Schmitz and Motani measured the eye-socket size and the external and internal scleral ring diameter in 33 Mesozoic archosaur species, and compared them with similar measurements from 1401 living species of mammals, birds, lizards and snakes with known activity patterns.
“There’s certainly nothing wrong with applying that to dinosaurs.” “If you have a particularly circular orbit and a pretty circular scleral ring you can get a reasonably good idea of the size of the eyeball and the size of the iris within the eye,” says David Hone of University College Dublin in Ireland, who was not involved in the study. This means they can avoid overstimulating the retina and contract the iris for better depth of focus. Fossils with a large internal scleral ring diameter relative to the diameter of the eye socket, or orbit, probably had a large pupil too – suggesting they were nocturnal.īy contrast species active by day have a small pupil for a given eye size. Living nocturnal species tend to have a large pupil for the size of their retina, and this helps them see in dim light. In modern archosaurs, the size and shape of this ring relative to the eye socket varies depending on whether the animal is diurnal, nocturnal or is active mainly at dawn and dusk.
The soft tissues of the eye, including the iris, rarely fossilise, but most archosaurs have a ring of bones within the eye called the scleral ring, which is preserved in some fossils.
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