To me the "quill" scenario would only make sense if they started on
the head and neck, and then quickly spread down the still vulnerable spine
of the animal.
However, the spine defense strategy in vertebrates seems to be a bunch
of shorter spines more scattered over the body (like prickly pufferfish) or
dorsally scattered in those that can curl up into a ball (like spiny
hedgehogs and echidnas). Even in porcupines (with the longer barbed
variety) it is a relatively scattered whole-body strategy.
That's one reason I prefer hypotheses of predatory evasion "by
deception" rather than a purely physical spiny defense in the initial
stages. An attractively colored and expendable tail tip is still used by
some lizards today, and if filaments extended beyond the tail tip, the
predator may have gotten nothing but filaments.
And if such filaments became barbed and painful to the mouth of
predators, their color would then serve to frighten off any predator that
had a previous painful experience (and then spreading up the spine of the
animal would become an effective strategy). But this kind of strategy would
only work if it began on an relatively expendable tail, not a critical area
like the head, neck or spine.
I will think about quills starting on head and necks and then spreading
down the back, but right now I prefer a "tail deception" strategy which was
exapted and then spread up the back. Quills and spines could have easily
been exapted from such filaments, and even if spines were the original
function, they could have begun on the tail (as in spiny-tailed lizards like
Uromastix).
In any case, I think predatory evasion (of some sort) preceded
exaptation for thermoregulation (which in turn apparently preceded the
exaptation for aerodynamics).
----------Ken
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From: "philidor11" <philidor11@snet.net> Reply-To: philidor11@snet.net To: <twilliams_alpha@hotmail.com>, <dinosaur@usc.edu> Subject: Re: feather tracts Date: Sun, 15 Jul 2001 10:00:15 -0400
Ken Kinman wrote: <The aerodynamic function was undoubtedly the last major exaptation. Before that was insulation. But the tendency of feathers to grow in tracts would indicate some function even before the insulation phase.> and Tim Williams replied: <<snip> Besides, even if you are correct, the growth and arrangement of feathers in tracts may reflect a developmental trait, and may have no evolutionary (i.e. selective) significance whatsoever.>
Though it may provide a clue about the prior condition. (The absence of facts provides a comfortable space for pure speculation. Here I am.) Anything immediately wrong with the following scenario? The hard patches on dino integument begin to extend as protection against attack, becoming higher, wider, thicker. As they do, instead of a single smooth surface they split into individual quill-like units. These quills would not have to cover the whole body, but would be effective as just rows. When these rows get long enough to cover the remaining skin, they also have value as insulation. Does this 'feathers began as protective quills' idea make sense? One useful consistency would be if the harder 'studs' on dino skin align as rows, and I'm not sure if that's true. Should I forget about this now, or develop a fondness for the idea?
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