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Re: bauplan convergence

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.

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