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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.
The Tarphyceratidae are tightly coiled, evolute Tarphycerida with ventral siphuncles. The dorsum is characteristically impressed where the whorl presses against the venter of the previous. The Tarphyceratidae are derived from Bassleroceras or possibly from some member of the Estonioceratidae.
Acleistoceras is a genus of the oncocerid, nautiloid family Acleistoceratidae that lived in the shallow seas that covered much of North America during the Devonian; living from 409—383.7 mya, existing for approximately 25.3 million years.
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.
The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian.
The Centroceratidae is the ancestral family of the Trigonoceratoidea and of the equivalent Centroceratina; extinct shelled cephalopods belonging to the order Nautilida
Laureloceras is a genus of the tarphycerid family Plectoceratidae that lived during the Middle Silurian in what is now North America.
The Barrandeoceratidae is a family of coiled nautiloids included in the Tarphycerida that lived from the Middle Ordovician to the Middle Devonian, characterised by mostly compressed shells with a subcentral siphuncle composed of thin-walled segments that may become secondarily ventral.(Flower and Kummel 1950, Sweet 1964).
The Uranoceratidae is a family of Silurian barrandeoceroids that tend to become uncoiled with age and in which siphuncle segments tend to be expanded into the camerae.
The Sactorthoceratidae comprise Orthocerataceaen genera with a subcentral suborthochoanitic siphuncle composed of slightly expanded segments and free of organic deposits. The camerae (chambers) of the phragmocone likewise have organic deposits that are typically retarded or sparse.
The Barrandeocerina comprise a suborder of Early Paleozoic nautiloid cephalopods, primitively coiled but later forms may be cyrtoconic, gyroconic, torticonic, and even breviconic, all having empty siphuncles with thin connecting rings. The Barrandeocerina were originally defined as a separate order by Rousseau Flower, but since then have been united within the Tarphycerida as a suborder. Derivation is from the Tarphyceratidae.
Avilionella is a Middle Ordovician tarphyceroid genus consisting of closely coiled, compressed shells with a small perforation in the center, shallow dorsal impression, and subventral tubular siphuncle with thin connecting rings. Chambers are very short, separated by closely spaced, dish-shaped septa. Coiling becomes loose in the mature, adoral, part of the shell.
The Apsidoceratidae is a family of Middle and Upper Ordovician Barrandeocerina,, characterized by curved or coiled, smooth, transversely marked, or laterally costate shells, with a conspicuous hyponomic sinus. Early whorl sections tend to be subtriangular; become broader and dorsally impressed in closely coiled forms. Sutures are with lateral lobes in primitive forms but are without lateral lobes, but with ventral lobes in more advanced species with broader sections. Siphuncles are between the center and ventral margin, but not close to either.
Barrandeoceras is a large, coiled, Middle Ordovician nautiloid cephalopod and part of the Tarphycerida. The shell is serpenticonic with whorls touching but not embracing. The adult body chamber becomes freed of the preceding whorl, a rather common character among tarphycerids. Whorl section is oval, somewhat more narrowly rounded ventrally, on the outer rim, than dorsally, on the inner rim. Prominent lateral ribs, at least on inner whorls. Grow lines show a distinct hyponomic sinus. Sutures have lateral lobes. The siphuncle is subcentral.
Plectoceras is a genus of nautiloids included in the tarphycerid suborder Barrandeocerina that lived during the Middle and Late Ordovician. It has been found widespread in the Middle and Upper Ordovician of North America.
Haydenoceras is a genus of middle Devonian cephalopods originally placed in the Barrandeocerida. The genus can be recognized by its strongly compressed, gradually expanding exogastrically curved shell with a strongly rounded dorsum and acutely angled venter, calling attention to the general form of earlier Bassleroceras. The siphuncle is tubular, central.
Charactoceras is a tarphycerid genus cephalopod which lived during the Late Ordovician.
References
- Flower, (1984). Bodeiceras; a New Mohawkian Oxycone, with Revision of the Older Barrandeocerida and Discussion of the Status of the Order. Journal of Paleontology v. 58, no.6, pp 1372–1379, Nov. 1984.
- Walter C Sweet, 1964 Nautiloidea-Barrandeocerida; Treatise on Invertebrate Paleontology Part K Mollusca 3. Geological Society of America and University of Kansas Press.