''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 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 ...

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. ''Cyrtogomphoceras'' seems to be derived from ''

Strandoceras ''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. The body chamber of ''Stradoceras'' is straighter and more sle ...

'' by a reduction in the degree of endogastric curvature and may have given rise to '' Landeroceras'' by a further evolutionary straightening of the shell. ''Cyrtogomphoceras'', named by Foeste, 1924, has been found in middle and upper Ordovician beds in North America and Greenland. Fossils identified as such have also been found in the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

of Estonia. The type species is ''

Oncoceras ''Oncoceras'' is a genus of oncocerids, family Oncoceratidae from the middle and upper Ordovician of North America and Europe. The shell, or conch, of ''Oncoceras'' is relatively short, a curved, compressed brevicone with a maximum width in the ...

magnun'' Whiteaves 1890.

References

* Flower, Rousseau H. and Curt Teichert, 1957; The Cephalopod Order Discosorida, in University of Kansas Paleontological Contributions, Mollusca Article 6, pp 1–144 (plates, figs) July 1, 1957. * Curt Teichert 1964. Nautiloidea-Discosorida; Treatise on Invertebrate Paleontology Part K, Endoceratoidea, Actinoceratoidea, Nautiloidea.. Geological Society of America. {{Taxonbar, from=Q16981204 Prehistoric nautiloid genera Middle Ordovician first appearances Silurian extinctions Discosorida Paleozoic life of Manitoba Paleozoic life of the Northwest Territories Paleozoic life of Nunavut