The primitive and ancestral
Plectronoceratidaeare 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
Cambrianof China and Manchuria and of North America (Texas, New Mexico?). Two families are recognized (Flower, 1964), the generally straight to endogastric Plectronoceratidaeand the slightly exogastric Balkoceratidae. Endogastricrefers to a condition in which the lower or ventral side is concave and the back or dorsum is convex, in other words -belly in. Exogastricrefers 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
orthoconesand endogastric cyrtoconeswith close spaced septaseparating 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
benthicanimals 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.
* 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 –
Wikimedia Foundation. 2010.
Look at other dictionaries:
Cephalopod — Cephalopods Temporal range: Devonian – Recent (Stem groups from Cambrian) … Wikipedia
Nautiloid — Nautiloids Temporal range: 495–0 Ma … Wikipedia
Discosorida — Temporal range … Wikipedia
Ellesmerocerida — Taxobox name = Ellesmerocerida fossil range = U Cambrian Ordovician regnum = Animalia phylum = Mollusca classis = Cephalopoda subclassis = Nautiloidea ordo = Ellesmerocerida ordo authority = Flower 1950 The Ellesmerocerida comprise early… … Wikipedia
Nautiloidea — Nautiloides Escala temporal: Cámbrico Tardío Reciente … Wikipedia Español
Ammonite — For other uses, see Ammonite (disambiguation). Ammonites Temporal range: 400–65.5 Ma … Wikipedia
Orthoceras — For the orchid genus, see Taxonomy of the orchid family. Orthoceras Temporal range: 488–199.6 Ma … Wikipedia
Orthoceratidae — Temporal range: Ordovician Permian (possibly Triassic) Orthoceras Scientific classification Kingdom … Wikipedia
Orthocerida — Temporal range: Lower Ordovician–Upper Triassic … Wikipedia
Ceratitida — Ceratites nodosus Upper Muschelkal Germany Scientific classification … Wikipedia