Re: sclerotic rings

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>Questions: what is the function of sclerotic rings in dinosaurs?
>Ornithologist consulted had no ideas, except that owls have very large
>rings, cannot rotate their eyeballs, so rotate their heads instead. They
>attribute this to carnivory. But the dinos with sclerotic rings are
>herbivors,and some are large.

The standard story for the purpose of sclerotic rings is to change the
shape (and thus the focus) of the eyeballs.  A band of muscle attaches to
the ring - when the muscles contract, the ring shrinks, the eyeball becomes
more oval, and the focal angle changes.  When the muscles relax, the ring
expands, and the eye resumes the normal shape.  This would give quite an
effective control of visual focus to the animal.  However, I don't know how
well studied this structure is in modern sauropsids.

Sclerotic rings are found in many dinosaur groups, not just hadrosaurids
(for example), including predators.  They are very rarely preserved or
found, however, due to their delicate nature.

This feature is characteristic of many reptilian groups.  If you want to
see some big sclerotic rings, check out the eyes of ichthyosaurs.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.