Tsunyidiscus
Tsunyidiscus is a trilobite belonging to the Suborder Eodiscina. "Tsunyidiscus" appeared near the end of the Lower Cambrian, during the late Atdabanian stage of geologic time and some collections suggest it may have survived into the Botomian. The genus is very small (up to 7mm), oculate and isopypous with a narrow dome-shaped glabella and a narrow bullet-shaped pygidial axis. Thorax consists of three segments. "Tsunyidiscus" is the only genus currently attributed to the family Tsunyidiscidae. Description. Like other Agnostida the exoskeleton of "Tsunyidiscus" is diminutive and isopygous with 3 fulcrate thoracic segments. The cephalon has a  strongly parabolic outline and maximum width (tr.) usually anterior the to genal angles. Glabella extremely narrow, lateral glabellar furrows usually obscure, with a rounded and expanded frontal glabellar lobe. The occipital ring (LO) is at least as long as L1, usually expanded laterally, and may bear a sharp posteriorly directed spine. Long, curved posterior fixigenal spines may be present. The occipital furrow is transverse and uninterrupted. Facial sutures proparian. Librigenae are 0.5% as long as the cephalon. The pygidium has a narrow multi=segmented axis (with five, six or more segments). The thoracic and pygidial axial segments may carry  nodes. Taxonomy. Ancestors. "Tsunyidiscus" is the oldest known eodiscoid. The glabella of "Tsunyidiscus" is extremely similar to that of "Dipharus clarki", and distinct from all other eodiscoids. "D. clarki" is thought to represent an immature stage of the redlichioid "Bulaiaspis" rather than an eodiscoid. This is because of the dominant palpebroocular ridges, extremely long librigenae, and free pleural tips on the pygidium of variable numbers of segments. In short: "Tsunyidiscus" is thought to have developed through paedomorphosis from "Bulaiaspis". Descendants. Three lineages are thought to have evolved from "Tsunyidiscus". First the Hebediscidae, that themselves gave rise to the Weymouthiidae, which  contain "Tannudiscus", the probable ancestor of the Agnostina. Second the Yukoniidae, who sprouted the Eodiscidae. And finally the Calodiscidae.