Tsunyidiscus

Tsunyidiscus Temporal range: Late Atdabanian PreЄ Є O S D C P T J K Pg N ↓
linedrawing of Tsunyidiscus niutitangensis
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Trilobita
Order:	Agnostida
Suborder:	Eodiscina
Family:	Tsunyidiscidae Zhang, 1980
Genus:	Tsunyidiscus W. Chang, 1966
Species
T. niutitangensis (Chang ,1964) (Type) T. aclis (Zhou, 1975) T. acutus (Sun, 1983) T. armatus (Zhang & Zhu, 1980) T. chintingshanensis (Lu, 1942) T. limbanodus Qian in Zhang et al., 1980 T. longquanensis (Zhang and Zhu in Zhang et al., 1980) T. pengshuiensis Zhang & Clarkson, 2012 T. pertenus Lin et al., 2004 T. yanjiazhiensis S. Zang et al. in Yin and Li, 1978 And see text

And see text

Tsunyidiscus is a genus of eodiscinid agnostid trilobites. 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 (up to 7mm), 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.

Like other Agnostida the body of Tsunyidiscus is diminutive, the headshield (or cephalon) and tailshield (or pygidium) 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 (fulcrate). The cephalon has a deeply parabolic outline and maximum width (tr.) 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 (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.

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.

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