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 Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.
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.
The Lowoceratidae is a small family of discosorids, early nautiloid cephalopods, from the Middle Silurian in which the characteristic bullette is found only in early growth stages. Lowoceratids were first found in Southampton Island in the Canadian arctic.
Mesoceras is a mid Silurian discosorid from central Europe named by Barrande in 1877, with a short, anteriorly contracted body chamber and an aperture that is a long transverse slit.. The cross section is slightly oval with the dorso-ventral dimension slightly less than the lateral. Sutures are straight and transverse, the siphuncle is subcentral.
Winnipegoceras is an extinct nautiloid genus from the Ordovician belonging to the Order Discosorida.
Ulrichoceras is recognised as the basal cyrtogomphoceratid genus, which is the source for the rest of the Cyrtogomphoceratidae as well as for the Westonoceratidae. The Cyrtogomphoceratidae are endogastric, the Westonoceratidae, exogastric.
Westonoceras is an extinct nautiloid genus from the Discosorida that lived during the Middle and Late Ordovician that has been found in North America, Greenland, and Northern Europe. It is the type genus for the Westonoceratidae
The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian.
Mandaloceratidae is a family in the nautiloid cephalopod order Discosorida, from the Middle and Upper(?) Silurian characterized by short, essentially straight shells referred to as breviconic, typically with a faintly exogastric shape produced by the profile of the body chamber.
The Cyrtogomphoceratidae are a family in the cephalopod order Discosorida that comprises genera commonly with compressed, endogastrically curved shells. Siphuncles lie close to the ventral side, segments are broadly inflated, connecting rings thick and apically expanded thick bullettes. Chambers are short, separated by shallow, dish shaped septa. Apertures are generally simple.
Strandoceras is a strongly curved cyrtogomphoceratid with a laterally compressed cross section from the middle and upper Ordovician of Northern Europe; Estonia, Norway, the U.K.
Westonoceratidae are exogastric, mostly compressed, Discosorida of moderate size from the Middle Ordovician to the Lower Silurian.
Teichertoceras is a discosorid genus in the family Westonoceratidae characterized by an endogastric curvature to the early portion of the phragmocone.
Reedsoceras is a genus of large discosorids (Nautiloidea) in the family Westonoceratidae from the middle and upper Ordovician of North America.
Simardoceras is a genus in the discosorid family Westonoceratidae from the Middle Ordovician of Quebec.
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.
Cyrtogomphoceras is a genus of nautiloid cephalopods, recognized by its large breviconic shell with a notable endogastric curvature. The shell is fusiform in profile, reaching maximum width at or near the base of body chamber, which narrows toward the aperture. The siphuncle is large and slightly removed from the ventral side, that with the concave longitudinal profile. Siphuncle segments are short, as are chambers; septal necks recurved, connecting rings thick, bullettes at the apical end of the rings swollen. Cameral deposits are lacking.
Kiaeroceras is a slender, nearly stright shelled, cyrogomphoceratid (Nautiloidea-Discosorida) from the Upper Ordovician of northern Europe. The cross section of the shell is compressed, height greater than width. The body chamber is slightly contracted so as to narrow toward the aperture, which in some is slightly flared. The venter, narrowly rounded. The siphuncle is close to the venter, septal necks short, connecting rings thick, bullettes prominent.
Discosoridae comprise a family of endogastric discosorids,, with endocones in the siphuncle, ranging from the Middle Silurian to Middle Devonian.
