''Tannudiscus'' Pokrovskaya (1959) is a genus of Eodiscinid trilobites belonging to the family

Weymouthiidae The Weymouthiidae Kobayashi 1943 are an extinct family of eodiscinid agnostid trilobites. They lived during the late Lower Cambrian and earliest Middle Cambrian (Botomian to Delamaran) in the so-called ''Olenellus''- and ''Eokochaspis''-zon ...

Kobayashi T. (1943), Order Agnostida (Salter 1864). It lived during the late Lower Cambrian, with remains found in Canada (Newfoundland), China (Gansu), The United Kingdom (England), and the Russian Federation (Tuva, Gorno-Altayskaya). TYPE SPECIES: ''T. tannulaicus'' Pokrovskaya, 1959.

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 and the occipital ring divided into basal glabellar lobes. Both the glabella and pygidial axis are isolated from the border furrow, and the latter is undivided. It is therefore considered likely that the Agnostina descended directly from a species assignable to ''Tannudiscus''. The earliest known Agnostina is ''

Archaeagnostus ''Archaeagnostus'' is an extinct genus from a well-known class of fossil marine arthropods, the trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest ...

'' (

Peronopsidae The Peronopsidae (which may also be called peronopsids) comprise the earliest family of the Agnostina suborder. Species of this family occurred on all paleocontinents. The earliest representatives of this family first occur just before the start ...

). According to Cotton and Fortey (2005) ''Tannudiscus'' is

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

Description

''Tannudiscus'' lacks eyes and has a thorax of three segments. The glabella almost touches the anterior border and consists of two inflated lobes, the frontal one slightly wider than the posterior lobe—a characteristic that is further developed in the Condylopygidae. Posterior glabellar lobe lacks a spine. The occipital ring is formed into two basal lateral lobes. The border is convex, moderately wide, more so in front of the glabella and extends to rear of the glabella in some species. Pygidial axis is conical, with seven or eight rings, and may or may not reach posterior border furrow. The furrows defining the axial rings are indistinct or obsolete. Pygidial border 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 (= Stage 4, upper of two stages subdividing the un-named Cambrian Series 2), Shivelikskaya Suite, Tuva, Shivelik-Khem River, eastern Tannu-Ola Mountains). * ''T. balanus'' was originally described in the Lower Cambrian of England. Holotype is A 57119, held at the Sedgwick Museum of Earth Sciences, University of Cambridge; collected by Rushton from about 450m above base of the Purley Shale Formation, Camp Hill, St. Paul's Church, Stockingford, Nuneaton, Warwickshire, England losest ICS interval: Cambrian Series 3 – Terreneuvian Epoch The species also occurs in the Toyonian, upper Redland Cove member, Brigus Formation of southeastern Newfoundland and also has been found in the Lower Cambrian of the Russian Federation (Russian-Kazakhian Botomian Stage, Sanashtykgolskogo horizon, Shashkunarskaya suite). * ''T. conicus'' has been recovered from the late Lower Cambrian of China (Huocheng Formation, Borohoro district). * ''T. extensus'' is present in the Middle Cambrian of Russian Federation (Amgaian, Berikul Formation, Kiya River section, Kuznetsk Alatau).

References

{{Taxonbar, from=Q16992939 Weymouthiidae Agnostida genera Cambrian trilobites Fossils of Russia Fossils of Great Britain Cambrian genus extinctions