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.
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. It is an extinct genus of nautiloid cephalopod that thrived in the warm waters of the United States during the Paleozoic era.
Ormoceras is an actinocerid nautiloid genus and type for the family Ormoceratidae, found in North America from the late Chazyan through the early Cincinnatian of the Middle and Upper Ordovician, but which continued through the Devonian worldwide.
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.
Cyrtonybyoceras is a genus of slightly exogastric members of the Wutinoceratidae, a family of actinocerids and probably derived from an earlier Wutinoceras. The shell of Cyrtonybyoceras is curved slightly upwardly and is slightly compressed. Sutures slope toward the aperture, from the dorsum to the venter. The siphuncle is ventral but not marginal, in general form like that of Nybyoceras. Upper and lower septal necks are recumbent or narrowly free. The canal system is reticular, characteristic of the Wutinoceratidae.
The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial canals and a narrow space along the inner side of the connecting ring known as a paraspatium. Septal necks are generally short and cyrtochoanitic, some being recumbent, some hook shaped. Most grew to lengths of about 60 to 90 cm but some, like the Huroniidae of the Silurian grew significantly larger.
Williamsoceras is an endocerid that Rousseau Flower (1968) added to his Allotrioceratidae on the basis of having a vertical partition within the siphuncle, known as a ventral process, with inter-connecting tubule-like structures along its margin where intercepted by endocones. Three species are named and described from the Garden City limestone of Whiterockian age near Logan and northern Utah, including the genotype Williamsoceras adnatum. Two other species come from the Juab limestone of near equivalent age in the southern Confusion Range in the Ibex area in western Utah.
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.
Bactroceras is a genus of the family Baltoceratidae, orthoceroid cephalopods that lived during the early Middle Ordovician, from about 472—464 mya, existing for approximately 8 million years.
Rhabdiferoceras is an extinct genus of orthocerids belonging to the Baltoceratidae that lived in what would be North America during the Cassinian Stage at the end of the Early Ordovician, existing for approximately two million years from about 474 -472 mya.
Oncoceratidae is a family of nauatiloid cephalopods in the order Oncocerida established by Hyatt, 1884, that range from the Middle Ordovician to the Upper Silurian.
The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian.
The Wutinoceratidae are a family of early actinocerids defined by Shimazu and Obata in 1938 for actinocerids with thick connecting rings and a complex irregular canal system. Actinocerids are generally straight shelled nautiloid cephalopods with a siphuncle composed of expanded segments, typically with thin connecting rings, in which the internal deposits are penetrated by a system of canals.
The Actinoceriatidae are a family of actinocerids named by Saemann in 1853 for those that grew to have large shells with blunt apices and large siphuncles with widely expanded segments and a generally arcuate endosiphucular canal system. Their range is from the upper Middle Ordovician to the Lower Silurian. Actinocerids are generally straight-shelled nautiloid cephalopods with a siphuncle composed of expanded segments, typically with thin connecting rings, in which the internal deposits are penetrated by a system of canals
The Armenoceratidae are a family of early Paleozoic nautiloid cephalopods belonging to the order Actinocerida.
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.
