Plectronocerida
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| Plectronocerida Fossil range: Upper Cambrian |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Nautiloidea |
| Order: | Plectronocerida Teichert 1988 |
The Plectronocerida are a primitive order which can be considered to be basal to,[1] or a stem group to,[2] the cephalopods.
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[edit] Ecology
All plectroneocerids, from the earliest onwards, possess a siphuncle, which would have permitted them to fill the chambers of their phragmocone with gas instead of water, thus controlling their buoyancy.[3] They were not, however, adapted for jet-powered swimming.[3]
Plectronocerids were probably benthic animals that crawled along the bottom in search of food or safety, facing downwards, with the shell carried above. Nothing is known of their specific soft part anatomy or to what extent tentacles, if any, had developed; nor whether the gastropod-type foot had evolved into a siphon by that time.
[edit] Occurrence
Plectronoceratids 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.
[edit] Evolution
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.
[edit] References
- ^ Flower, Rousseau H. (1955). "Saltations in Nautiloid Coiling" (PDF). Evolution 9: 244. doi:. http://www.jstor.org/stable/pdfplus/2405647.pdf.
- ^ Otto H. Schindewolf ; translated by Judith Schaefer ; edited and with an afterword by Wolf-Ernst Reif ; with a foreword by Stephen Jay Gould. (1993). Basic questions in paleontology : geologic time, organic evolution, and biological systematics. Chicago: University of Chicago Press. ISBN 0226738353.
- ^ a b Mutvei, Harry; Zhang, Yun-bai; Dunca, Elena (2007), "Late Cambrian Plectronocerid Nautiloids and Their Role in Cephalopod Evolution", Palaeontology 50 (6): 1327–1333, doi:
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This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (April 2009) |
- 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 –
