dinosaurs and color vision

[Mickey Rowe <rowe@psych.ucsb.edu>]

As HP's Kirkaldy and Pharris noted, I wrote to the editor of _Discover_ in
response to their article by William Speed Weed on how dinosaurs might
have looked.  My original letter was substantially longer and more
detailed than the printed version, but I was reasonably satisfied with
what got published (my main goal was that they not attribute to me any
comments with which I disagreed).  I am not as satisfied with
Mr. Weed's response.

Weed begins with a gross mischaracterization of the extant
phylogenetic bracket (EPB) method.  What HP Witmer did in formalizing
EPB was attempt to rescue the term "speculation" from the pejorative
connotations that frequently ride along with it.  Witmer said, in
effect, that we can categorize various claims (speculations, if you
will) about extinct animals according to the types of empirical data
that we have which support those claims (or significantly fail to
refute them -- I add this for the Popperians :-)  With Witmer's
classifications we can discuss the certainty with which we hold a
statement to be correct.  Weed's caricature would leave naive readers
with the belief that application of EPB leaves you with a statement
which is either fully supported or merely a "guess".  IMHO a much
better assessment of the situation with respect to scleral ossicles
and color vision appears in my own paper on the subject:

     [...] if we ultimately reject the logical linkage between scleral
     ossicles and diurnal activity patterns - and hence the retinal
     characteristics that appear linked to diurnality in reptiles -
     then my inferences about Deinonychus' retina merely become Level
     I' [as opposed to Level I -- MPR ] in Witmer's (1995)
     terminology.  That is, we would still have good reason to infer
     that dinosaurs had retinas more similar to those of modern birds
     than those of say, modern mammals, but our confidence in the
     inference would not be quite as strong.

My reference to "Level I" and "Level I'" refer to two of the
categories Witmer created in his description of the EPB method.
Nowhere in Witmer's description is "Level I'" equated to a "guess".

Let me back up a bit and return to the original claim with which I
took issue.  Weed stated that imagining dinosaurs with camouflage
relies on "unscientific assumptions about the ability of dinosaur eyes
to distinguish colors".  I took that usage of "unscientific" to mean
that either a) there is no empirical evidence which could be used to
analyze statements like "dinosaurs could distinguish lights based on
the light's spectral qualities" or worse b) there is empirical
evidence suggesting that the statement in a) is incorrect.  However,
our current understanding of evolution and the biochemistry of color
vision indicates that the statement: "dinosaurs could not make hue
discriminations" is generally false.  That is, it could not have been
true unless parakeets and goldfish do not form a phylogenetic bracket
around any dinosaurs.  Accepting current consensus -- that birds are
direct descendents of (other) animals we'd call "dinosaurs" -- then
the best fallback position would be that every lineage of dinosaurs
not on the direct line to modern birds lost their capacity for color
vision immediately after the branch point that separated their
ancestors from those of birds (read: it's slightly more tenable to
assert "most dinosaurs could not make hue discriminations").  While
this is a logical possibility it is certainly not anywhere near
parsimonious.  Parsimony indicates that all dinosaurs were
well-endowed for making color discriminations.  Note that this doesn't
make it the TRUTH(tm), but it is what the science indicates.

Does Weed stand by his statement that it is "unscientific" to claim
dinosaurs could see in color?  From his response I can't tell.  I have
no idea what his reference to "common sense" means in this context.
Albert Einstein defined common sense as the set of preconceptions that
people build up before their 18th birthday.  Although I don't know as
that I'd put an 18-year limit on it, I'd agree that another name for
common sense should probably be "prejudice".  In any case, IMHO, the
phrase "common sense" should be eschewed in discussions about science.
All I care about is making a case for or against a hypothesis based on
all the relevant evidence.  Best evidence indicates that dinosaurs had
color vision.  Hence, contra Weed's original phrasing, it is not
unscientific to assume dinosaurs had color vision.  Whether that
statement is in line with I or anyone else thinks "ought" to be true
is irrelevant.

I also note in passing that Weed's original logic was flawed since
dinosaurs might have needed to be camouflaged from animals besides
other dinosaurs.  Weed claimed : "Most dinosaur colorations [...] are
based on guesses about the landscape the creature lived in and its
need for camouflage.", and later on in disparaging that artistic
trend: "we cannot know how they evolved to hide from one another."  As
has been noted on this list in the past, cephalopods are near the top
of the list of animals that use camouflage well.  And current best
evidence indicates that they themselves do not see in color.

Finally, to pick a nit...  Weed states: "Finding an osteological
correlate to color vision cannot be accomplished by genetic analysis,
because we have no dinosaur DNA."  Apparently Weed does not understand
that "osteological" refers to the study of bones.  It might have been
better to say "Finding an osteological correlate to color vision
cannot be accomplished by genetic analysis because genes aren't
bones."  I know that was a cheap shot, but I really was disappointed
by the lack of scholarship in Mr. Weed's response, and it concerns me
that anyone unfamiliar with the subject matter will think he actually
knows what he's talking about.

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

P.S. While I'm here, I should mention that the dinosaur list is moving
to another computer.  The addresses for the list and its
administration will *NOT* change, so you should not notice anything as
a result of this move.  But accent the "should".  The list was down
for a while this morning because of a hard drive failure on the new
machine.  If you sent a message and it was returned to you with a
"User Unknown" error, please resend it.

References:

Rowe, M.P. 2000. Inferring the Retinal Anatomy and Visual Capacities
     of Extinct Vertebrates.  Palaeontologia Electronica, vol. 3,
     issue 1, art. 3: 43pp., 4.9MB.
     http://www-odp.tamu.edu/paleo/2000_1/retinal/issue1_00.htm

Witmer, L.M. 1995. The extant phylogenetic bracket and the importance
     of reconstructing soft tissues in fossils, p. 19-33. In Thomason,
     J.J. (ed.), Functional Morphology in Vertebrate Paleontology,
     Cambridge University Press, Cambridge.