''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 to the 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.Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997

Species and distribution

* ''Tsunyidiscus aclis'' (Zhou, 1975) :: = ''Emeidiscus planilimbatus'', ''Mianxiandiscus badaowanensis'', ''M. emeiensis'', ''M. gaoqiaoensis'', ''M. jinningensis'', ''M. sichuanensis'' :Collected in the Lower Cambrian of China (Atdabanian: Jinning, 24.7° N, 102.7° EA. Yang, M. Zhu, J. Zhang and G. Li. 2003. Early Cambrian eodiscoid trilobites of the Yangtze Platform and their stratigraphic implications. Progress in Natural Science 13(11):861-866 , cited in=PBDB and Maotianshan 24.0°N, 102.0°E, Yuanshan Formation, Yunnan). * ''Tsunyidiscus acutus'' (Sun, 1983) :Present in the Lower Cambrian of China (Atdabanian, Shuijingtuo Formation, Yichang and Zgui, Yangtze Gorge Area, Hubei, 111°E, 30.5°N) * ''Tsunyidiscus armatus'' (Zhang & Zhu, 1980) :Occurs in the Lower Cambrian of China (Atdabanian: Weng'an, Longshancun Section, Niutitang Formation, Guizhou, 27.1°N, 107.5°E) * ''Tsunyidiscus chintingshanensis'' (Lu, 1942) :: = ''T. kaiyangensis'', ''Eodiscus chintingshanensis'', ''Guizhoudiscus chintingshanensis'', ''G. kaiyangensis'', ''H. chintingshanensis'' :Occurs in the Lower Cambrian of China (Zhongxin). * ''Tsunyidiscus limbanodus'' Qian ''in'' Zhang et al., 1980 * ''Tsunyidiscus longquanensis'' (Zhang and Zhu ''in'' Zhang et al., 1980) :: = ''Shizhudiscus longquanensis'' * ''Tsunyidiscus niutitangensis'' (Chang, 1964) :: = ''Hebediscus niutitangensis'' :Known from the Lower Cambrian of China (Atdabanian: Jinning, 24.7° N, 102.7° E and Malong, 25.4° N, 103.4° E, Yuanshan Formation, Yunnan; Zhijin, Gezhongwu Section, and Weng'an, Longshancun Section, Niutitang Formation, GuiZhou, 26.7°N, 105.8°E. * ''Tsunyidiscus pengshuiensis'' Zhang & Clarkson, 2012 was extracted from the Lower Cambrian of China (Qiongzhusi Formation). * ''Tsunyidiscus pertenus'' Lin et al., 2004 :: = ''T. orientalis'', ''Hebediscus orientalis'', ''Hupeidiscus orientalis'' :Found in the Lower Cambrian of China (Atdabanian: Huanglian Member, Jiumenchong Formation, 28.2°N, 109.2°E, Taijiang, Wuhe Member, Jiumenchong and Bianmachong Formations, 26.7° N, 108.3° E); Botomian: Weng'an, Longshancun Section, Mingxinsi Formation, Guizhou, 27.1°N, 107.5°E; Atdabanian: Pangwangcun Member, Huangboling Formation, Anhui, 30.1°N, 117.0°E * ''Tsunyidiscus yanjiazhiensis'' S. Zang et al. ''in'' Yin and Li, 1978 * ''Tsunyidiscus'' sp. :Present in the Lower Cambrian of China (Atdabanian: Shuijingtuo Formation, Yangtze Gorge, Hubei, 30.8° N, 111.3° E).S. Weiguo. 1986. Late Precambrian Pennatulids (sea pens) from the Eastern Yangtze Gorge, China: Paracharnia gen. nov Precambrian Research 31:361-375, cited in=PBDB

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.

References

{{Taxonbar, from=Q7850239 Eodiscina Cambrian trilobites Fossils of China