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



Tim Donovan wrote-

> There has been no published evidence for any
> Erlikosaurus or Segnosaurus
> remains outside of their holotypes and (in
> Segnosaurus) paratypes.  Sure
> Currie and Eberth reported elements which were
> 'identical' in 1993, but then
> in 2001 and 2002 we get two new Iren Debasu
> therizinosauroids.  Until the
> old remains are compared with these new taxa, the
> referral is very weak.

  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, 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, and four of Therizinosaurus. How is Therizinosaurus rare again? Or looking at it from a stratigraphic perspective, more than six Iren Debasu specimens, six Baynshiren specimens, 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).
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. At least they come complete, in larger samples, and aren't affected by ontogeny.


Mickey Mortimer