Tannudiscus

Tannudiscus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the upper Lower Cambrian, with remains found in Canada (Newfoundland), China (Gansu), The United Kingdom (England), and the Russian Federation (Tuva, Gorno-Altayskaya).[1]

Tannudiscus Temporal range: Toyonian PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Trilobita
Order:	Agnostida
Suborder:	Eodiscina
Superfamily:	Eodiscoidea
Family:	Weymouthiidae
Genus:	Tannudiscus Pokrovskaya, 1959
species
T. tannulaicus Pokrovskaya, 1959 (type) T. altus Repina, 1964 T. balanus Rushton, 1966 T. bulla Gabova, 2004 T. conicus Zhou, 1982 T. dingus Zhuravleva, Zadorozhnaya, Osadchaya, Pokrovskaya, Rodionova & Fonin, 1967 T. extensus Zhuravleva, Zadorozhnaya, Osadchaya, Pokrovskaya, Rodionova & Fonin, 1967

Taxonomy

The Weymouthiid family shows a trend of shortening the occipital ring. In Chelediscus and Tannudiscus the occipital ring is entirely obliterated as in the Agnostina suborder. Tannudiscus has some other characters it shares with the Agnostina, such as a glabella consisting of two lobes, the occipital ring divided into basal glabellar lobes, both the glabella and the pygidial axis isolated from border furrow, and undivided pygidial axis. It is therefore considered likely that the Agnostina descended directly from a species assignable to Tannudiscus. The earliest known Agnostina is Archaeagnostus (Peronopsidae).[1] According to Cotton and Fortey (2005) Tannudiscus is polyphyletic.[2]

Description

Like in all Agnostida the outline of the headshield (or cephalon) and tailshield (or pygidium) of Tannudiscus are nearly identical (or isopygous). Like all other Weymouthiidae it lacks eyes and has a middle part (or thorax) with three segments. In Tannudiscus the central raised area of the cephalon or glabella almost touches the anterior border. It consists of two inflated lobes, the frontal one slightly wider than the rear one, a character that is further developed in the Condylopygidae. The posterior lobe lacks a spine. The occipital ring has been squeezed aside into two basal lateral lobes. The border is convex, moderately wide, more so in front of the glabella and extends back to back of glabella in some species. The axis of the pygidium is conical, has seven or eight rings, and may or may not reach posterior border furrow. The furrows defining the rings are indistinct or obsolete though. The border of the pygidium is almost flat, but similar in width to the cephalic border.

Distribution

• T. tannulaicus is known from the Lower Cambrian of the Russian Federation (Toyonian, Shivelikskaya Suite, Tuva, Shivelik-Khem River, eastern Tannu-Ola Mountains).[1]
• T. balanus has been found in the Lower Cambrian of the Russian Federation (Botomian, Sanashtykgolskogo horizon, Shashkunarskaya suite).
• T. balanus occurs in the Lower Cambrian of England (Toyonian, Purley Shales, Warwickshire),[1] and Canada (Toyonian, upper Redland Cove member, Brigus Formation, southeastern Newfoundland).[3]
• T. conicus has been excavated from the late Lower Cambrian of China (Huocheng Formation, Borohoro district).[4]
• T. extensus is present in the Middle Cambrian of Russian Federation (Amgaian, Berikul Formation, Kiya River section, Kuznetsk Alatau).[1]

References

1. W.B. Wittington; et al., eds. (1993). Treatise on Invertebrate Paleontology, Part O., Revised.
2. Cotton, T.J.; Fortey, R.A. (2005). "Comparative morphology and relationships of the Agnostida". In Koenemann, S.; Jenner, R. (eds.). Crustacea and Arthropod Relationships. Crustacean Issues. 16.
3. Rushton, A.W.A. "8> Midland microcraton". A Revised Correlation of the Cambrian Rocks in the British Isles. Bath: the geological Society Publishing house. Retrieved 27 November 2013.
4. Huanling, Lin; Zhiyi, Zhou; Yunan, Ni; zhiqiang, Zhou. "III. Stratigraphic framework - 11. Cambrian". Phanerozoic Geology of Northwest China. Beijing. p. 135. Retrieved 27 November 2013.