Serrodiscus

Serrodiscus; Temporal range: Lower Cambrian PreꞒ Ꞓ O S D C P T J K Pg N
	pygidia of Serrodiscus silesius
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Trilobita
Order:	Agnostida
Suborder:	Eodiscina
Superfamily:	Eodiscoidea
Family:	Weymouthiidae
Genus:	Serrodiscus; Richter & Richter, 1941
species
	S. serratus (Richter & Richter, 1941) (type species) = Eodiscus (Serrodiscus) serratus ; S. bellimarginatus ; S. speciosus (Ford, 1873) = Microdiscus speciosus ;
Synonyms
	Paradiscus

Serrodiscus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It has been collected from the Lower Cambrian of Canada (Nova Scotia), Germany (Silesia), Poland (Holy Cross Mountains), Russia (Kuznetsk Alatau), the United Kingdom (Wales) and the United States (Massachusetts, New York State). It is named for the spines on the ventral side of the tailshield (or pygidium, which give it a serrated impression.

Distribution

S. bellimarginatus is known from the Lower Cambrian of Canada (Atdabanian, Callavia broeggeri zone, Branchian Series, Brigus Formation).[1]
S. speciosus has been found in the Lower Cambrian of Poland (Atdabanian, Ocieseki Sandstone and Kamieniec Shale, Holy Cross Mountains).[2]

Description

Like all Agnostida, Serrodiscus is diminutive. Like all Weymouthiidae, Serrodiscus lacks eyes and rupture lines (or sutures). The cephalon is semi-elliptical. The central raised area (or glabella) is conical, tapering forward, or is parallel sided. Usually it has weak or effaced furrows. The front of the glabella does not touch the furrow that defines the border, permitting the cheeks to join anteriorly in short preglabellar field or these are separated by a depression. As most backward part of the glabella the occipital ring is defined by a furrow. It may or may not carry a backward-directed spine. The border defined by a furrow is convex, with up to eight pairs of tubercles laterally. The articulate middle part of the body (or thorax) has three segments. The tailshield (or pygidium has a wide, subconical axis of more than eight rings. The pleural areas to the right and left of the axis lack furrows or are extremely weakly furrowed (on the internal mold only). The border becomes narrower further backwards and usually carries about 8 ventral spines at each side.[3]

Behaviour

Like all trilobites Serrodiscus could protect its soft underside by rolling and it has been demonstrated that the ventral spines on the pygidial doublure fit beneath the nodes on the cephalic border, probably improving the defence of its belly.

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References

Landing, E. (1995). "Upper Placentian-Branchian series of mainland Nova Scotia (middle-upper Lower Cambrian) faunas, paleoenvironments, and stratigraphic revision". Journal of Paleontology. 69 (3): 475–495. cited on Phil Borkow. "NeR loc., Branchian Series, Brigus Fm, 59.0m above Ner Sect. base - Landing 1995". Fossilworks.
Orlowski, S. (1975). "Jednostki litostratygraficzne kambru i gornego prekambru Gor Swietokrzyskich [Cambrian and Upper Precambrian lithostratigraphic units in the Holy Cross Mountains]". Acta Geologica Polonica. 25 (3): 431–448. cited on Uta Merkel. "Holy Cross Mountains, Ocieseki Sandstone". Fossilworks.
Whittington, H. B. (1997). Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. Treatise on Invertebrate Paleontology. Part O, Revised. pp. 397–398.

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[1] Landing, E. (1995). "Upper Placentian-Branchian series of mainland Nova Scotia (middle-upper Lower Cambrian) faunas, paleoenvironments, and stratigraphic revision". Journal of Paleontology. 69 (3): 475–495. cited on Phil Borkow. "NeR loc., Branchian Series, Brigus Fm, 59.0m above Ner Sect. base - Landing 1995". Fossilworks.

[2] Orlowski, S. (1975). "Jednostki litostratygraficzne kambru i gornego prekambru Gor Swietokrzyskich [Cambrian and Upper Precambrian lithostratigraphic units in the Holy Cross Mountains]". Acta Geologica Polonica. 25 (3): 431–448. cited on Uta Merkel. "Holy Cross Mountains, Ocieseki Sandstone". Fossilworks.

[Treatise-3] Whittington, H. B. (1997). Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. Treatise on Invertebrate Paleontology. Part O, Revised. pp. 397–398.