The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula. In the case of the cuttlefish, the siphuncle is indistinct and connects all the small chambers of that animal's highly modified shell; in the other cephalopods it is thread-like and passes through small openings in the septa (walls) dividing the camerae (chambers). Some older studies have used the term siphon for the siphuncle, though this naming convention is uncommon in modern studies to prevent confusion with a mollusc organ of the same name.
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.
Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian. Their shells were variable in form. Some were straight (orthoconic), others curved (cyrtoconic); some were long (longiconic), others short (breviconic). Some long-shelled forms like Endoceras attained shell lengths close to 6 metres (20 ft). The related Cameroceras is anecdotally reported to have reached lengths approaching 9 metres (30 ft), but these claims are problematic. The overwhelming majority of endocerids and nautiloids in general are much smaller, usually less than a meter long when fully grown.
Orthocerida, also known as the Michelinocerida, is an order of extinct orthoceratoid cephalopods that lived from the Early Ordovician possibly to the Late Triassic. A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous, and the Eocene fossil Antarcticeras is sometimes considered a descendant of the orthocerids although this is disputed. They were most common however from the Ordovician to the Devonian.
Cameroceras is an extinct genus of endocerid cephalopod which lived in equatorial oceans during the entire Ordovician period. Like other endocerids, it was an orthocone, meaning that its shell was fairly straight and pointed. It was particularly abundant and widespread in the Late Ordovician, inhabiting the shallow tropical seas in and around Laurentia, Baltica and Siberia.
The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longer juvenile portion and an inflated short adult portion that separate sometime in maturity.
Michelinoceras is the oldest known genus of the Michelinocerida, more commonly known as the Orthocerida, characterized by long, slender, nearly cylindrical orthocones with a circular cross section, long camerae, very long body chambers, and a central or near central tubular siphuncle free of organic deposits. Septal necks are straight; connecting rings cylindrical and thin. Cameral deposits are well developed. A radula has been found in one species, with seven teeth per row. It had ten arms, two of which formed longer tentacles.
Armenoceras is a genus of actinocerid nautiloid cephalopods whose fossils ranged from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian. It is the type genus of the family Armenoceratidae.
Lituitida is an order of orthoceratoid cephalopods. They correspond to the family Lituitidae of the Treatise, reranked as an order and combined with other orthoceratoids. They are considered to be more closely related to the Orthocerida than to the Ascocerida or Pseudorthocerida which are also included.
Pseudorthocerida is an order of generally straight longiconic orthoceratoids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining.. Cameral deposits are common and concentrated ventrally. Apices typically have a slight to moderate exogastric curvature
Bactroceras is a genus of orthoceratoid cephalopods that lived during the early Middle Ordovician, from about 472—464 mya, existing for approximately 8 million years.
Eosomichelinoceras is an extinct genus from the actively mobile carnivorous cephalopod family Baltoceratidae that lived in what would be Asia and South America during the Ordovician from 461—452 mya, existing for approximately 9 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 Armenoceratidae are a family of early Paleozoic nautiloid cephalopods belonging to the order Actinocerida.
The Orthocerataceae is a superfamily of orthocerid cephalopods that lived from the late Early Ordovician to the Early Cretaceous, but is no longer in general use.
Rangeroceras is an extinct orthoceratoid cephalopod genus that lived in what is now western North America during the latter part of the Early Ordovician.
Veneficoceras is a genus of the rod-bearing Baltoceratidae, an extinct cephalopod family with characteristics of the orthoceratoid Dissidocerida, found in Cassinianage, Lower Ordovician, limestone in western Utah.
Rangeroceratidae is a family of orthoceroid cephalopods known from the late Early Ordovician in eastern North America and Great Britain, included in the order Dissidocerida. The family was named by Evans, 2005, and contains three genera: Rangeroceras, Anrangeroceras, and Cyclorageroceras.
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.
