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Uncinate Processes in Early Tetrapods



Hey all,
Jenny Clack's book _Gaining Ground_ (heavily recommended, by the way!) mentions 
very briefly small, triangular flanges on the midpoint of the shaft on certain 
thoracic ribs of dissorophid tetrapods (among them _Dissorophus_ itself).  
_Eryops_ also bears similar, although larger, flanges that have often been 
associated with shoulder musculature.  Clack calls these flanges uncinate 
processes, but includes a brief disclaimer regarding their definition.

Also mentioned are similar processes in a whole suite of early tetrapods, among 
them _Whatcheeria_, _Eucritta_, _Baphetes_, _Kotlassia_ (a seymouriamorph),  
_Seymouria_, _Discosauriscus_, _Greererpeton_, and others.  Amazingly, 
processes that might be related to those seen in the above forms are also seen 
in _Acanthostega_.

My question is this: is there any justification for calling these flanges 
uncinate processes?  Some authors refer to them as UPs, but many others do not. 
 Based on the figures I have seen, there seems to be little correlation between 
the true UPs of birds, _Velociraptor_, and (possibly...maybe) _Thescelosaurus_. 
 I doubt they are homologous, but I might be wrong.  While avian UPs are 
separate ossifications, these flanges appear to be true processes.  

Any ideas on why these structures are referred to as uncinate processes?

Steve

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Steve Brusatte-DINO LAND PALEONTOLOGY
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