Re: Sclerotics and nose bending (fwd)

[Betty Cunningham <bettyc@flyinggoat.com>]

forwarded with John's permission

John V Jackson wrote:
> Hi -
> Maybe it will turn out that the sclerotic ossicles are the bits of bone the
> rings are composed of [yup - it did!], but as for the function of the rings
> themselves, that's probably the thing I've wondered about for longer than
> any other paleo question.
> 
> I think it's to help make the eye non-spherical.  If the eye were perfectly
> spherical, the best place for the lens to be optically would be at the very
> middle of the eye, but it isn't - not even in mammals which do have fairly
> spherical eyes.  In birds (and some of their reptilian relatives) the eye
> isn't shaped like a whole sphere but more approaches a half grapefruit with
> a (very) large cherry sitting in the middle of the cut side representing the
> lens.
> Although a full sphere maintains its shape automatically by internal
> pressure, the half-sphere shape needs some kind of internal mechanics which
> is what the sclerotic ring does.  The shape this allows in turn lets all
> parts of the retina be in focus at once, something that we pathetic mammals
> don't enjoy.
> 
> "Accomodation" is adjusting the shape and maybe the position of the lens to
> get the focus right, but this isn't exactly what the sclerotic rings do;
> they merely allow accomodation when it occurs to produce a sharp image all
> over the retina at once.
> 
> Sclerotic rings tend to be, I think, better developed in animals such as
> woodpeckers and gannets where the eyes are subject to considerable forces.
> Amongst my bird skulls, the woodpecker has the most substantial.  (I have no
> gannet (sob); I found one on the beach at Brighton in 1974 and hung its head
> in a sock out of my bedroom window to reduce, but the landlord found it and
> threw it away!)
> 
> One possibility for the gaps in Coelophysis skull joints being bigger in
> adults might be that the younger ones have softer, perhaps thinner bones, so
> that the bones themselves do some of the bending.  In adults the joints have
> to do all or more of the work so they have to be better developed.  Although
> I still find the kinesthesis of bird skulls confusing (though Chatterjee has
> a good description of it in his Protoavis paper) I think in modern birds
> some of it is due to bending bones - particularly the one joining the top of
> the "nose" to the part between the eyes.  If this is true in modern birds it
> may have been true to some extent in Coelophysis, perhaps supporting the
> 'differential bending of bones with age' idea.
> 
> JJ

-- 
Flying Goat Graphics
http://www.flyinggoat.com
(Society of Vertebrate Paleontology member)
-------------------------------------------<,D,><