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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.
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.
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.
Winnipegoceras is an extinct nautiloid genus from the Ordovician belonging to the Order Discosorida.
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
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.
Rioceras is an extinct genus of orthocerid cephalopods of the family Baltoceratidae that lived in what would be North America during the Early Ordovician from 480—472 mya, existing for approximately 8 million years.
Orthoceratoidea is a major subclass of nautiloid cephalopods. Members of this subclass usually have orthoconic (straight) to slightly cyrtoconic (curved) shells, and central to subcentral siphuncles which may bear internal deposits. Orthoceratoids are also characterized by dorsomyarian muscle scars, extensive cameral deposits, and calciosiphonate connecting rings with a porous and calcitic inner layer.
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.
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.
Cartersoceras is a genus of nautiloid cephalopods placed in the Orthocerid family Baltoceratidae, established by Rousseau Flower (1964), for species formally included in part in Murrayoceras, and in part in "Sacotoceras". This group is typical of Murrayoceras in all features except that the siphuncle segments are convex in outline. It is known from the Middle Ordovician of the eastern United States.
Murrayoceras is a nautilid cephalopod included in the orthocerid family Baltoceratidae, widespread in the Middle Ordovician of North America, characterized by a depressed orthoconic shell with a subtriangular cross section and flattened venter and a proportionally large ventral siphuncle, 0.15 to 0.3 the dorso-ventral shell diameter. Septa are close spaced with sutures forming broad lobes on the upper flanks and ventral surface.
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.
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.
References
- Rousseau H Flower and Curt Teichert, 1957. The Cephalopod Order Discosorida. University of Kansas Paleontological Contributions. Mollusca Article 6. July 1, 1957.
- Rousseau H Flower 1964. The Nautiloid Order Ellesmeroceratida (Cephalopoda). New Mexico Bureau of Mines and Mineral Resources Memoir 12.
- Björn Kröger, Yunbai Zhang, & Mare Isakar 2009. Discosorids and Oncocertids (Cephalopoda) of the Middle Ordovician Kunda and Aseri Regional Stages of Baltoscandia and the early evolution of these groups. Geobios, Vol. 42, No. 3 (May 2009) pp. 273–292.