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The Plectronoceratoidea is a superorder of primitive, mostly Late Cambrian, cephalopods, comprising four recognized orders: Plectronocerida, Ellesmerocerida, Proactinocerida, and Yanhcerida. Most so far have been found in the Fengshanian of northeastern China, which is in large part equavalent to the North American Trempealeauan.
Of the plectronoceratoids, only the ellesmerocerid family, Ellesmeroceratidae, persisted with perhaps as few as only two genera into the Ordovcian. Most, including most of the Ellesmeroceratiae, had become extinct by the end of the Cambrian.
The concept of the superorder Plectronoceratoidea was advanced by Mary Wade, 1988, as which time she proposed logical, phylogenetically defensible superorders for the major groups of fossil cephalopods. Up to then at least fossil cephalopod taxonomy had often been careless, resulting in disparit forms, such as Orthocerida and Ellesmerocerida, or Discosorida and Tarphycerida being combined in embracing taxa.
The Plectronoceratoidea is characterized by small, even tiny, commonly straight or endogastrically curved, laterally cmpressed shells, but some exogastrically curved. Septa are close spaced, often less than 1mm apart. Siphuncles are generally large in proportion and ventral in location. Segments may be straight or expanded; septal necks highly variable, orthochoanitic, loxochoantiic, cyrtochoanitic, holochoanitic, recumbent, etc; connecting rings layered or thin; diaphragms common; organically precipitated calcareous deposits possible. The included orders are determined in large part by differences in siphuncular structure.
Of the four plectronoceratoid orders, the Plectrocercerida is the oldest, first appearing as Plectronoceras in the late Franconian and continuing into the Trempealeauan (Fengsanian of China), and giving rise to the Ellesmerocerida and Protactinocerida; the Yanhederida possibly derived from the Ellesmerocerida. Two ellesmeroceratid genera, Clarkoceras and Ectenolites , carry the Plectronoceratoidea into the Ordovician wherein advanced cephalopod orders have their beginning.
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Late Cambrian Cephalopods of North China, Plectroncerida, Protactinocerida (Ord, Nov.) and Yanhecerida (Ord, Nov.); Chen Jun-yuan et al., Acta Palaeontologica Sinica, 1979-02
Cambrian Cephalopods; Chen Jun-yuan and Curt Teichert, 1983. Geology November 1983
Nauatiloids and their discentants: cephalopod classification in 1986; Mary Wade, 1988. New Mexico Bureau of Mines and Mineral Resources Memoir 44.
Biostratigraphy of China, by Wen-tang Shang, Pei-ji Chen, A.R Palmer, 2003; Elsevier
Credits: JM 6/28/2011