Article: Exceptionally well-preserved isolated eyes from Cambrian 'Orsten' fossil assemblages of Sweden
Eyes other than those of trilobites are rarely preserved in the fossil record. We describe here a set of six tiny, isolated, three-dimensionally preserved compound eyes. These secondarily phosphatized eyes were etched from ‘Orsten’ limestone nodules dated to the Agnostus pisiformis Biozone from the Cambrian Alum Shale Formation of Sweden. The ovoid eyes arise from an elongated stalk, their surface being covered by a mosaic of regular and hexagonal-shaped facets representing the surface of ommatidia. Facet size and pattern change within the same specimen from the posterior to the anterior end. With regard to some morphological criteria, we grouped the material in two different morphotypes, type A and B, the first being represented by specimens of two different developmental stages. From stage to stage, mostly growth in overall size and addition of new ommatidia was noticed. Among the meiobenthic ‘Orsten’ arthropods, only the crustacean Henningsmoenicaris scutula has been described as possessing stalked eyes, but the eyes of the largest specimen with preserved eyes of this species are much smaller than the new eyes and do not display any kind of ommatidia on their visual surface. However, fragments of larger specimens of H. scutula and the co-occurrence of this species with the new isolated eyes in the sieving residues make it likely that the latter belong to this species but belong to more advanced stages than those described previously of H. scutula. Ontogenetically, the eye stalks of this fossil crustacean elongate progressively, while the regular hexagonal facets, lacking in early stages, appear later on.