Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living Nautilus and Allonautilus. Fossil nautiloids are diverse and species rich, with over 2,500 recorded species. They flourished during the early Paleozoic era, when they constituted the main predatory animals. Early in their evolution, nautiloids developed an extraordinary diversity of shell shapes, including coiled morphologies and giant straight-shelled forms (orthocones). No orthoconic and only a handful of coiled species, the nautiluses, survive to the present day.
Septa are thin walls or partitions between the internal chambers (camerae) of the shell of a cephalopod, namely nautiloids or ammonoids.
Discosorida are an order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and connects the camerae (chambers) in cephalopods, which unlike those in other orders is zoned longitudinally along the segments rather than laterally. Siphuncle structure indicated that the Discosorida evolved directly from the Plectronoceratida rather than through the more developed Ellesmerocerida, as did the other orders. Finally and most diagnostic, discosorids developed a reinforcing, grommet-like structure in the septal opening of the siphuncle known as the bullette, formed by a thickening of the connecting ring as it draped around the folded back septal neck.
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.
Brevicoceras is an extinct nautiloid genus from the order Oncocerida with wide distribution in the Middle Devonian in Eastern North America, Russia and Morocco. 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.
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 Tripteroceratidae is a family of depressed, straight to slightly curved nautiloid cephalopods from the middle and upper Ordovician with generally flattened venters and empty siphuncles with straight to inflated segments included in the Oncocerida.
The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian.
The Naedyceras group comprises three similar and closely related openly coiled, gyroconic, genera within oncocerid family, Brevicoceratidae: Naedyceras, Gonionaedyceras, and Gyronaedyceras.
The Valcouroceratidae is a family within the Oncocerida, nautiloid cephalopods from the middle and upper Ordovician, established by Rousseau Flower in 1945.
Blakeoceras is a nautiloid cephalopod from the Oncocerida family Nothoceratidae with a curved shell that lived in shallow seas from the Silurian to the Middle Devonian in what has become Europe.
Didymoceras nebrascense was an extinct species of heteromorph ammonite from the upper Campanian age. It was sexually dimorphic, with two adult sizes averaging at 270 mm (11 in) and 180 mm (7.1 in) high for females and males respectively. It exhibited three distinct growth stages. The first growth stage was composed of one or two straight sharply bending sections and a gently curved third. The second growth stage is composed of around three and a half loosely coiling whorls. The last (adult) growth stage is composed of a U-shaped bend facing upwards.
Manitoulinoceras is a genus of Late Ordovician oncocerid nautiloid cephalopods, found in North America. The shell is cyrtoconic, exogastric, more strongly curved than in Kindleoceras. The cross-section is somewhat wider than high, with a somewhat flattened upper side, known as the dorsum. Actinosiphonate deposits in the siphuncle are confined to the early part of the phragmocone.
Minganoceras is a genus in the oncocerid family, Valcouroceratidae, named by Foeste, 1938, from the Middle Ordovician of Quebec, found on Mingan Island.
Galtoceras is a cyrtoconic nautiloid from the Middle Silurian of North America, named by Foerste in 1934.
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.
Macrodomoceras is a genus of oncocerids, family Polyelasmoceratidae, from the Middle Devonian of Australia.
Paldoceras is a genus of nautiloid cephalopods found in the middle Ordovician of Estonia and Sweden, included in the discosorid family Apocrinoceratidae. Two species are recognized; Paldoceras paldiskense found northwest of Paldiski in northern Estonia and Paldoceras neptunsakerense found on Öland Island, Sweden.
