Growth pattern and visual acuity of the Caypullisaurus eye (Ichthyosauria: Ophthalmosauria)

FERNÁNDEZ, M S; ARCHUBY, FERNANDO M; TALEVI, M; EBNER, R

Göttingen, Alemania

Congreso; 74. Jahrestagung der Paläontologischen Gesellschaft ?GEOBIOLOGIE? (74o Congreso Anual de la Sociedad Alemana de Paleontología ?GEOBIOLOGÍA?); 2004

Deutsche Paläontologischen Gesellschaft (Sociedad Alemana de Paleontología )

In the eyes of many extant reptiles there is an osseous ring embedded within the sclera and around the cornea. This sclerotic ring was well developed and is frequently preserved in ichthyosaurs. Significant information can be drawn from it, such as eyeball diameter and relative size of the cornea. Estimation of these parameters in extinct forms is significant as they are closely related to vision capability. We present here insights into the growth pattern and vision capabilities of some Jurassic ichthyosaurs as inferred from the analysis of the sclerotic rings and orbit sizes. The suggested growth pattern of Caypullisaurus eyes is similar to that of human eyes. Thus, the cornea did not grow as much as the rest of the eye. The relative size of the sclerotic ring and orbits can be used as a complementary criterion for estimating relative age in some ichthyosaurs. In juveniles the aperture of the sclerotic ring represents approximately 20% of the orbital area. This percentage decreases in adults. The most striking property of the parvipelvian ichthyosaur eye is its large absolute size. The absolute size of an eye is very important as it is absolute rather than relative size which determines visual acuity. An increase in the eyeball diameter results in an increase of the resolution of the details of the image.  If the simplest way to increase the visual acuity is to have a large eye, then all parvipelvian ichthyosaurs must have had a good visual performance. Previously, it has been suggested that the large eyes allowed Ophthalmosaurus to hunt small, fast-moving prey. This could be also true not only for this genus but for other parvipelvians (like Caypullisaurus), as squid-like cephalopds (belemnites) seem to be an important item in fish-shaped ichthyosaur diet. It is noteworthy that in some belemnites, the presence of an ink sac has been suggested. Animals that fed on them must have had high visual acuity. Also it must be considered that ichthyosaurs were probably intermediate species within a food web, so a good visual performance could be significant not only for prey detection but also for escaping from predators.