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Re: TYRANNOSAURIAN IMPLOSION [long; part 1 of 2]



In a message dated 1/26/01 1:16:08 AM EST, danchure@easilink.com writes:

<< one might be wary of
 assuming how unlikely it is to have a  number of sympatric species. While a 
dozen
 DOES seem unlikely, there might well be more than one.  In the Morrison, 
there are
 a number of theropods which reach very large adult size (Saurophaganax,
 Ceratosaurus, Torvosaurus, and Allosaurus) and these are found in the same
 quarries, implying that they were living close enough together to be buried
 together.  While these animals may represent several different lineages in 
the
 Morrison (neoceratosaur, spinosauroid, and allosaurid), they are all large
 predators, and with the exception of the tremendously long maxillary teeth in
 Ceratosaurus, they don't show a great deal of  difference in the business 
end of
 the axial skeleton.
 
 Similarly, the Triceratops argument, while probably true, should be viewed 
with
 some caution, as are at least 4 different genera of large sauropod which 
occur in
 the DNM quarry, showing that one can get overlap in animals with very similar
 biology (as far as we can determine what that is). >>

Six months ago, I would have said that there were three tyrannosaurids each 
("papa bear," "mama bear," and "baby bear") in Mongolia and in the Lance/Hell 
Creek of western North America. Now I've turned into a lumper. In the case of 
the Morrison taxa, the distinctions among the sympatric species are pretty 
clear; but in the case of Tyrannosaurus rex, the distinctions among the 
specimens are muddy indeed.

Perhaps the most interesting way to attack this problem would be to run 
cladistic analyses of >the specimens themselves<. Get as many characters as 
possible from the specimens, plug these into a computer, and see how the 
specimens group together. If you get lots of unresolved polychotomies, you're 
probably dealing with a single species with a fair amount of individual 
variation. If you get clumps, these may represent definable species or 
subspecies. If you get a Hennigian comb, you may be seeing the staged 
evolution of a single species. And so on.