The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species. All told, between 22 and 34 families and 165 to 184 genera have been recognised, making this the largest order of the subclass Nautiloidea.
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 Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial canals and a narrow space along the inner side of the connecting ring known as a paraspatium. Septal necks are generally short and cyrtochoanitic, some being recumbent, some hook shaped. Most grew to lengths of about 60 to 90 cm but some, like the Huroniidae of the Silurian grew significantly larger.
The Brevicoceratidae is a family of oncocerids that contains genera characterized by exogastric gyrocones, brevicones, and torticones. that tend to develop vestigial actinosiphonate deposits and subtriangular transverse sections. The Brevicoceratidae are derived from Oonoceras (Oncoceratidae) and range from the mid-Silurian to the Upper Devonian.
Uranoceras is a barrandeocerid genus from the Middle Silurian belonging to the family Uranoceratidae, characterized by its loosely coiled, gyroconic shell of 1.5 to 2 subquadrate whorls.
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.
Hemiphragmoceratidae is a family of endogastrically brevconic oncocerids characterized by elaborately visored apertures in which the hyponomic sinus in mature specimens is on a spout-like process and there may be lateral and dorsal salients.. Shells are compressed with the apical portion curved and the anterior straight. Siphucles are nummuloideal with expanded spheroidal segments and continuously actinosiphonate interiors.
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
The Aipoceratoidea are a superfamily within the order Nautilida characterized by rapidly expanding, smooth to ribbed, cyrtoconic to coiled shells with rounded or sometimes dorsally flattened or impressed whorls, nearly straight sutures, and a ventral and marginal siphuncle. Septal necks are orthochoanitic ventrally and orthochoanitic or cyrtochoanitic dorsally.
Oxygonioceras is a genus in the Oncocerid family, Brevicoceratidae, from the Middle Silurian of North America and Europe.
Jolietoceras is a compressed, annulate, lituiconic nautiloid included in the derived Tarphycerid family, Uranoceratidae. The shell is gyroconic in the early stage, becoming straight and more rapidly expanded in the later. sutures are straight and transverse. Surface annuli slope strongly to the rear, dorso-ventrally, in the early gyroconic stage but a lacking in the later straight segment.
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).
Lechritrochoceratidae is a family of derived tarphycerids from the middle and upper Silurian, once included in the now largely abandoned Barrandeocerida.
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.
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.
The Plectoceratidae is a family of tarphycerids in the suborder Barrandeocerina established as a place for the genus Plectoceras; defined simply as coiled, costate barrandeocerids with subcentral adult siphuncle.
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.
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.
