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Re: Tarbosaurus?




AS you yourself said in an earlier topic (I believe it was the evolution of flight in birds), you said that we should stick to what facts we have at hand, and not digress into speculation. The speculation in that case was behavior, and is the same here. The data for the geologic setting can be directly proven. (rain drop impressions=rain) However, assuming that "clumsieness" in sub-adult asian tyranosaurs was their main cause of death cannot be proven (Well, I suppose if someone did a study showing that the joints on the legs weren't well developed until adulthood, making walking clumsy...). Yes, there are behavioral possibilities for the discrepency, like juveniles/sub-adults preferring different territory. However, looking at the geology of the region can also give you potential possibilites that can be tested.
As for work in Montana: In the Two Medicine formation, any therapod remains are scarce, but there are some. For a weeks worth of exploring, between 5-7 albertasaur teeth will show up. Occasionaly, we found a troodon tooth, and once a vert. However, across the state, in the Judith River formation, at the MOR JDM quarry, there was a number of small tyranosaurid teeth present at one outcropping. However, we never found anything from any smaller carnivorous dinosaurs. Does this mean that they weren't there? And why are there few or no skeletal remains of tyranosaurids from the Two Medicine? What behavioral factors would cause scores of hadrosaurs to die off, but not one albertasaur.
Peace,
Rob
Student of Geology
Northern Arizona University
AIM: TarryAGoat
From: Dinogeorge@aol.com
Reply-To: Dinogeorge@aol.com
To: abcoulso@unity.ncsu.edu
CC: dinosaur@usc.edu
Subject: Re: Tarbosaurus?
Date: Mon, 24 Jul 2000 19:41:33 EDT

In a message dated 7/24/00 5:59:14 PM EST, abcoulso@unity.ncsu.edu writes:

<< We're very real, we're the ones trying to factor in the real world, rather
than totally ignore the depositional context the tyrannosaur assemblages are
found in. If you want a reason for the bias in juvenile preservation, the
sedimentology should be the first thing you investigate, rather than the
first thing you gloss over >>


No, the first thing that one should investigate is the possibility of
behavioral or other factors that would influence the ratio of subadults to
adults living in a particular depositional locality. The principal variable
that governs the ratio of fossilized subadults to adults at a locality has to
be the ratio of subadults to adults in the original population. I would say
that this variable >overwhelms< the effects of all the other variables and
sedimentological factors, etc., particularly because subadults and adults are
not that different in size and bone structure. All the other factors may
modify this ratio, but they must start with the original population.
Certainly local sedimentology and geology will affect whether or not an
animal will be fossilized at a particular time and place, but that is >not<
what I'm discussing.


If, for example, soil acidity preferentially removes subadults from the local
fossil record, then there will indeed be fewer subadults than adults, as in
the Hell Creek; but then you have to explain what kind of soil would >retain<
subadults preferentially to adults, which is what we have in the Nemegt. And
then you have to go to the Nemegt and to the Hell Creek, where there are
probably a dozen different kinds of paleosol deposits in each locality, try
to figure out what the original soil acidity might have been in each place
where a skeleton was excavated, and see whether these data fall into line
with your hypothesis. (They probably won't, by the way; nature has a way of
doing that.)


Then you have to do this for all the other sedimentological variables that
you have come up with. In the end, I think you'll come up with just what I've
been saying: that none of these variables is as important to understanding
the ratio of subadult to adult Nemegt and Hell Creek tyrannosaurids as is the
original ratio of subadults to adults in their respective populations. If
you'd like to prove me wrong, I'd be most interested in reading the results
of your field trips to Mongolia and Montana.

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