Not titanosaurs ...

["Matthew Bonnan" <mbonnan@hotmail.com>]

Tim:

You said:
But
> _Alamosaurus_ is
> a poorly known genus, and since there are at least two Early Cretaceous
> titanosaurs from N. America (_Pleurocoelus_ and one to be named), there is 
> a
> possibility that _Alamosaurus_ was home-grown.

Well, it depends on whose classification scheme for sauropods you go by.  
The two recent ones (Wilson and Sereno, 1998; Upchurch, 1998) don't appear 
to consider Pleurocoelus outright, but Wilson and Sereno mention that 
Pleurocoelus would fall closer to Brachiosaurus than the traditional 
titanosaurs.  Upchurch did a genus-level taxonomy of the sauropods, but 
Pleurocoelus is not included, probably because it is so fragmentary.

Having looked at Alamosaurus and Pleurocoelus forearms, I can tell you there 
are big differences between the two.  Pleurocoelus has an ulna like that of 
Brachiosaurus, with a reduced and flattened olecranon process, whereas 
Alamosaurus has a secondarily-developed large olecranon process that points 
posteriodorsally behind its proximal articular surface.  That secondarily 
developed olecranon is considered to be a character of titanosaurs or the 
Somphospondyli of Wilson and Sereno.

In any case, it is amazing how similar the ulna and humerus of Alamosaurus 
look to that weird sauropod from Poland, Opisthocoelicaudia, which is now 
also regarded as a titanosaur.  Based on this, I would have doubts about 
Alamosaurus being "home grown."  It seems more likely it is a southern 
immigrant.

Two cents from a guy obsessed with sauropod feet,

Matt Bonnan
Dept Biological Sciences
Northern Illinois University



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