Tsunyidiscus Temporal range: | |
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linedrawing of Tsunyidiscus niutitangensis | |
Scientific classification ![]() | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Class: | † Trilobita (?) |
Order: | † Agnostida |
Suborder: | † Eodiscina |
Superfamily: | † Eodiscoidea |
Family: | † Tsunyidiscidae Zhang, 1980 |
Genus: | † Tsunyidiscus W. Chang, 1966 |
Species | |
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 is the only genus currently attributed to the family Tsunyidiscidae. [1]
Like other Agnostida the body of Tsunyidiscus is diminutive, the headshield (or cephalon) and end-section (pygidium) are of approximately same size (isopygous), with 3 thoracic segments in-between, each consisting of a horizontal inner portion that abruptly passes into an inclined outer portion (fulcrate). The cephalon has a deeply parabolic outline and maximum width (tr.) usually anterior to (to the front of) genal angle. Glabella extremely narrow, lateral glabellar furrows usually obscure, with rounded and expanded frontal glabellar lobe. The most backward lobe of the glabella (called occipital ring or LO) at least as long as the lobe in front of it (L1), 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 (SO) is transverse and uninterrupted. The facial sutures are proparian. The free cheeks or (librigena) are ½× as long as the cephalon. The pygidium has a narrow, multisegmented axis (with five, six or more segments). The thorax and pygidial axis segments may carry nodes. [1] [2]
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. [1]
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. [1]