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Septa are thin walls or partitions between the internal chambers (camerae) of the shell of a cephalopod, namely nautiloids or ammonoids.
The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian, in which the connecting rings are thin and siphuncle segments are variably expanded. At present the order consists of some 16 families, a few of which, such as the Oncoceratidae, Brevicoceratidae, and Acleistoceratidae contain a fair number of genera each while others like the Trimeroceratidae and Archiacoceratidae are represented by only two or three.
The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician to the Middle Devonian. Some, such as Aphetoceras and Estonioceras, are loosely coiled and gyroconic; others, such as Campbelloceras, Tarphyceras, and Trocholites, are tightly coiled, but evolute with all whorls showing. The body chamber of tarphycerids is typically long and tubular, as much as half the length of the containing whorl in most, greater than in the Silurian Ophidioceratidae. The Tarphycerida evolved from the elongated, compressed, exogastric Bassleroceratidae, probably Bassleroceras, around the end of the Gasconadian through forms like Aphetoceras. Close coiling developed rather quickly, and both gyroconic and evolute forms are found in the early middle Canadian.
Pseudobrevicoceras is an extinct genus of nautilitoid cephalopods in the order Oncocerida. The familial position is undetermined.
Jovellania is a genus of extinct prehistoric nautiloids from the order Oncocerida known from the Lower Devonian of Europe. Nautiloids form a broad group of shelled cephalopods that were once diverse and numerous but are now represented by only a handful of species in two genera.
Zittelloceras is an extinct genus of nautiloids from the order Oncocerida which are among a large group of once diverse and numerous shelled cephalopods, now represented by only a handful of species.
Shuranoceras is a genus in the extinct oncocerid family Karoceratidae that plied the shallow sea floor from the Early Silurian to the Early Devonian. Shuranoceras is characterized by a smooth, compressed, slowly enlarging orthoconic shell with a ventral submarginal siphuncle composed of complex segments. Shuranoceras was found in middle Silurian strata in Ferghana, central Asia.
The Tainoceratoidea are a superfamily in the cephalopod order Nautilida characterized by straight to loosely coiled shells, generally to a degree such that the width is greater than the height, to quadrate whorl section. Many bore spines, ribs, frills, wings, or nodes. In early forms, the siphuncle is generally near ventral, but more variable in advanced forms.
Basslerocerida is an order of nautiloid cephalopods from the Ordovician comprising exogastric longiconic cyrtocones, that is no longer in common use.
The Graciloceratidae is a family of nautiloid cephalopods from the Middle and Upper Ordovician belonging to the Oncocerida, characterized by exogastric cyrtocones that expand slightly or moderately and have thin walled, orthochoanitic marginal or subventral, tubular siphuncles.
Oncoceratidae is a family of nauatiloid cephalopods in the order Oncocerida established by Hyatt, 1884, that range from the Middle Ordovician to the Upper Silurian.
Oonoceras is an extinct genus of fossil cephalopods included in the nautiloid order Oncocerida and the family Oncoceratidae from the Middle Ordovician to Middle Silurian of North America and Europe, arbitrarily included in the Slender Oncoceratidae.
Conostichoceras is a genus of exogastric, breviconic oncocerids included in the family Nothoceratidae, known from the Middle Devonian of central Europe and Upper Devonian of Australia. It probably lived on and swum above the sea floor.
The Nautilitoidea is a superorder within the subclass Nautiloidea, comprising the phylogenetically related Nautilida, Oncocerida, and Tarphycerida.
Nothoceratidae is a family of nautiloid cephalopods in the orthoceratoid order Oncocerida in which shells are exogastrically or endogastrically breviconic, planospiral, or torticonic; often with a constricted or visored aperture; and a siphuncle commonly composed of concave segments and occupied by actinosiphonate deposits. Some ten genera have been described which lived during the time between the Early Silurian and Late Devonian. The ancestral form is probably Perimecoceras which is known from the Upper Silurian of central Europe and which is similar in external form the Oonoceras from the Oncoceratidae.
Fayettoceras is a genus in the nautiloid family Valcouroceratidae, part of the order Oncocerida, Fayettoceras has a shell which is a depressed cyrtocone with a ventral cyrtochoanitic siphuncle of elongated ovoid segments strongly contracted at the septal necks. The internal structure is unknown.
Actinomorpha is a genus of valcouroceratid, order Oncocerida, from the Middle Ordovician of central North America, named by Rousseau Flower, 1943. The shell is breviconic, short and laterally compressed; the venter more narrowly rounded than the dorsum, having the effect of a modest keel in horizontal orientation. The shell is gibbous adorally, such that the body chamber is widest well behind the aperture. The siphuncle is ventral, which could also be indicative of a horizontal orientation in life, with broad, slightly inflated segments and continuous, radial, actinosiphonate, deposits.
Augustoceras is a genus of nautiloid cephalopods included in the order Oncocerida and family Valcouroceratidae. It is known form the Middle and Upper Ordovician of Kentucky and Ohio in the US.
Karoceratidae is a family of nautiloids within the order Oncocerida, characterized by straight or curved, laterally narrow, shells and slender, ventral siphuncles that are empty except in Karoceras. Siphuncle segments are inflated ventrally but straight dorsally. Septal necks are cyrtochoanitic, outwardly curved; connecting rings are thin.
Codoceras is a genus of nautiloids belonging to the Polyelasmoceratidae, an Oncocerida family.
References
- Megaloceras Paleobiology Database 7/9/12
- Sepkoski's list of Cephalopods
- Walter C. Sweet, 1964. Nautiloidea-Oncocerida. Treatise on Invertebrate Paleontology, Part K. Geological Society of America and University of Kansas Press.
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