Nautiloids are a large and diverse group of marine cephalopods (Mollusca) belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus and Allonautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, but only a handful of species survive to the present day.
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.
Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.
The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.
Baltoceras is a member of the Ellesmerocerida, included in the family, Baltoceratidae. The shell of Baltoceras is slender with a subcircular cross section, straight transverse sutures, and a large siphuncle in contact with the venter. Septal necks are short but not vestigial; connecting rings are thick; endosiphonal organic deposits are unknown.
Bathmoceras is a primitive cephalopod genus from the Middle and Upper Ordovician, type for the family Bathmoceratidae, order Ellesmerocerida.
Ectenolites is a genus of small, slender, cylindrical Ellesmeroceratids that resemble Ellesmeroceras but are smaller and proportionally narrower. Septa, as typical for ellesmerocerids, are close spaced with shallow lobes on either flank. The body chamber is proportionally long, the shell itself slightly compressed. The dorsal side at the beginning of the shell, opposite the aperture and body chamber, is strongly convex so to produce a sense of endogastric curvature with the apex and siphuncle aligned.
Clarkoceras is a genus of breviconic ellesmerocerid cephalopods, one of only two genera known to have crossed from the Late Cambrian, Trempealeauan, into the Early Ordovician, Gasconadian. ; the other being Ectenolites.
The Gasconadian Stage is the first stage of the Ordovician geologic period in North America and of the Lower Ordovician Canadian Epoch, coming immediately after the Late Cambrian Trempealeauan and preceding the middle Canadian Demingian Stage. The Gasconadian is equivalent to the European Tremadocian and roughly to the Skullrockian of the Ibexian series.
The Ellesmeroceratidae constitute a family within the cephalopod order Ellesmerocerida. They lived from the Upper Cambrian to the Lower Ordovician. They are characterized by straight and endogastric shells, often laterally compressed, so the dorso-ventral dimension is slightly greater than the lateral, with close spaced sutures having shallow lateral lobes and a generally large tubular ventro-marginal siphuncle with concave segments and irregularly spaced diaphragms. Connecting rings are thick and layered, externally straight but thickening inwardly with the maximum near the middle of the segment so as to leave concave depressions on internal siphuncle molds. Septal necks are typically orthochoanitic but vary in length from almost absent (achoanitic) to reaching halfway to the previous septum (hemichoanitic) and may even slope inwardly (loxochoanitic).
The Reudemannoceratidae are the ancestral and most primitive of the Discosorida, an order of cephalopods from the early Paleozoic. The Reudemannoceratidae produced generally medium-sized endogastric and almost straight shells with the siphuncle slightly ventral from the center.
Baltoceratidae is an extinct family of orthoconic cephalopods belonging to the subclass Nautiloidea endemic to what would be Asia, Australia, Europe, North America, and South America during the Ordovician living from about 480–460 mya, existing for approximately 20 million years .
Protcycloceratidae is an extinct family of slender, commonly annulate, members of the cephalopod order Ellesmerocerida that lived during the Early Ordovician.
Tajaroceras is an extinct slender cephalopod from the uppermost Lower Ordovician of western North America, belonging to the Orthocerid family Troedssonellidae.
Jiagouceras is a genus of early primitive cephalopods from the Upper Cambrian of China, assigned to the Plectronoceratidae. The shell is small, nearly straight with a slight endogastric curvature and compressed cross section. The siphuncle is close to the ventral margin, with segments expanded into the chambers.
Paraplectronoceras is a genus of very early nautiloids from the middle Late Cambrian, named by Chen and Qi, 1979, type species Paraplectronoceras pyriforme, for small, endogastrically curved forms found in the upper Yenchou and Wanwankou members of the Fengshan Formation of northeastern China.
Shungtangenoceras is a conical plectronoceratoid cephalopod from the Upper Cambrian of north-eastern China, described by Sun (1937) as a primitive endoceroid. Because of its apparently poor preservation its ordinal and familial position is uncertain. It could be included in either the Plectronocerida, family Plectronoceratidae, or the Ellesmerocerida, family Ellesmeroceratidae. On the other hand, Teichert (1964) included Shungtangendoceras in the Ellesmoerocatidae.
Apocrinoceratidae constitutes a family of Middle Ordovician nautiloid cephalopods characterized by straight or slightly curved, transversely ribbed shells having siphuncles composed of expanded segments, short recurved septal necks, and thick connecting rings. Derivation is from the Protocycloceratidae, a family of ellesmerocerids, which differ in having straight or concave siphuncle segments, but are otherwise similar in form.
Discosoridae comprise a family of endogastric discosorids,, with endocones in the siphuncle, ranging from the Middle Silurian to Middle Devonian.
Eothinoceratidae is a family of Lower Ordovician nautiloid cephalopods included in the Ellesmerocerida originally established for the genus Eothinoceras.
