Related Research Articles
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 walls dividing the chambers.
The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period with questionable origins in Pragian stage before 409 million years ago, and persisted until Carnian pluvial event in the upper middle Carnian stage of the Triassic period. They are considered ancestors of the ammonoids, as well as of the coleoids.
Endoceras is an extinct genus of large, straight shelled cephalopods from the Middle and Upper Ordovician that gives its name to the Nautiliod order Endocerida. Shells grew to be as long as 3.5 meters The cross section in the mature portion is slightly wider than high, but is narrower laterally in the young. Sutures are straight and transverse. Endoceras has a large siphuncle, located close to the ventral margin, composed of concave segments, especially in the young but which may be tubular in the adult stage. Endocones are simple, subcircular in cross section, and penetrated by a narrow tube which may contain diaphragms reminiscent of the Ellesmerocerid ancestor.
Suecoceras is an endoceratid that lived during the Middle Ordovician. It is characterised by a long, straight, slender shell with a slightly expanded tip that curves slightly downwards.
Pseudohaloritidae is the larger of two families that form the goniatitid superfamily Pseudohaloritoidea, the other being the monogenerc Maximitidae. They are part of the vast array of shelled cephalopods known as ammonoids that are more closely related to squids, belemnites, octopuses, and cuttlefish, than to the superficially similar Nautilus.
Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.
Trocholites is a tarphycerid genus in the family Trocholitidae from the Middle and Late Ordovician with a gradually expanding, weakly ribbed shell; whorls in contact, dorsum slightly impressed; cross section depressed, venter and sides rounded; siphuncle close to but not at the dorsal margin.
The Tarphyceratidae are tightly coiled, evolute Tarphycerida with ventral siphuncles. The dorsum is characteristically impressed where the whorl presses against the venter of the previous. The Tarphyceratidae are derived from Bassleroceras or possibly from some member of the Estonioceratidae.
Sactoceras is an extinct nautiloid cephalopod that lived during the Ordovician and Silurian in what would become North America, Europe, and Asia.
Solenochilus, type genus of the Solenochilidae is an extinct cosmopotilian nautilid from the Lower Pennsylvanian to the Lower Permian with a rapidly expanding, coiled globular shell with few whorls, from which prominent spines extend laterally from the umbilical area at maturity. Solenochilus is derived from the Upper Mississippian Acanthonautilus, principally through evolutionary changes in the siphuncle.
Orthoceratoidea is a subclass, formerly considered an infraclass or a superorder, that comprises Cephalopoda orders that have orthoconic to slightly cyrtoconic shells and central to subcentral siphuncles in which there may be internal deposits. Currently, Orthoceratoidea comprises the orders Dissidocerida, Ascocerida, Pseudorthocerida, Lituitida and Orthocerida.
Wutinoceras is a genus of now extinct nautiloid cephalopods of the Wutinoceratidae family. It exhibits orthoconic actinocerids with ventral siphuncles composed of broadly expanded segments.
Nybyoceras is an actinocerid genus assigned to the Armenoceratidae and similar to Armenoceras except for having a siphuncle close to the ventral side of the shell.
Syringonautilidae is a family of Nautiloidea from the middle to late Triassic. Syringonautilidae comprise the last of the Trigonoceratoidea and are the source for the Nautilaceae which continued the Nautiloidea through the Mesozoic and into the Cenozoic right down to the recent. Syringonautilidae is a strictly Triassic family, derived early in the Triassic from the Grypoceratidae.
The Centroceratidae is the ancestral family of the Trigonoceratoidea and of the equivalent Centroceratina; extinct shelled cephalopods belonging to the order Nautilida
Protcycloceratidae is an extinct family of slender, commonly annulate, members of the cephalopod order Ellesmerocerida that lived during the Early Ordovician.
Dakeoceras is a genus of simple cyrtoconic ellesmeroceratid cephalopods in the fossil record. All known species come from the Lower Canadian epoch (Gasconadian) of North America.
Ophioceras is a genus of closely coiled tarphycerid nautiloid cephalopods, the sole representatives of the family Ophidioceratidae, characterized by an evolute shell with narrow, subrounded, annulated whorls and a subcentral siphuncle composed of thin connecting rings that show no evidence of layering. The mature body chamber is strongly divergent and is the longest proportionally of any tarphycerid. The aperture has a deep hyponomic sinus and ocular sinuses, and so resembles some lituitids.
Plectoceras is a genus of nautiloids included in the tarphycerid suborder Barrandeocerina that lived during the Middle and Late Ordovician. It has been found widespread in the Middle and Upper Ordovician of North America.
Kentlandoceras is a genus of middle Ordovician Oncocerids. Its shell is curved exogastrically, such that the ventral margin is longitudinally convex, but less so than in Loganoceras, and with a submarginal ventral siphuncle instead. The siphuncle in Loganoceras is subcentral. The related Romingoceras is more curved, also with a ventral siphuncle.
References
- Rousseu H Flower, 1964. The Nautiloid Order Ellesmeroceratida (Cephalopoda) New Mexico Bureau of Mines and Mineral Resources Memoir 12.