Plectronocerida

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The primitive and ancestral Plectronoceratidae are included in the suborder, Plectronoceratina (Flower, R.H. 1964) of the Ellesmerocerida, but sometimes included in a separate order known as the Plectronocerida.

The Plectronoceratina are known from the Upper Cambrian of China and Manchuria and of North America (Texas, New Mexico?). Two families are recognized (Flower, 1964), the generally straight to endogastric Plectronoceratidae and the slightly exogastric Balkoceratidae.

Endogastric refers to a condition in which the lower or ventral side is concave and the back or dorsum is convex, in other words -belly in. Exogastric refers to the opposite condition in which the ventral side is instead convex and the back, concave, that is – belly out.

The Plectronoceratidae gave rise to the rest of the ellesmerocerid families and to the unique Discosorida. The Balkoceratidae which are unrelated to later exogastric forms died out by the end of the Cambrian and left no progeny.

The Plectronoceratidae, which typify the suborder, represented by Plectronoceras are characterized as follows. Plectronoceratidae are minute, generally compressed orthocones and endogastric cyrtocones with close spaced septa separating short chambers and a ventrally marginal siphuncle. Septal necks vary from very short to extending back almost to the previous septum in mature portions of conchs -that is may be subholochoanitic. Connecting rings are thick and typically expanded into the adjacent chambers as siphuncular bulbs where not confined by septal necks. The connecting rings are poorly calcified and fragile, being of chitiniferous organic material. Genera are defined on the basis of overall form and internal details.

Plectronocerids were probably benthic animals that crawled along the bottom in search of food or safety , facing down with the shell carried above. Nothing is known of their specific soft part anatomy or to what extent tentacles, if any, had developed, or whether the gastropod-type foot had evolved into a siphon by that time.

Small beginners, for what has turned out to be a large and diverse class of animals, the molluscan cephalopods.

[edit] References

Rousseau H Flower, 1964, The Nautiloid Order, Ellesmeroceratida (Cephalopoda), Memoir 12, New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico
W.M. Furnish and Brian F Glenister, 1964, Nautiloidea - Ellesmerocerida; Treatise on Invertebrate Paleontology, Vol K, p K129 –