RE: Sclerotic rings

["Al Fraser and Laurie Fletcher" <alflaurf@abs.net>]

Ron Orenstein asked:


"Since sclerotic rings are not precisely linked to diving but are linked to
large eye size (in birds and ichthyosaurs), could they not simply help
either to maintain the shape of the eyeball under normal circumstances, or
to protect it against physical injury, or both?"

.......

I have scant knowledge of the comparative anatomy of animals with and
without sclerotic rings, and I have read no functional morphology papers
about them.  However, I will hazard some tentative replies and thoughts.

I think that sclerotic rings likely do exist for mechanical support of eyes.
If there is collagenous tissue from the ring to the eye (is that true,
anatomists?), then even tensile and shear loads would be distributed from
the eye to the ring in a way that would reduce deformation.  These supported
loads would include those due to gravity and those due to acceleration of
the animal.  The loads due to moving through the water can also be
appreciable, and these loads are vectors (not isotropic like hydrostatic
pressure) that push in a particular direction.  At twenty knots (how fast is
your ichthyosaur?) the peak pressure on the head would be about half of an
atmosphere (this force varies with the square of the speed).

Your statement that the rings are linked to eye size would argue in favor of
a simple ring-as-support hypothesis.  The weight of the eye (for a given
shape) is proportional to the cube of its dimensions, and the surface to
support it and transfer forces due to acceleration is proportional to the
square of the dimensions.  Thus, external support would be more useful in
larger eyes.

To try to sort out this matter I would ask the following questions (not
necessarily expecting answers in this forum):

Is there a trend towards presence of sclerotic rings in animals with flatter
eyes or ones that are less encased in a bony cavity?

Is there a trend towards sclerotic rings with faster swimmers (or animals
that plunge into the water from airborne?)

Is there a trend towards sclerotic rings in animals that routinely undergo
higher accelerations?  (Are there bird data available for this question,
Ron?)

Al Fraser