7-621: And see text 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
14-440: 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. Tsunyidiscus
21-446: Is the only genus currently attributed to the family Tsunyidiscidae . 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)
28-443: 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
35-489: Is the still unnamed third stage of the Cambrian . It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4 , although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approximately to the first appearance of trilobites , about 521 million years ago, though the globally asynchronous appearance of trilobites warrants the use of a separate, globally synchronous marker to define
42-779: Is thought to have developed through paedomorphosis from Bulaiaspis . 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. Atdabanian Cambrian Stage 3
49-545: The base. The upper boundary and beginning of Cambrian Stage 4 is informally defined as the first appearance of the trilobite genera Olenellus or Redlichia around 514 million years ago. The International Commission on Stratigraphy has not officially named the 3rd stage of the Cambrian. The stage approximately corresponds to the " Atdabanian ", which is used by geologists working in Siberia . The oldest trilobite known
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