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Re: AMARGASAURUS SATTLERI



In a message dated 8/21/00 9:33:58 AM EST, darren.naish@port.ac.uk writes:

<< Finished reading Leo 
 Salgado's diplodocimorph macroevolution paper the other day (ref 
 below): in it he suggests that _Dicraeosaurus_ is paraphyletic and that 
 _D. sattleri_ is closer to _Amargasaurus_ than to _D. hansemanni_ 
 (I'm hoping there that I have the species the right way round). In fact, 
 _D. sattleri_ might be regarded as a second species of _Amargasaurus_ 
 and thus become _Amargasaurus sattleri_ he says (he does not use this 
 combination - I'm hoping both names are the same gender).  >>

D. sattleri would be correct; it has taller neural spines than D. hansemanni 
and is not the type species of Dicraeosaurus.

It's difficult (though not impossible) to avoid paraphyletic genera in this 
situation. If D. sattleri and A. cazaui are sister groups, then D. sattleri 
could indeed be placed in the genus Amargasaurus. But then what happens to 
the common ancestor of Dicraeosaurus and Amargasaurus? If it's a 
Dicraeosaurus sp., then Dicraeosaurus remains paraphyletic; if it's an 
Amargasaurus sp., then Dicraeosaurus becomes a senior synonym of Amargasaurus 
and all three species belong to Dicraeosaurus, with Amargasaurus sunk. If 
it's an unnamed third genus, then it's paraphyletic because it doesn't 
include its descendant genera Dicraeosaurus and Amargasaurus. Maybe D. 
sattleri requires its own genus? I think Scott Sampson ran into this problem 
with Styracosaurus-Einiosaurus-Achelousaurus-Pachyrhinosaurus, and Tom Lehman 
with Chasmosaurus and Pentaceratops.