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 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.
The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian.
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.
Glyptodendron is a Lower Silurian westonocerid characterized by compressed cyrtocones with a narrowly rounded dorsum and greatest width in the ventrolateral region. Sutures slope forward from the dorsum which is on the longitudinally concave side. The siphuncle is slightly ventral from the center. Segments are subspherical in the young; equally broad but shorter in the adult. No endosiphuncular deposits are known. The surface of the shell is covered by obliquely intersecting rows of scale-like pits.
Teichertoceras is a discosorid genus in the family Westonoceratidae characterized by an endogastric curvature to the early portion of the phragmocone.
Hectoceras is a genus in the nautiloid cephalopod order Discosorida from the Upper Ordovician of Australia (Tasmania), known from a few isolated siphuncle specimens.
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.
Sinclairoceras is a nautiloid cephalopod in the discosorid family Westonoceratidae from the Middle Ordovician Simard Limestone of Quebec. The shell is laterally compressed, upwardly curved, and short; an exogastric brevicone with the venter narrowly and dorsum broadly rounded. The dorsum, at the upper surface, is only slightly concave longitudinally. The venter, at the lower surface, is more strongly convex longitudinally, drawing away from the dorsum and forming a hump at the anterior end of the phragmocone where Sinclairoceras reaches its greatest width.
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.
Jaregoceras is a genus of nautiloids from the Upper Devonian discosorid that probably belongs to the family Discosoridae.
Lavaloceras is a genus of middle Ordivician discosorids included in the Westonoceratidae. The shell as nearly straight, with a slight exogastic curvature, and is rather short, breviconic.
