Vision in a Middle Ordovician trilobite eye

• We analysed 1011 lenses in eye of Cyclopyge sp. and measured eye parameter (p).
• The value of p ranged from 0.02 μm rad to 1.75 μm rad.
• Cyclopyge sp. was adapted to strongly lit epipelagic conditions.
• Cyclopyge sp. was probably not a predator.
• Epipelagic niche of trilobites was invaded by at least the Middle Ordovician.

Exceptionally well-preserved compound eyes of a Middle Ordovician trilobite, Cyclopyge sp., show remarkable detail, including each of the lenses and their distribution. Based on a new analysis using computed tomography and computer software, it has been established that the Cyclopyge sp. had an acute zone of enhanced visual acuity. The eye parameters (p) of the left eye of the specimen ranged from 0.02 μm rad to 1.75 μm rad (75th percentile = 0.70 μm rad) in the dorso-ventral direction and 0.26 μm rad to 1.57 μm rad (75th percentile = 1.05 μm rad) in the antero-posterior direction. These parameters indicated that the organism was adapted to strongly lit epipelagic conditions, which is comparable to adaptations exhibited by many present-day diurnal crustaceans. The results also suggest that this trilobite was not a diurnal migrant, because the eyes would have become inefficient under darker conditions. The lowest values of p were distributed in the ventral and posterior areas. This part of the eye was an acute zone with a wide field of view covering the lateral region, which indicated that the Cyclopyge sp. was probably not a predator. According to previous studies, early Middle Ordovician species inhabited the epipelagic as well as mesopelagic realms. From the viewpoint of the trilobite analysis, this study supports previous geological and palaeontological evidence that suggests that the epipelagic niche was invaded by at least the Middle Ordovician.