''Cyrtogomphoceras'' is a genus of

nautiloid Nautiloids are a group of 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. Th ...

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 biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

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 slen ...

'' 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 The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

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 23.5 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 third and shortest period of t ...

of Estonia. The type species is '' Oncoceras 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 The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

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