Re: An open letter to Dr. David Unwin on pterosaur origins

[Tim Williams <twilliams_alpha@hotmail.com>]

(Hang on to your hats - this is going to be one BORING message...)

David Peters wrote:

> And if its not the Macrocnemus > Longisquama clade, then can we assume that 
> the various apparent synapomorphies found in various members of that clade 
> -- including the elongated naris, antorbital fenestra, multi-cusped teeth, 
> elongated cervical ribs, deep dorsal ribs, high sacral count, attenuated 
> caudal series, elongated scapula, tall coracoid, sternal complex (sternum + 
> keeled interclavicle + clavicles), magnified deltopectoral crest, migrated 
> centrale, elongated fourth finger, reduced fifth finger, elongated ilium, 
> fused ischium+pubis, prepubis, sprawling femora, attenuated > fibula, 
> mesotarsal ankle, elongated lateral toe, and uropatagia -- are ALL 
> convergent with pterosaurs?

This list looks impressive at face value.  However, it may be very 
misleading.  Even assuming that ALL these characters are actually evidenced 
in individual members of this clade, this is less important than whether 
these characters are synapomorphies that are shared with the Pterosauria.  
If Pterosauria evolved from within the _Macrocnemus_-_Longisquama_ clade (as 
you contend), then we would expect at least some of these characters to be 
shared between Pterosauria and certain members of this clade.  In other 
words, the presence of these characters would be resolved as diagnostic for 
individual clades in a cladogram: Pterosauria and successively more 
inclusive clades whcih contain Pterosauria.

If, however, these listed characters are autapomorphic for individual taxa 
within the _Macrocnemus_-_Longisquama_ clade, and/or they are convergent 
(homoplastic) between members of the _Macrocnemus_-_Longisquama_ clade and 
Pterosauria, then they are not phylogenetically significant in terms of 
pterosaur origins.  This is the value of a parsimony-based phylogenetic 
analysis, compared to simply compiling lists of shared characters.  I think 
this is essentially what David M. was saying, but I think it bears 
repeating.

Cheers

Tim

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