Ontogeny of the eodiscid trilobite Shizhudiscus longquanensis from the lower Cambrian of China

The eodiscid Shizhudiscus longquanensis occurs abundantly in the Lower Cambrian Shuijintuo Formation, Pengshui, Sichuan, China. Phosphatized specimens in excellent preservation have been isolated from a single limestone lens, and include a few protaspides and disarticulated examples of all subsequent growth stages. From this material the ontogeny of S. longquanensis is described and reconstructed. Of particular note are two large spines on the axis of the transitory pygidium which eventually become those of the second and third thoracic segments. The visual surface first appears in the earliest meraspid, with a single lens flanked by two half lenses. An immature specimen of Shizhudiscus sp. from Shaanxi Province shows that this trilobite genus was already capable of full enrollment in the degree 0 meraspid stage.