Re: Sclerotic rings

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

If you want to investigate scleral ossicles, I invite you to find:

Motani, R., Rothschild, B.M., andWahl Jr., W. (1999). "Large eyeballs
     in diving ichthyosaurs", _Nature_, 402:747.

(A URL to Motani's work was already listed here...)

Hall, B.K. and Miyake, T. (1992). "The membranous skeleton: the role of
     cell condensations in vertebrate skeletogenesis", _Anatomy
     and Embryology_, 186:107-124. 

(IMHO there is a big question as to whether or not all bony rings
commonly referred to as scleral ossicles are in fact homologs of each
other.  Knowing in which lineages they were evolved and lost might help us
understand their function in the different groups that have them.  The
above discusses how they are formed during embryogenesis in some
groups.)

Canavese, B. Fazzini, U. and Colitti, M. (1994). "Morphometric analysis
     of the scleral bony ring with different numbers of ossicles in
     the eye of Coturnix coturnix japonica", _Anatomia, Histologia,
     Embryologia: Journal der Weltvereinigung der
     Veterinearanatomen_, 23:128-136.

(The number of ossicles constituting a ring ranges from 4 to something
like 28.  The above looks at the development of the bones in Japanese
Quail; IIRC they were interested in looking at how variation in the
number of plates might be used in phylogenetic reconstruction, but I
don't have the paper with me.)

Andrews, K.D. (1996). "An endochondral rather than a dermal origin for
     scleral ossicles in Cryptodiran turtles", _Journal of Herpetology_,
     30:257-260.

(Contrary to the previous papers (including Canavese et al.)
indicating that reptilian ossicles form by direct ossification, the
above claims ossicles in turtles develop by ossification of cartilage
as they do in fish -- compounds the problem in understanding homology
of ossicles in comparing different groups.)

Suburo, A.M. and Scolaro, J.A. (1990). "The eye of the Magellanic Penguin
     (Sphiniscus magellanicus): structure of the anterior segment", _The
     American Journal of Anatomy_, 189:245-252.

(A major reason for the existence of ossicles in reptiles (including
birds) appears to be that they anchor a kink in the eye where the
cornea meets the sclera.  Without the ossicles, the eye would tend to
balloon outward at that kink because the pressure inside is higher
than the pressure outside, and the sclera and cornea are not rigid.
Penguins retain ossicles without retaining the kink, thus putting a
kink in that explanation for why reptiles have ossicles.)

Nakamura, K. And Yamaguchi, H. (1991). "Distribution of scleral ossicles
     in Teleost fishes", _Memoirs of Faculty of Fisheries of Kagoshima
     University_, 40:1-20.

(Looks at the distribution of ossicles in fish and finds they
predominate in the eyes of fish like tuna which are large and fast-
swimming.  Suggests as Al Fraser did that differential pressure across
the front of the eye required an evolutionary response.  These animals
also tend to have large extraocular muscles, and the ossicle may
provide support for them.)

and still probably the most important one:

Walls, G.L. (1942). _The Vertebrate Eye and Its Adaptive Radiation_, The
     Cranbrook Press, Bloomfield Hills, Michigan.

(If you ever want to know anything at all about comparative anatomy of
eyes you should start here and then look for papers that cite it and
deal with whatever group(s) you're interested in.  The explanation
Matt Bonnan attributed to Hildebrand, 1995 actually belongs to Walls,
1942.  There's more detail than provided in the extract from
Hildebrand; if you want a deeper understanding of how ossicles are
used by reptiles in accommodation and supporting the corneal-scleral
sulcus (the kink I mentioned above), you should find Walls and read
the relevant sections.)

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