Vision in a Middle Ordovician trilobite eye Gengo Tanaka a, ⁎, Brigitte Schoenemann b , Khadija El Hariri c , Teruo Ono d , Euan Clarkson e , Haruyoshi Maeda f a Center for Marine Environment Studies, Kumamoto University, 6061 Matsushima-cho, Kami-amakusa, Kumamoto 861-6102, Japan b University of Cologne, Departments of Biology Education (Zoology) and Neurobiology/Animal Physiology, Gronewaldstrasse 2, D-50931 Köln, Germany c Université Cadi Ayyad, Département des Sciences de la Terre, Ave Abdelkrim Khattabi, B.P. 549, 40000 Marrakech, Morocco d 1552-141, Honden, Mizuho, Gifu 501-0236, Japan e School of Geosciences, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, Scotland, UK f Kyushu University Museum, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan abstract article info Article history: Received 6 October 2014 Received in revised form 22 April 2015 Accepted 19 May 2015 Available online 27 May 2015 Keywords: Palaeoecology Functional morphology Trilobite Paleozoic Eye 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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Trilobites were the most ubiquitous arthropods during the Palaeozoic era. They appeared in the early Cambrian (ca. 522 Ma), and they became extinct in the Permian after successfully surviving for more than 250 Ma. From the beginning, most trilobites possessed spectacular compound eyes, sometimes with hundreds (if not thousands) of facets that ensured high visual resolution. These lenses were usually covered by a pellucid membrane (the cornea), and the so-called holochroal eyes were kidney-shaped and orientated to the horizon. They subtended a wide field of view that extended to the front and rear, and they often had a broad latitudinal range as well. In the Early Ordovician, schizochroal eyes originated in the Phacopina suborder (ca. 488–359 Ma). These were compound eyes with large, separated, and less numerous lenses, which were probably very sophisticated visual systems (Clarkson and Levi-Setti, 1975). Thus, trilobites were well adapted for the ‘arms race’ between prey and predator. The ability to see undoubtedly triggered early evolution in ecological systems, as was suggested by Parker in his “Light Switch Theory” (Parker, 1998, 2003). Initially, during the Cambri- an, most, if not all, trilobites were bottom dwellers. Providing a conspic- uous amount of protein, they must have been the target of numerous predators, such as anomalocaridids and subsequent cephalopods and other raiders, as revealed by lesions in their carapaces (Owen, 1985). Enrolment, as a defence, was operational in some Middle Cambrian trilobites (Esteve et al., 2010), but it became greatly effective in the Ordovician and later periods. The great explosion in the diversity of trilobites arose in the Early Ordovician when species became much more numerous, and new ecological niches were invaded. Thus, the tiny trilobite Ctenopyge ceciliae (Clarkson & Ahlberg, 2002) invaded the planktonic realm (Schoenemann et al., 2010), and pelagic forms arose in the Telephinidae, Opipeuteridae, Bohemillidae, and Cyclopygidae, including Carolinites killaryensis utahensis (Hintze, 1952) and the Bohemian Pricyclopyge binodosa (Salter, 1859)(Fortey, 1985; McCormick and Fortey, 1998). Compared to other trilobites, the latter are quite large (up to several centimetres), and during such times, these strong swimmers became widely distributed at the margin of the Gondwana continent. As mentioned above, most trilobites had holochroal compound eyes composed of many lenses (Clarkson et al., 2006). The lenses of the ommatidia of the trilobite eye are composed of calcite, which, though Palaeogeography, Palaeoclimatology, Palaeoecology 433 (2015) 129–139 ⁎ Corresponding author. E-mail addresses: gengo@gpo.kumamoto-u.ac.jp (G. Tanaka), B.Schoenemann@uni-koeln.de (B. Schoenemann), elhariri@fstg-marrakech.ac.ma (K. El Hariri), tethys1950@yahoo.co.jp (T. Ono), euan.clarkson@ed.ac.uk (E. Clarkson), maeda@museum.kyushu-u.ac.jp (H. Maeda). http://dx.doi.org/10.1016/j.palaeo.2015.05.013 0031-0182/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo