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The presence or absence of sterna and uncinate processes on the ribs 
in fossil archosaurs probably are due to variable ossification of 
these features. Paul Davis, who is apparently not convinced that 
_Archaeopteryx bavarica_ is truly distinct from _A. lithographica_, 
has argued that ossification of the sternum was subject to 
individual variation within members of the genus (verbal report, 
Portsmouth 1998). There are numerous similar examples of variable 
ossification in other tetrapod taxa. 

As for furculae, TJW (what is your proper name?) writes...

> I'm guessing the same holds true for the furcula ("wishbone").  
> Norell and Clark (1997) described a furcula in a _Velociraptor_ 
> skeleton from Tugrugeen Shireh, the first for any dromaeosaurid.  
> Norell and Clark mention that this specimen is a particularly large 
> individual, and so probably fully-grown.  I had assumed that 
> ossification of the furcula was perhaps an age-related thing, with 
> younger theropods having a purely cartilaginous furcula (or 
> clavicles).

You may be interested to hear that furcular ossification sometimes 
never happens in some extant volant birds - budgies and at least some 
owls for example - even though they have well developed cartilaginous 
furculae. I am not aware of bird species where furcular ossification 
pends on individual variation BUT in one of the Hawaiian moa-nalos, 
_Thambetochen_, one population has an ossified furcula, while another 
population does not (Olson and James 1985). This is based on a small 
sample size, but Bradley Livezey does cite it as reliable in one of 
his anseriform relationships papers.

> I wonder, is the one _Archaeopteryx_ specimen which shows an 
> ossified breastplate any larger than the other specimens in body 
> size?

No, the Aktien-Verein specimen (type for _A. bavarica_) is smaller 
than the Solnhofen specimen at least.