Re: bauplan convergence

["Scott Hartman" <scott_hartman@hotmail.com>]

> > > Neither display nor insulation provides a >primary cause< for the 
> > > evolution of feathers. When an organism has no feathers, it cannot use 
> > > them for display, for brooding, or for insulation(!). This would require 
> > > the organism to predict the future (e.g., "I will need insulation and to 
> > > show off, so I'll evolve feathers"). Rather, the first feathers evolved 
> > > for some other reason, and as soon as or very shortly after they 
> > > appeared, they would have found secondary uses (exaptations) for display 
> > > or for insulation (or for parachuting). But the primary cause must have 
> > > been something positive and internal. The best suggestion I've read so 
> > > far is that feathers appeared as a means of sulfur excretion through 
> > > molting. Excess sulfur is used up in the
keratin proteins of the feathers, which are shed from time to time during 
the animal's life. The excess sulfur itself may have resulted from a 
metabolic shift toward homeothermy in the archosaur lineage.<<<<

    I've always been fond of the excess sulfur speculation on the origin of 
feathers, although (or is that "because" ;-) it's largely untestable for the 
forseeable future.  Here's an equally plausible scenario:
    Endothermic dinosaur species X has a hard time sheding excess heat in 
its warm Mesozoic locale.  One individual has elongate scales, which 
increases surface area, enhancing cooling behaviors such as sitting in the 
shade.  It doesn't (much)increase the surface area visible to the sun, so 
there is little negative impact on thermodynamics even when dinosaur X is 
forced into the midday sun.  As the scales continue to elongate, the surface 
area for cooling increases, further benifiting our dinosaurs descendants. 
(Note that I'm not refering to long spatulate scales as is often pictured in 
discussions of feather evolution, but rather thin conical ones that would 
develop from elongating roundish pebbled dinosaur skin, rending the annimal 
with a bristly appearance  not un similar to the proto feathered critter 
Luis Rey painted for your "Birds Came First" article in Omni) Further 
elongagation of these scales would continue to provide cooling benifits to 
descendants, until the scales started to overlap and trap air beneath them, 
at which point they could start to serve as insulation.  Everyone thinks of 
insulation as keeping animals warm, but it was hot in the Mesozoic, and 
small animals are more prone to overheating than large animals.  A small, 
tropical, endothermic dinosaur would have benefitted greatly from protection 
against the sun (if only because it would have increased midday foraging 
time during the summer), after which its descendants could have exapted it 
for use in cooler climates, to look sexy, to fly with, etc.
    I admit I'm more or less pulling this out of thin air here, but th idea 
does generate testable predictions (i.e. future fossil finds), and there is 
an interesting bit of speculation I can pull from feather ontogeny that may 
be wonderfully consistent with it.  During development, the feather is 
encased in a keratinous sheath that is filled with a blood supplied pulp 
cavity.  As the feather further develops, the pulp cavity leaves behind a 
series of division in the calamus (quill) called pulp caps.  Of course, 
these are filled with air.  I mention this because a blood filled epidermal 
structure would do a superb job of cooling an animal in the shade, while a 
hollow tip would protect the blood supply from direct sunlight, while 
simulaneously pre-adapting the structure for insulation once an air layer 
could be trapped underneath the pellage.

    Just somethin' to think about.

Scott
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