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RE: "The X Digit"...a Pteroid bone??





Larry Febo wrote... 
< I am a minimalist when it comes to
Convergence. When it involves  the use of different morphological structures 
fine, but when the same skeletal constituants are involved I become extremely 
doubtful, especially when the types being compared are close together in 
phylogeny. (in this case being both archosaurs) >

      A close relationship also means that two different groups evolving a 
similar adaptation are likely to use the same parts, since they have the same 
parts at their disposal.  In order to fly, both pterosaurs and birds needed to 
develop some kind of paired aerodynamic structures.  The only real coincidence 
that one might argue represents homology is that they decided to use their 
forearms, instead of their ribs (as in _Icarosaurus_ and _Draco_) or legs (as 
in _Sharovpteryx_).  
     Since both "decided" to use their arms, they would have been taking 
advantage of the same arm bones, pectoral girdle bones, and muscles.  Flapping 
was aerodynamically advantageous for developing true flight.  Since the 
archosaur arm already has a nice joint between the humerus and shoulder, it 
isn't surprising that both groups chose to use it; and not surprisingly, the 
muscles used to move the arm in the proper flapping motion, being homologous 
and attaching to roughly the same place on the humerus and pectoral girdle, 
developed similarly. 
     Also note that the convergence is far from complete; feathered wing of the 
bird and the membranous wing of the pterosaur represent a pretty fundamental 
difference, and one that must have begun pretty early in the evolution of 
flight in both these groups.  Otherwise, one group must have radically changed 
the wing structure at some point; if this is the case, what was the common 
ancestral wing like?  Was it feathered, in which case the pterosaurs opted for 
a different design, extending the fourth digit out and developing the membrane 
while simultaneously reducing the feathers?  Or was it membranous, in which 
case the birds gradually reduced the fourth finger and membrane,  
simultaneously developing feathers?  Presumably, the switchover was to develop 
a more aerodynamically efficient wing, which seems a little difficult to 
imagine happening since the intermediates with their combinations of 
fundamentally different wing designs would likely have been relatively clumsy.  
  
          
< I do believe that endothermy to the point of egg incubation evolved only 
once, and most likely began in the early Anapsid forms whence it either was 
improved upon by later mammals and birds and relinquished by other forms that 
assumed a condition of secondary ectothermy, such as squamates, turtles, 
crocodiles.>

     By relinquished you mean "re-evolved convergently".  Why is that easier to 
believe in then endothermy evolving convergently?  The cellular basic for 
endothermy is pretty simple; cell membranes are "leaky", allowing Na+ and K+ 
ions to get across more easily (this forces the Na+-K+ pump to work a lot 
harder to fight the ionic gradient, consuming more oxygen and producing heat in 
the process).  Endothermy actually amounts to a simple cellular defect which 
produces an advantageous side effect when it is compensated for.  For more 
detailed information, see:

Else, P.L., and A.J. Hulbert. 1981. Comparison of the "mammal machine" and the 
"reptile machine": energy production.  American Journal of Physiology 240:R3-R9

---------------------------------------.  1985. A allometric comparison of the 
mitochondria of mammalian and reptilian tissues: the implications for the 
evolution of endothermy. J. Comp. Physiol. B Biochem Syst. Environ. Physiol. 
156:3-11.

--------------------------------------. 1987. Evolution of mammalian 
endothermic metabolism: "leaky" membranes as a source of heat. American Journal 
of Physiology 253:R1-R7

Hulbert, A.J., and P.L. Else. 1990. The cellular basis of endothermic 
metabolism: a role for "leaky" membranes? NIPS  5:25-28

     I've only read the first and the last, and those were given as handouts by 
my animal physiology professor, so I don't know what NIPS is.  Apparently, the 
authors have looked at both birds and mammals. 
  
LN Jeff