Tsunyidiscus is a genus of eodiscinidagnostidtrilobites. Tsunyidiscus occurs near the end of the Lower Cambrian, during the late Atdabanian stage period and some collections suggest it may have survived into the Botomian. They are very small, have eyes, and equal sized head and tail shields, with a narrow dome-shaped glabella and a narrow bullet-shape pygidial axis. It has a thorax of three segments. Tsunyidiscus is the only genus currently attributed to the family Tsunyidiscidae.
Description
Like other Agnostida the body of Tsunyidiscus is diminutive, the headshield and tailshield are of approximately same size, with 3 thoracic segments in-between, each consisting of a horizontal inner portion that abruptly passes into an inclined outer portion. The cephalon has a deeply parabolic outline and maximum width usually anterior to genal angle. Glabella extremely narrow, lateral glabellar furrows usually obscure, with rounded and expanded frontal glabellar lobe. The most backward lobe of the glabella at least as long as the lobe in front of it, and is usually expanded laterally, may bear sharp, posteriorly directed spine. Long, curved posterior fixigenal spine may be present. The furrow between those two lobes is transverse and uninterrupted. The facial sutures are proparian. The free cheeks or are ½× as long as the cephalon. The pygidium has a narrow, multisegmented axis. The thorax and pygidial axis segments may carry nodes.
Species and distribution
Tsunyidiscus aclis
Tsunyidiscus acutus
Tsunyidiscus armatus
Tsunyidiscus chintingshanensis
Tsunyidiscus limbanodus Qian in Zhang et al., 1980
Tsunyidiscus longquanensis
Tsunyidiscus niutitangensis
Tsunyidiscus pengshuiensis Zhang & Clarkson, 2012 was extracted from the Lower Cambrian of China.
Tsunyidiscus yanjiazhiensis S. Zang et al. in Yin and Li, 1978
Tsunyidiscus sp.
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.
