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Re: Tarbosaurus?



On Mon, 24 Jul 2000 Dinogeorge@aol.com wrote:

> 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.

        not if juveniles are being preyed upon/heavily scavenged, or
again, are environmentally-partitioned from the adults

 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,

        the kind found in a sub-environment frequented only by the
juveniles (I don't mean to advocate this suggestion necessarily, it just
pops to mind)

 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.

        no one said science was easy. Besides, if we approach it simply
from a population dynamics angle as you suggest, then we still have to
work out difficult considerations like population structure, clutch size,
life span, juvenile mortality etc. Then we have to work out the same
variables for each species involved, which at the very least is one Asian
and one North American species. These are all important factors that
should not be considered any more or less potentially informative than the
suite of sedimentological characters. I certainly don't envy the person
who tackles life tables for a 65 million year-old predator. Since the
fossil record seems to be skewed in terms of the population preserved, it
will be even more difficult to get at the true parameters of the
population when it was alive (i.e., what does the number of juvenile and
the number of adult specimens found tell us?). 

 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.
> 
        are you offering to pay for my field work? :) I don't think anyone
here means to belittle the importance of the biological/population
characters, we only mean that the sedimentary record should not be
overlooked, as it is potentially capable of shedding much-needed light on
this intriguing quandry.


Alan Blake Coulson
3938-C Marcom St
Raleigh NC 27606
abcoulso@unity.ncsu.edu
PhD student at North Carolina State University