8-557: Tannudiscus Pokrovskaya (1959) is a genus of Eodiscinid trilobites belonging to the family Weymouthiidae 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. The Weymouthiid family shows
16-695: A stock within the Hebediscidae . The Weymouthiidae are a paraphyletic family because the Agnostina suborder is nested within it, particularly in the clade that further consists of the genera Mallagnostus , Chelediscus , Tannudiscus and Jinghediscus . The trend in the Weymouthiidae to reduce the occipital ring is carried furthest in Chelediscus , Tannudiscus and the Agnostina with
24-400: 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
32-477: Is long and strongly tapered with 10 or more axial rings. The segmentation of the axis, however, is often effaced. The Weymouthiidae include forms with the rear of the glabella roundly expanded over the occipital ring or with a vertical spine, forms in which the occipital ring cannot be discerned because the glabella is expanded, and forms with a primitive occipital structure but with a greatly increased numbers of axial segments. This Agnostida -related article
40-554: The axial rings are indistinct or obsolete. Pygidial border is almost flat, but similar in width to the cephalic border. Weymouthiidae The Weymouthiidae 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 -zones in the former paleocontinents of Laurentia , Avalonia , Gondwana . The Weymouthiidae are all blind and lack free cheeks. The Weymouthiidae have developed from
48-533: 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
56-427: 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 ( Peronopsidae ). According to Cotton and Fortey (2005) Tannudiscus is polyphyletic . Tannudiscus lacks eyes and has a thorax of three segments. The glabella almost touches the anterior border and consists of two inflated lobes,
64-829: The occipital ring divided into basal lobes. Most taxa in this family are small (1–2 cm or 0.4–0.8 inches long), but this is large for the Eodiscina. The glabella is wide at its base, normally parallel sided but may taper gently or be at its widest at half length. Glabellar furrows are mainly absent, but incomplete furrows may be present in some species and deeply impressed transglabellar furrows also occur. Free cheeks (or fixigenae) are normally confluent in front of glabella. The occipital ring may be simple or split into two lateral lobes (e. g. in Chelediscus and Tannudiscus ). All Weymouthiidae lack eyes. The thorax consists of three rings when known ( Mallagnostus , Marocconus , Serrodiscus , Tannudiscus and Weymouthia ). The pygidial axis
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