Eyes and vision in the coeval Furongian trilobites Sphaerophthalmus alatus (Boeck, 1938) and Ctenopyge (Mesoctenopyge) tumida Westergård, 1922, from Bornholm, Denmark

The two olenid species Sphaerophthalmus alatus (Boeck, 1838) and Ctenopyge (Mesoctenopyge) tumida Westergård, 1922, occur together in the Ctenopyge tumida Zone (Zone 19) of the Furongian of Scandinavia. Material from Bornholm, Denmark, forms the basis of this study of the morphology and partial ontogeny of the eyes. The eyes of both species are directed laterally and have virtually panoramic vision, looking out sideways like those of a rabbit. The eye of S. alatus is comparatively smaller, with fewer lenses and a larger eye parameter; calculations show that this trilobite was adapted for dim light intensity, possibly suggesting a vagrant benthic habit. Ctenopyge (Mesoctenopyge) tumida, with a smaller eye parameter, was adapted for a higher light intensity, and this trilobite was most likely a pelagic swimmer. The two species, although preserved together, inhabited different levels in the water column.