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Re: The iguanodont paper



In a message dated 12/9/07 9:18:08 PM, twilliams_alpha@hotmail.com writes:

<< 
 Take the example of the old Dinosauria, which excluded birds. People made 
all sorts of sweeping statements, such as: "Dinosaurs never became aquatic or 
marine" and "All dinosaurs became extinct at the end of the Cretaceous".  We 
now 
know that both statements are untrue, and (more importantly) were *never* 
true, given that birds are a subset of dinosaurs.  But if we were to have a 
taxonomic group that was limited to the *traditional* dinosaurs (i.e., without 
birds), these clangers would return with a vengeance.  Ditto for "Thecodontia", 
and so forth.
 
 Comparing a paraphyletic "Iguanodontidae" (= non-hadrosaur iguanodonts, or 
"spiky-thumb" group) with hadrosaurs is apples and oranges.  In evolutionary 
terms, hadrosaurs are a subset of iguanodonts.  There's nothing hadrosaurs have 
that wasn't either present in the "iguanodontids", or secondarily lost by the 
hadrosaurs (like the spiky thumb).  Thus, by saying that "iguanodontids" were 
at least as widespread or diverse as hadrosaurs is potentially misleading, 
because it is impossible to discuss the success of iguanodonts without 
including 
the hadrosaurs. >>

The above arguments can be flipped to show why they are simplistic. For 
example, it remain extraordinary that among all the dinosaur groups only one, 
the 
flying birds, have developed marine forms, unlike mammals which have spawned a 
series of marine forms from nonflying types. This important truth is obscured 
by the silly notion that saying that just because one group of dinosaurs 
evolved marine forms that this is typical of the entire clade. In any case the 
old 
exclusion of birds from Dinosauria was not a taxonomic issue, but a 
phylogenetic one due to the mistaken consensus that birds evolved from early 
archosaurs 
independently of theropods. 

Hadrosaurs were not a mere subset of iguanodonts. They had more complex 
dental batteries -- the most sophisticated among dinosaurs - a tendency to 
develop 
nasal passage crests, modified ilia and the like. Lumping them together is 
rather like lumping bison and cattle. Of course, we could call bison noncattle 
bovids. Or are cattle nonbison bovids? Is suppose will have to check a 
cladogram 
and see. 

GSPaul 

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