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Re: more dinosaur extinction stuff



I'm not sure exactly where the pineal gland story might lead, but
there are a few things I want to tackle in flying goat's message (just
out of curiosity--is there a story to that nome de net?)

>Then I wondered what this group would have in common that dinosaurs
>did not, and truth to tell, I woke up one day wondering why the
>remains of the primitive third eye seemed to survive so late into
>modern times, since it seemed such a vestigal thing.

In what animal are you suggesting that the eye is vestigial?  In some
animals, particularly tiger salamanders and fringe toed lizards, it
has been postulated that it is the pineal gland which is responsible
for the animal's perception of optical polarization (I'll fill in
details if there's interest).  The pineal gland of these animals has
working photoreceptors, with molecular machinery quite similar if not
identical to that operating in the retinal photoreceptors.  The fact
that the third eye is postulated to have strong adaptive effects on
behavior suggests it is anything but vestigial in these animals.

In other animals (like us) with opaque tissue overlying the pineal
gland, said gland appears to function as a chronobiological endocrine
organ only.  That is, it lost some of the direct sensitivity to light,
but it maintained the function it had as a time giver anyways.
Although the pineal photoreceptors are gone, connections between the
retina and the pineal still offer timing information that allow the
gland to retain some of the function it probably always had.

>Then I tried to remember what creatures' behaviors currently is
>believed to be affected by the effects of light on the receptors of
>the pineal gland, and realised that these were mammals, lizards,fish,
>birds, etc.

Is it true that in all of these cases there are representatives with
pineal photoreceptors?  If it's true of birds, then parsimony suggests
that it would have been true of some of the other dinosaurs as well
(yes, I recognize that you covered this possibility by suggesting that
the death of those without could have triggered the extinction of
those hypothetically few with, but that still leaves questions such as
"why birds and not other theropods?" untouched).  I think it's
actually not true of any mammals--my impression is that all cyclic
activity in the pineal glands in that group is due to the effects of
information flowing from the retina to the pineal.  I think that's
actually true of birds as well.  In any case it would likely be true
of dinosaurs also since the pineal as an interpreter of zeitgebers
(i.e. timing cues) is clearly an ancient condition.

Finally, I'm curious if you have any kind of mechanism in mind.  If
anything it seems to me that animals relying on circadian and seasonal
cues would be more effected by sudden changes in light cycles than
would those that didn't make use of such cues.  Why would a pineal
gland with or without a third eye protect a clade from the ravages of
an asteroid winter?

I hope I don't seem antagonistic here.  Such is not my intent.  I'm
only attempting to probe both the data and your hypothesis.

-- 
Mickey Rowe     (rowe@lepomis.psych.upenn.edu)