Article: The eyes of Lower Cambrian eodiscid trilobites

The oldest known well-preserved trilobite eyes are described in the Lower Cambrian eodiscids Neocobboldia chinlinica Lee and Shizhudiscus longquanensis S. G. Zhang and Zhu, the latter being slightly older. The material, from south central China, is preserved as moulds or by partial replacement in phosphate, and gives fine details of lens structure. Shizhudiscus eyes have biconvex lenses, polygonal and closely packed as in normal holochroal eyes, whereas those of Neocobboldia have round, separated lenses, and compare with Middle Cambrian eodiscid eyes previously described as 'abathochroal'. The validity of 'abathochroal eyes' as a separate eye-type is discussed. The lenses of Neocobboldia, like those of some phacopids, conform to the ideal model of an aplanatic lens originally described by Des Cartes. Ontogenetic changes in the eyes are described for both species. These show that the programmes for growth of the visual surface and lens emplacement are separate and that the two sometimes got out of phase. The eyes of eodiscids are similar enough to those of other trilobites to suggest that this group is truly trilobitan. They also indicate how advanced the visual organs were at this early stage in trilobite history.