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Re: Origins (was: Re: Sharovipteryx)




Brian McCarthy <philidor11@snet.net> wrote:

> For contrast, the Very Honored, Patient, and Tolerant Mickey Rowe commented:

   >blush<  Thank you.

>> If you find that, for example, parakeet, anole and goldfish
>> acquired one of the principal components of color vision from their
>> most recent common ancestor then you can infer that dinosaurs
>> should have inherited that component as well.

> We can find:
> -the most recent common ancestor of parakeets, anoles, and goldfish
> -what a principal (essential?) component of color vision might be
> -a way of determining that this component was inherited straight through to
> the modern species
> -the connection to dinosaurs
> -the dominance of this component so well proven that we can infer that dinos
> must have inherited it.
> I know you were joking, but it's a wonderful example of rampant assumptions.

Actually, the only part that was a joke was the "somebody ought to
write a paper" part.  That was a joke because I just did write such a
paper.  The main reason for my footnote was that I realized as I
generated the example that it might seem like I was holding up my own
work and saying "see, I do science and you don't", but I did not
intend that at all.  I started to use other examples, but as I tried
to flesh them out I felt uncomfortable with the proposition of not
really knowing what it was I was talking about (for instance I didn't
want to talk about stegosaur plates since I haven't read that paper of
Jim's; sorry Jim and Nick).  Plus I was in a hurry, so it seemed
standing on familiar ground was more than prudent.

Anyhoo, to address some of your concerns -- you need not find the most
recent comon ancestor of the three animals in question.  You merely
need to have valid (i.e., tested) reasons for accepting that, for
example parakeet and anole or parakeet and goldfish form a
phylogenetic bracket around most if not all dinosaurs.  The principal
component I had in mind was a complement of visual pigments -- when I
wrote yesterday I was just trying to stay away from jargon.  Since you
seem to want more... All known visual pigments appear to have derived
from a single molecule, but those pigments have diversified between
and within many lineages.  One necessary component of color vision (as
it's constructed in all animals, so far as we know) is that they have
at least two visual pigments.  Read my paper if you want details, but
phylogenetic reconstructions of the visual pigment gene histories
strongly indicate that those three animals derived from a common
ancestor that had four pigments.  It is possible to argue that
multiple visual pigments is not enough for color vision to exist, but
I don't yet know of any animal which has multiple pigments and no
ability to make spectral (read color) comparisons.  Certainly the
animals I named all make color discriminations...  We can farther test
the original hypothesis "dinosaurs had color vision" by testing to see
if the neural mechanisms used to make color comparisons in the three
animals are the same.  If they differ in some substantial way then the
hypothesis would be weakened because you could argue that dinosaurs
had the four pigments but we can't make a strong case for how they
made use of them.  The 'way of determining that the component was
inherited "straight through"' appears to me to be a misunderstanding.
However, if it means anything to you, I can restate what I wrote above
as: best evidence indicates that the four pigment genes in the three
animals I named are orthologs of each other.  The connection to
dinosaurs is the phylogenetic bracket.  Read Larry Witmer's papers for
a discussion of that topic and how it allows us to qualify the
confidence we have in our assessments of various aspects of the
biology of extinct animals.  I'd give you a more complete reference,
but I think Ralph is planning to do that.  And for the last point, we
again have... the phylogenetic bracket.  I also proposed that scleral
ossicles provide an osteological correlate which would allow us to
infer several things about the anatomy of dinosaur retinas.  If you
really want to attack my example, I suggest you read my paper in its
entirety.  I'd welcome critical feedback.  You can find the article
at:

http://www-odp.tamu.edu/paleo/toc.htm

If you find my article has "rampant assumptions" and you think any of
them are even a little unreasonable I'd like to know about it.

-- 
Mickey Rowe     (rowe@psych.ucsb.edu)