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Re: Elongated Pupils

On Tue, 23 Apr 1996, JCMcL wrote:

> Thank you for this posting, Mr. Hardenbrook.  HOrizontally elongate
> pupils occur in animals that have to scan the horizon, or large areas
> around them, for potential trouble like predators.  Hence this sort of
> pupil is common in herbivores like goats, horses, etc.
> In restoring my archosaurian herbivores with such pupils, I presumed that
> they inhabited a world as dangerous to them as that of, say, Cape
> buffalo today, and that they might well have had such
> horizontally-scanning pupils.
> John C. McLoughlin

Hello All,

This is my first post to this list. I'm not a paleontologist but an
ornithologist who studies color vision in birds. There is no need
for me to retrace the evolution of birds and mammals since the
thecodont-therapsid split for a group such as this. The above text
equates "animals" and "herbivores" with mammals, however, it is the
avian eye that will be closer to that of dinosaurs, as you know.

It is my view that the largely diurnal birds inherited their eye
from diurnal theropods, with apologies to proponents of the
crocodilian and archosaur ancestry hypotheses (I'm a student of
Ostrom :-)). Regardless, the avian eye developed from a diurnal
eye, not from the mammalian eye which became highly modified from
its diurnal ancestor through 100 million years of nocturnality.
Though the mammals mentioned here are now diurnal, their eye is
secondarily adapated to daylight environments. The retinas of
diurnal mammals differ from that of diurnal birds in several very
important respects. Now, I missed part of this thread. If John
is referring specifically to non-dinosaurian archosaurs, it will
still be the case that the avian eye, not the mammalian eye, is to
be expected to be the better model.

I have no expertise on aspects of the avian eye anatomy, morphology,
or function outside of its capacity for color vision. Thus, the text
below is paraphrased from published sources which are given at the

There are three general shapes of bird eyeballs, flattened, round
(or globose), and tubular. The majority of birds have the flattened
eye, this is also what is found in lizards. Birds requiring high
resolution at great distances, analogs to John's herbivorous
archosaurs, have round eyes. Owls, and a few other birds have
tubular eyes (Walls, 1942). Martin (1985) states, "the avian pupil
is generally spherical, but there are reports of slightly ellipsoid
pupils." The only birds with slit pupils are the three species of
skimmers (Rhynchops spp.). The pupil of the King Penguin (Aptenodytes
patagonica) can contract to a perfect square, the only known bird
with a square pupil. In the limited time I've had to look into this,
I was not able to find a discussion of horizontally elongate pupils
in birds, nor can I ever recall seeing any and I've looked at a lot
of bird eyes. Walls (1942, pp. 161-162) says:

  "The weak amphibian sphincter pupillae is replaced by a much more
   powerful one in the reptiles and here, as in birds also, the
   iris and ciliary muscles are of the *striated* variety. ... At
   any rate, the sauropsidan iris is capable of extremely rapid
   action, though particular species do not necessarily ever tax
   this capacity. ..."

  "Bird pupils are very active, but the photochemical changes have
   made a phylogenetic 'come-back' in this group. The paradox is
   resolved when one notes the lack of precise adjustment of the
   avian pupil to illumination. It plays so much that, although
   experimental proof is as yet lacking many workers have suspected
   it of being under the bird's voluntary control. At any rate, it
   is easy to understand why in the birds the retina has had to
   re-assume the responsibility of regulating its own stimulation -
   the pupil can not be trusted to do so."

Of related interest is Graham R. Martin's schematic eye models for
birds (Martin, 1993), which will be helpful to artists. In this
chapter he discusses pupil size and image brightness, also stating
"only three species of birds, the Skimmers (Rhynchopidae), have
pupils that are not circular (Zusi and Bridge, 1981)."

And, finally, an additional body of vision literature which may be
of interest to you is that of visual fields. Again, refer to Graham
Martin, and his collaborator, Gadi Katzir. One particularly good
avian model to study is the Hirundinidae (swallows) which feed on
the wing and have an eye adapted to scanning the horizon (but I'm
unable to locate the references tonight). These and other birds
have two foveae per retina, associated with binocular or monocular
visual fields.

Selected references:

Walls, Gordon L. 1942. The vertebrate eye and its adaptive radiation.
Cranbrook Institute of Science Bulletin No. 19: Bloomfield Hills, MI.
[This is one of the 20th century classics of vertebrate literature.]

Martin, G. R. 1985. Eye. Pp. 311-373 (Chapter 7) in, A.S. King and J.
McLelland (eds.), Form and Function in Birds, vol. 3. New York:
Academic Press. [This is the best single chapter for an introduction
to the avian eye in all its aspects.]

Martin, G. R. 1993. Producing the image. Pp. 5-24 (Chapter 1) in,
H. P. Zeigler and H.-J. Bischof (eds.), Vision, brain, and behavior
in birds. Cambridge, MA: A Bradford Book, The MIT Press. [Presents
his schematic eye model and discusses pupil size & image brightness,
eye position & visual fields, among other topics. A very good and
informative chapter, places everything in perspective.]

Martin, G. R. 1994. Visual fields in woodcocks Scolopax rusticola
(Scolopacidae; Charadriiformes). J. comp. Physiol. A 174: 787-793.
[I include this just to give a good example closely related to
John's objective. All the references I'm including here have
excellent bibliographies in which other papers on this topic are
to be found.]

Meyer, D. B. 1977. The avian eye and its adaptations. Pp. 549-611
(Chapter 10) in, F. Crescitelli (ed.), Handbook of sensory
physiology, vol. VII/5, The visual system in vertebrates. [Similar
in objective to Martin, 1985. Well worth looking at, more
physiological than Martin, good figures & bibliography. Meyer will
be familar to those who refer to P.D. Sturkie's "Avian Physiology"
chapter on vision.]

Polyak, S. 1957. The vertebrate visual system. Chicago: University
of Chicago Press. [Another classic. A very interesting evolutionary
synthesis based upon the study of the visual system. Polyak, a
vision researcher, clearly assumes birds evolved from dinosaurs (in
1957) and discusses this in relation to the modifications required
in the eye and central nervous system, especially for eye-wing

Well, I hope this will be useful to John and other artists and also
to all paleopersons trying to analyze soft body parts. Now, if we
can just get the AMNH to stop putting feathers on Mononykus!

Byron K. Butler         "A liberal education ... frees a man from
58 Windfall Lane        the prison-house of his class, race, time,
Guilford, CT 06437      place, background, family and even his
byron.butler@yale.edu   nation."     ---  Robert Maynard Hutchins