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Re: AMARGASAURUS SATTLERI (pretty long)



In a message dated 9/2/00 11:40:10 PM Pacific Daylight Time, Dinogeorge 
writes:

<< Have to worry about it now, lest one or the other genus be rendered 
paraphyletic. If the common ancestor is Amargasaurus, then that genus becomes 
paraphyletic because it excludes its descendant genus Dicraeosaurus; if it's 
Dicraeosaurus, then it's paraphyletic because it excludes its descendant 
genus Amargasaurus. If it's a third genus, it's paraphyletic because it 
excludes both descendant genera Dicraeosaurus and Amargasaurus. >>

No, I don't have to worry about it.  The species (or, more accurately, 
collection of individuals) from which _hansemanni_, _sattleri_, and _cazaui_ 
descend does not have to have a genus, or even species, name--certainly not 
before it is excavated and described, which will almost certainly never 
happen.

Taxonomy only works because we have a finite collection of individuals to 
classify.  If we were confronted with the grand parade of every individual 
organism that ever lived, any classificatory scheme would totally break down.

If I assign the species _sattleri_ to the genus _Dicraeosaurus_, I am 
implicitly making the statement that, to the best of my knowledge, "the 
individual whose remains form the type of the species _sattleri_ was more 
closely related to the individual whose remains form the type of the genus 
_Dicraeosaurus_ than it was to the individual whose remains form the type of 
any other (named) genus".

While this was true in 1914, when _sattleri_ was named, it appears that this 
is no longer the case: as far as we can tell, the type individual of 
_sattleri_ was more closely related to the type individual of _Amargasaurus_ 
than it was to the type individual of _Dicraeosaurus_.  Thus, our terminology 
is implicitly making false statements of relative relationship; and a 
taxonomic change is in order.

The situation can be rectified in any of three ways.

1)  The individual whose remains comprise the type of _Amargasaurus_ can be 
referred instead to _Dicraeosaurus_.  The type individual of _Amargasaurus_ 
would then cease to by a generic type individual, and the statements implied 
by taxonomy would be true, since, to the best of our knowledge, the type 
individuals of both _sattleri_ and _(Amargasaurus) cazaui_ were more closely 
related to the type individual of _Dicraeosaurus_ than to any other generic 
type.

2)  The individual whose remains comprise the type of _sattleri_ could be 
referred to the genus _Amargasaurus_, satisfying the relative proximity 
requirement.

3)  The individual whose remains comprise the type of _sattleri_ could be 
made the type individual of a third genus, satisfying the relative proximity 
requirement vacuously.

Which of the three methods to use is a matter of personal taste, since all 
three will produce taxonomy which correctly (though with varying degrees of 
detail) describes the interrelationships of the individuals involved.

And if Dinogeorge, or anyone else, brings me the remains of (remains of, mind 
you, not theoretical arguments concerning) some other dicraeosaurid 
individual, I can apply the same reasoning to determine how it should be 
classified and named.

Nick P.