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Re: Molecular clocks and avian diversification



> > Ah, thanks for pointing this out! The possible
> > presence of a cartilaginous sternum in the Munich
> > specimen nonwithstanding (that's the way I
> remember it
> > anyways) -
> 
> It's the Berlin specimen. (It's amazing what people
> can overlook for 150 
> years. The Berlin specimen was recently shown to
> preserve a postorbital, 
> too...)

No, I meant Munich alright, hence "possible". Should
have written "speculated", maybe.

> The issue is that dromaeosaurs, troodontids (except
> *Jinfengopteryx*), and 
> oviraptorosaurs all have ossified sterna. Therefore,
> no matter what their 
> exact relationships, the lack of ossified sterna in
> *Archaeopteryx*, 
> *Jinfengopteryx* and *Sapeornis* (assuming the
> absence in *S.* is real --  
> none of the specimens is fully articulated) is their
> own peculiarity 
> (autapomorphy), and the presence of ossified sterna
> is the ancestral 
> condition for the clade composed of birds and their
> closest relatives. 

But *is* an ossified sternum a monophyletic trait? I'd
rather be tempted to put it on hold in the "avian
characters theropods were prone to evolve one way or
another" list. Just as with feathering, flight, beaks
or fusion of distal caudal vertebrae.

(I would in a pinch not even accept stem theropods
w/ossified sternum as proof, until I have not seen and
compared the specimens with my own eyes.)

I'd have to see specimens to decide whether ossified
sterna are symplesiomorphies or parallelisms. There
are far too many of the latter in the whole theropod
clade to make me cautious. Archie and Rahonavis both
had flight-enhancing pelves, but looking at specimens
it is quite obvious they are very much homoplasies as
far as adaptations for flight are concerned (the
hypopubic cup of Archie and the hypertrophied pubic
boot of Rahonavis, to be precise). Possibly very much
equivalent - but not at all equal.

I agree for Jingfengopteryx BTW. Sapeornis, OTOH, ties
in too shakily into the whole lineage to make me want
to make anything of it.

Regards,

Eike


                
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