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Re: Everything you know about the Iren Dabasu age is (right)
--- Michael Mortimer <email@example.com>
> 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
> >Nemegtian. Iren Dabasu types appear very similar
> >Bayanshiree forms; both were medium sized, common
> >diverse, whereas Therizinosaurus was big and rare.
> Very simplistic comparison there.
> Regarding size, Segnosaurus overlaps with
Because the hindlimb specimen from Khermeen Tsav is
>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
> 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.
> 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
> 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
> > > Velociraptor have all
> > > been reported from Iren debasu as well, and are
> > > Djadockhta/Nemegt age.
> > > But Currie and Dong (2001) later modified Currie
> > > Eberth's referral from
> > > Saurornithoides to Troodontidae indet.. I've
> > > figures of the Iren
> > > Debasu Avimimus remains, and how are we to tell
> > > they're Avimimus
> > > portentosus, A. sp. nov., or another genus of
> > > avimimid? They differ a bit
> > > from the A. portentosus specimens, but
> > > what it means is pretty
> > > subjective.
> > Turnover among small theropods was apparently not
> >fast as turnover among larger dinosaurs.
> >is known from the Campanian to the end of the
> >Maastrichtian. It wouldn't be surprising if some
> >Dabasu theropods were very closely related to
> >counterparts, even if the temporal difference were
> 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
> > > 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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