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Re: Tawa hallae: everything you know about basal saurischians is wrong...



Evelyn Sobielski <koreke77@yahoo.de> wrote:


> IF they were homologous. Were they? And to what degree -
> were they present in the LCA, or (like the wings of birds
> and bats, which are homologous insofar as they are both
> formed by the forelimbs) was only the underlying genetic
> framework present but the actual trait realised
> independently?


Homology is an absolute concept.  There are never 'degrees' of homology.  If 
the integumentary structures of theropods and heterodontosaurs arose 
independently, they cannot be homologous.  Although bat wings and bird wings 
evolved independently, they are both formed by the forelimbs, which were 
present in their closest common ancestor; therefore the forelimbs (wings) are 
homologous.  


> _Tianyulong_ raises an important point: until its
> discovery, phylogenetic bracketing would have predicted it
> to be nude. It wasn't. So it serves to show that even type I
> inference-bracketing is unreliable in the absence of a
> comprehensive sample of taxa of close and known phylogenetic
> position to the taxon in question.


No, because in the case of feathers (or their homologs) phylogenetic bracketing 
only tells you the *minimum* phylogenetic distribution of this trait.  

Feathers were once thought to be exclusive to birds (i.e., the clade defined by 
_Archaeopteryx_).  Feathers have since turned up in some maniraptorans, in 
which integument was preserved.  Feather homologs have turned up in other 
coelurosaurs.  The phylogenetic bracket for feathers (or homologs) has expanded 
several times.  We have only negative evidence that this bracket doesn't extend 
all the way to the base of the Theropoda (or beyond).  Very few 
sites/formations provide us with the luxury of preserved integument (Solnhofen, 
Jehol, Karabastau, Crato, etc).  It's only because we are fortunate enough to 
have a Jehol heterodontosaur (_Tianyulong_) preserved with integument that we 
now know that this ornithischian clade could have fuzzy coats as well.


> Any physiological considerations need to take into account
> that the Mesozoic climate was quite a bit warmer than
> today's. So the drawbacks of an insulating integumentary
> covering would have been more severe then than they are
> now.


Yes, but the nights might have been cool.  This is especially true of arid 
desert environments (which you mention in the context of _Tawa_ and the 
American Southwest).  Also, an active, slender-bodied endotherm would still 
need to prevent loss of internally generated body heat, day and night.


Cheers

Tim