohugou locality, the fossilbearing
sediments are also found to overlie the Tuchengzi
Formation at a few sites¡±, but they have not cited the
coordinates of the sites where this occurs nor have they
documented this contact in a photograph of these sites."

"Contrary to He et al. (2004), more thorough field
investigation by Liu et al. (2004, fig. 2) has revealed
compelling evidence showing that the Tuchengzi
Formation disconformably overlies the volcanic rocks of
the Lanqi (Tiaojishan) Formation (observation site: N41¡ã
22.91¡ä E119¡ã10.03¡ä), stratigraphically some 270 m above
the Daohugou fossil beds. Liu et al.¡¯s (2004) publication
included detailed description of the stratigraphic
successions, measured geological sections, and clearly
marked stratigraphic and GPS positions from where rock
samples have been collected for radiometric dating."

Moreover...

"More recently, both 40Ar/39Ar and SHRIMP U-Pb dating
of trachyte and trachytic ignimbrite rock samples from the
Daohugou area (N41¡ã18.979¡ä, E119¡ã14.318¡ä) some 20 m
above the fossil-bearing shales yielded a date of 164 Ma
(Chen et al., 2004), a Middle Jurassic (Bathonian) age.
Before this, the stratigraphically higher Lanqi
(=Tiaojishan) Formation in western Liaoning has yielded a
K-Ar date of 158 Ma (Wang Si¡¯en et al., 2000), providing
an indirect support for the Middle Jurassic (Bathonian) age
of the underlying Haifanggou (Jiulongshan) Formation
(including the fossil beds exposed at the Daohugou area)."

Gao and Ren, 2006. Radiometric Dating of Ignimbrite from Inner Mongolia Provides
no Indication of a Post-Middle Jurassic Age for the Daohugou Beds. Acta Geologica Sinica. Vol. 80 No. 1 pp. 42¨C45.


I have the pdf if anyone wants it.

Mickey Mortimer