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RE: So many tyrannosaur questions, so little time




From: "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
To: "Tim Donovan" <msdonovan66@hotmail.com>
Subject: RE: So many tyrannosaur questions, so little time
Date: Thu, 31 Oct 2002 08:48:50 -0500

> From: Tim Donovan [mailto:msdonovan66@hotmail.com]
>
>   Right. I think the Nemegt is a bit younger, maybe early late
> Maastrichtian. S angustirostris is bigger than osborni, and big
> dinos often
> tended to get bigger over time. (I don't think the Mongolian
> environment was
> better than that of NA.)

First of all, where is your data for the "better environment" in North
America? The Nemegt has been interpreted as an Okavango-like delta, and the
Okavango is one of THE best spots for large-bodied animals in Africa today.

But Asia seems generally drier, even in Nemegtian times; Barungoyotin beds sometimes "interfinger" with those of the Nemegt.




Furthermore, we aren't comparaing "Mongolia" and "North America"; we're comparing the Nemegt and the Horseshoe Canyon. Major distinctions there, and the devil is in the details.

Environmental differences are important.  After all, the
Edmontosaurus-bearing beds of the Horseshoe Canyon are in the late
Campanian: if we were to use genus-level biostratigraphy, we would probably
want to argue that the Edmontosaurus-bearing beds were younger than the
Saurolophus-bearing beds, as Edmontosaurus is present in the Hell Creek.
However, these differences are most easily explained by the shifts in
climate *within* the Horseshoe Canyon.

Right. My interpretation of the age of the Nemegt would be weakened if Saurolophus were also known from Campanian and Lancian beds, but this doesn't appear to be the case.



Also, beware of a circular argument, because it will prevent you from
discovering any alternative to your own preconceptions. For example, if you
take it as an assumption that all dinosaur genera get bigger through time,
then if you use relative size between Saurolophus species as an age
indicator you will fail to be able to recover the possibility that the
Nemegt is as old or older than the Horseshoe Canyon. (Incidentally, I don't
believe it is; I just want to point out the danger of circularity here).


For that matter, the Campanian E. regalis is bigger than Maastrichtian E.
annectens (based on an admittedly small sample size of the former).
Therefore we have evidence of at least one case for hadrosaurids that does
not show the largest species last.


IIRC, it has been suggested that E. regalis, which also occurs in the Hell Creek, is the adult or larger sex, annectens the smaller.



We should go by what the data say, and ignore "belief". Personally I
"believe" that the Nemegt probably is younger than the Horseshoe Canyon, but
until such time as there is convincing evidence independant of the
large-bodied vertebrates I cannot reject the possibility it is the same age.


> That, and the presence of a tyrannosaur more
> Tyrannosaurus-like than any known from the Edmontonian is, IMHO, a pretty
> strong hint that the Nemegt is about intermediate in age between
> Horseshoe
> Canyon and Hell Creek.


Well, while I agree with you on the Tarbosaurus-Tyrannosaurus relationship,
others (who known their tyrannosaurs very well) argue on other details for a
Daspletosaurus-Tarbosaurus pairing or for a Daspletosaurus-Tyrannosaurus
pairing.


> >In fact, dinosaur biostratigraphy is (in my professional opinion)
> >practically useless. I am far more secure in dates established by other,
> >independant means. It seems as if the Russians & Mongolians are finally
> >beginning to pin down the dates for the Mongolian units like the Nemegt,
>
> Unfortunately it lacks datable materials, even if the other svitas and
> gorizonts have them. Biostratigraphy must play a big role.


Biostratigraphy, yes.  Dinosaur biostratigraphy, NO!  To do such gets you
(again) into a circularity argument.

Furthermore, what makes you think that the units *lack* datable materials?

I meant only the Nemegt.

There are there, as some of the newest work to come out has shown (the late
Shuvalov's 2000 paper, for instance).  What has been lacking is Soviet (and
post-Soviet) expertise in detailed stratigraphic work compared to the
(petroleum-company generated) advances in western stratigraphy.  Such is
beginning to be employed there, but it will take a lot of time until it is
done.

> > > It is certainly falsifiable-those guys have been trying hard. If
> > > Edmontonian sized adult ceratopsids or ankylosaurids ever
> turn up in the
> > > Hell Creek, that would sink it.
> >
> >Care to try for the Judithian? The Sam Noble Museum specimen of
> >Pentaceratops rivals the largest Lancian ceratopsids in postcranial
> >dimensions, and has arguabley the largest skull of any
> terrestrial animal.
> >(However, I did note that the rostral-occipital dimension of the
> skull was
> >shorter than in several Triceratops skulls I know).
>
> And I would point out its horncores look far less robust than those of
> Torosaurus and that ceratopsids its size seem the exception then.


Danger!  You set up a falsifiability statement, then when presented with
such falsification,

Above, I wrote that a relatively small Lancian adult ankylosaurid or ceratopsid would falsify it. Some Campanian hadrosaurs and ceratopsids became as large as Lancian ones for reasons other than matching predators. The giant Shantungosaurus evolved elevated neural spines- which Bakker equated with rearing ability-so it probably became large just to radiate into the kind of niche usually occupied by a sauropod. But those were exceptions; big ones were the rule only alongside Tyrannosaurus.


you retreat into an ad hoc explanation for it. Be VERY
careful of such argumentation: too much of that and you stop doing science.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796


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