Tannudiscus
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Tannudiscus Temporal range: Toyonian |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Trilobita |
| Order: | Agnostida |
| Suborder: | Eodiscina |
| Superfamily: | Eodiscoidea |
| Family: | Weymouthiidae |
| Genus: |
Tannudiscus Pokrovskaya, 1959 |
| species | |
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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).
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). According to Cotton and Fortey (2005) Tannudiscus is polyphyletic.
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.
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