THE VISUAL SYSTEM OF THE PTERYGOTID EURYPTERIDS: IMPLICATIONS FOR THEIR MODE OF LIFE
We investigated the lateral eyes of specimens of the pterygotid Acutiramus cummingsi from the late Silurian Bertie Formation, Ontario, Canada (approximately 423-419 Ma) in the collections of the Yale University Peabody Museum. Each specimen is around 20 cm from anterior to posterior. The eyes are crescent shaped with about 1300 lenses. The lenses reach 330 µm in maximum dimension and are packed onto the visual surface in a hexagonal manner (i.e., each lens has six immediate neighbors). Calculations show that the interommatidial angles (between the optical axes of adjacent lenses) are on the order of 1.50º to 2.00º, indicating a visual acuity comparable to that of many modern arthropods. As the size of the eye increases in larger specimens, the number of lenses remains relatively constant; the size of the lenses increases to accommodate the additional area. Like the modern Limulus a single cornea composed of cuticle varies in thickness on lenses across the eye. The lenses are best preserved where the cuticle was mineralized during diagenesis.
The visual system of Acutiramus shares properties with many living arthropods that are not high-level predators. The eyes of Acutiramus do not appear to represent the precise and efficient visual system required for high-level predation, a conclusion consistent with the recent interpretation of the biomechanics of the chelicerae.