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feathers and spines--another alternative?



Dear all,

It's not my area of expertise but, for some time, I've been musing over
the early evolution of feathers.

Recently, philidor11 sent the following:

> So, we need something that would have an advantage and which would share
> some characteristics with feathers.
> One possibility is quills, as on porcupines.  These are modified guard
> (long) hairs on the mammal, so the analogy is inexact, but if some
> dinosaurs
> had protective, hardened areas on their skin, the material used to produce
> those areas might well be unique.

That prompted me to toss in this bit of speculation (for whatever it's
worth). :-)

Rachis came first:

Regardless of whether they arose as modifications of  scales, scutes or
a novel structure, feathers first evolved as barbed, erectile, defensive
spines. And, they did so in an ectotherm.

Contour feathers were the first true feathers:

Contour feathers arose when such spines were exapted for temperature
control in a behavioral homeotherm. Of necessity, spines would have
altered airflow near the body changing the rate of heat loss while also
effecting the efficiency of heat uptake during basking. Furthermore,
their erectile nature would have given the animal some control over the
balance between the two factors. Elongating the lateral barbs while
developing a means of constraining them to a single plane could have
yielded a "best compromise" solution that maximized the potential for
insulation while retaining the ability to bask efficiently (and,
initially, retaining much of the defensive function). What I'm
visualizing, is an animal  that would hold its feathers perpendicular to
the body while basking in the morning, alter their position to "self
shade" selectively during the day to avoid over heating and flatten them
to the body at dusk to conserve heat and extend activity into the
evening.

At an early point, spines/contour feathers were exapted for display as
various elongated crests on the forelimbs, tail, ect. This opened the
possibility of a second level of postural temperature control (as well
as exaptation for brooding, generating drag, etc. as per recent
discussions).

With the transition to endothermy, relatively simple mutations altering
the density and length of the contour feathers as well as "mistakes"
resulting in suppression of the rachis and/or barbules could shift the
functional balance in favor of insulation.

So, the "dinofuzz" seen in the recent Chinese specimens would be derived
and probably homologous to the hair-like feathers of modern ratites
regardless of whether or not they were secondarily flightless.

Prediction:

"True" protofeathers, when found, will take the form of relatively
large, course, widely spaced spines with laterally elongated barbs.

Which brings to mind another recent post by Mickey Mortimer:

> Praeornis Rautian 1978
> P. sharovi Rautian 1978
> Late Jurassic
> Balabansai Formation, Kazakhstan
> Holotype- (PIN 2585/32) feather?
> Comments- Normally, I wouldn't include a feather taxon, but this one is 
> rather controversial.  A photo is > included and
> really doesn't look like a feather to me.  The shaft is much too wide (about 
> one fourth the total feather
> width) and the
> barbs are pointed spine-like structures without barbules.  Despite this, SEM 
> data (Glazunova et al. 1991) > and an
> analysis by Unwin and Bakhurina suggest it actually is a feather.  I'll have 
> to see those analyses to believe > it.

Perhaps? :-)

Back to lurking.

Cory Pittman
Cory@cet.com