Re: Everything you know about the Iren Dabasu age is (right)

[Tim Donovan <uwrk2@yahoo.com>]

--- Michael Mortimer <mickey_mortimer111@msn.com>
wrote:

> Tim Donovan wrote-
> >   The Iren Dabasu therizinosaur fauna still
> appears to
> >have much more in common with that of the
> >Bayanshirenian of Outer Mongolia than it does with
> the
> >Nemegtian.  Iren Dabasu types appear very similar
> to
> >Bayanshiree forms; both were medium sized, common
> and
> >diverse, whereas Therizinosaurus was big and rare.
> 
> Very simplistic comparison there.
> Regarding size, Segnosaurus overlaps with
> Therizinosaurus, 


 Because the hindlimb specimen from Khermeen Tsav is
relatively small?



>while the named 
> Iren Debasu taxa are smaller than Baynshiren or
> Nemegt taxa.  But then you 
> have the Iren Debasu forearm which is in the
> Baynshiren size range.  Of 
> course, this isn't very useful when we don't know
> the age of any of these 
> specimens.
> Regarding abundance, we have one specimen of
> Erlikosaurus and Erlianosaurus, 
> two of Neimongosaurus, four of Segnosaurus, 

 Is that all? I count 5 localities which yield
Segnosaurus or something similar. 


>and four
> of Therizinosaurus. 

 I thought there was one specimen from the Nemegt
Basin and two from Khermeen Tsav.

 
> How is Therizinosaurus rare again? 


 AFAIK, it is only known from a few specimens from two
Nemegtian localities out of about ten, whereas all
localities long considered of Bayanshiree age have
yielded therizinosaurs.


> Or looking at it
> from a stratigraphic 
> perspective, more than six Iren Debasu specimens,
> six Baynshiren specimens, 


 How recent are these stats? Do they take into account
work of the mid-late '90s in Outer Mongolia?


> and five Nemegt specimens.  I don't see any useful
> difference there.  Then 
> you have to consider collection bias (maybe big
> Therizinosaurus was rarer 
> than suggested because we're more likely to see and
> keep its bones; maybe 
> Iren Debasu taxa were rarer than suggested since
> we've explored it better or 
> more recently).


 There's no doubt that Nemegtian localities have
received more attention than Amtgai, Khar Hotol etc.



> And affecting both of these variables is
> paleoenvironment.  Who knows what 
> affect the climate, vegetation and/or topography had
> on what genera of 
> therizinosaur lived where.
> 
> > > Remember, Mononykus, Avimimus, Saurornithoides
> and
> > > Velociraptor have all
> > > been reported from Iren debasu as well, and are
> all
> > > Djadockhta/Nemegt age.
> > > But Currie and Dong (2001) later modified Currie
> and
> > > Eberth's referral from
> > > Saurornithoides to Troodontidae indet..  I've
> seen
> > > figures of the Iren
> > > Debasu Avimimus remains, and how are we to tell
> if
> > > they're Avimimus
> > > portentosus, A. sp. nov., or another genus of
> > > avimimid?  They differ a bit
> > > from the A. portentosus specimens, but
> determining
> > > what it means is pretty
> > > subjective.
> >
> >  Turnover among small theropods was apparently not
> as
> >fast as turnover among larger dinosaurs.
> Ornithomimus
> >is known from the Campanian to the end of the
> >Maastrichtian. It wouldn't be surprising if some
> Iren
> >Dabasu theropods were very closely related to
> Nemegt
> >counterparts, even if the temporal difference were
> >great.
> 
> Or maybe smaller taxa are more homogenous
> osteologically, or more likely to 
> be preserved as fragments which are mistakenly
> assigned to taxa.  For 
> instance, Late Maastrichtian Ornithomimus specimens
> consist of a few manual 
> and pedal elements (holotype and paratype of O.
> velox) or are undescribed 
> (which given the similarity between derived
> ornithomimids, makes them 
> useless for our case).  Maybe the taxon differs
> enough in its unknown 
> portions to warrant generic separation from O.
> edmontonicus.  Maybe the 
> single synapomorphy currently joining the species
> (metacarpal I longer than 
> II) is convergent.  Just more reasons dinosaurs suck
> for biostratigraphy.
> 
> > > Basically, the take-home message is that
> dinosaur
> > > biostratigraphy sucks and
> > > should not be used except very generally.
> > > Invertebrates, pollen,
> > > charophytes - those are useful.
> >
> >   Not necessarily. Recall J. Wagner's remark that
> >palynomorphs have a nasty habit of showing up at
> >different times in different places.
> 
> That just means palynomorphs suck for
> biostratigraphy too.  Less than 
> dinosaurs I'd bet though. 


 Not in Mongolia!

 At least they come
> complete, in larger samples, 
> and aren't affected by ontogeny.
> 
> Mickey Mortimer
> 
> 
> 



                
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