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Re: A Jurassic 'beaver'



<<Interestingly, however, the beastie is a docodont! If it is EK, that makes
it the youngest known docodont in the world by some 20 million years.>>

Thanks to a kindly List Member, I've got the description of /Castorocauda/,
the Supplementary Online Material, the commentry from Thomas Martin and a
Science Press Release.  The Supplementary Material has further information
on the basis of the age assessment.  There are two radiometric datings of
relevance and the following is from page 3.

"Geological Age and Stratigraphical Horizon:

The holotype specimen of JZMP04117 was discovered at the Daohugou Locality
(N41°18.979',

E119°14.318') of Ningcheng County of Inner Mongolia Autonomous Region. The
fossil is from

Bed 3 of the Jiulongshan Formation (sensu Ren et al. 2002). The
fossiliferous bed yielding

JZMP04117 and other fossil vertebrates are 20 meters below the volcanic ash
beds dated at

164.2±2.5Ma from feldspar by 40Ar/39Ar dating, and at 164.6±2.4Ma from
zircon by SHRIMP

206Pb/238U dating (Chen et al. 2004; Ji et al. 2004). Therefore the fossil
beds of JZMP04117

are approximately 164 ma and their age should be the Middle Jurassic."