Belongs within: Oncoceratida.
As defined by Sweet (1964), the Oncoceratidae is ancestral to a number of other oncocerid families; indeed, the oncoceratid lineage probably survives to the modern day in the Nautilidae. The Oncoceratidae excluding the descendant families were mostly cyrtoconic.
Oncoceras was a genus of oncocerid cephalopods found from the Middle to Upper Ordovician.
Cyrtorizoceras was a genus of small cyrtoconic oncocerids from the Middle Ordovician. Species have also been assigned to this genus from the Silurian, but probably represent convergent forms (Sweet 1964).
Characters (from Sweet 1964): Exogastric, ventromyarian brevicones and cyrtocones; siphuncle empty, on ventral side of chamber. Siphuncles suborthochoanitic and cylindrical in early stages of early forms, cyrtochoanitic with expanded segments in later stages and later forms. Connecting rings thin. Conch usually compressed, few species with circular or depressed section. Actinosiphonate deposits in a few advance forms.
<==Oncoceratidae [Inequilobates, Maelonoceratidae, Melonoceratidae, Ooceratidae] |--Asaphiceras Foerste 1928S64 | `--*A. schucherti Foerste 1928S64 |--Centrorizoceras Foerste 1930S64 | `--*C. slocomi Foerste 1930S64 |--Digenuoceras Foerste 1935S64 | `--*D. latum (Foerste 1929) [=Oxygonioceras latum]S64 |--Dunleithoceras Foerste 1924 [=Dunleitoceras (l. c.)]S64 | `--*D. dunleithense (Miller & Gurley 1896) [=Cyrtoceras dunleithense]S64 |--Ehlersoceras Foerste 1932S64 | `--*E. huronense Foerste 1932S64 |--Ekwanoceras Foerste & Savage 1927S64 | `--*E. breviconicum Foerste & Savage 1927S64 |--Kentlandoceras Foerste 1932S64 | `--*K. shrocki Foerste 1932 [=K. schrocki]S64 |--Loganoceras Foerste 1932S64 | `--*L. regulare (Billings 1857) [=Cyrtoceras regulare]S64 |--Miamoceras Flower 1946S64 | `--*M. shideleri Flower 1946S64 |--Paroocerina Zhuravleva 1961S64 | `--*P. podolskensis Zhuravleva 1961S64 |--Romingoceras Foerste 1932 [=Romingeroceras (l. c.)]S64 | `--*R. josephianum Foerste 1932S64 |--Scofieldoceras Foerste 1932 [=Schofieldoceras (l. c.)]S64 | `--*S. shumardi (Clarke 1897) [=Cyrtoceras shumardi]S64 |--Vaupelia Flower 1946S64 | `--*V. russelli Flower 1946S64 |--Maelonoceras Hyatt 1884 [=Meloceras (l. c.), Melonoceras (l. c.)]S64 | |--*M. praematurum (Billings 1860) [=Phragmoceras praematurum]S64 | `--M. discoideum [=Cyrtoceras discoideum]H84 |--Zittelloceras Hyatt 1884 [=Zitteloceras (l. c.); incl. Laphamoceras Foerste 1932]S64 | |--Z. hallianum (d’Orbigny 1850)S64 (see below for synonymy) | `--Z. clarkeanumP68 |--Beloitoceras Foerste 1924S64 | |--*B. pandion (Hall 1861) [=Oncoceras pandion]S64 | |--B. fremontense Foerste 1935P68 | |--B. kirtoniKE19 | |--B. landerense Foerste 1935P68 | |--B. popoagiense Foerste 1935P68 | `--B. ulrichi Flower 1946P68 |--Neumatoceras Foerste 1935S64 | |--*N. gibberosum Foerste 1935S64 | |--N. breviposticumP68 | |--N. canyonense Foerste 1935P68 | |--N. milleri Foerste 1935P68 | `--N. nutansS64 |--Cyrtorizoceras Hyatt in Zittel 1900 [=Cyrtorhizoceras (l. c.)]S64 | |--*C. minneapolis (Clarke 1897) [=Cyrtoceras minneapolis]S64 | |--C. auctidomum Foerste 1930P68 | |--C. barbense Foerste 1936P68 | |--C. byronense Foerste 1930P68 | |--C. cedarvillense Foerste 1934P68 | |--C. diagonale Foerste 1936P68 | |--C. falciforme Foerste 1934P68 | |--C. fosteriP68 | |--C. gasconsense Foerste 1936P68 | |--C. hyattianum Girty 1909P68 | `--C. longidomum Foerste 1930P68 |--Dalecarlioceras constrictumKE19 `--Oncoceras Hall 1847 [=Onoceras (l. c.)]S64 |--*O. constrictum Hall 1847S64 |--O. arcticum Schuchert 1900P68 |--O. canicameratumKE19 |--O. carltonenseKE19 |--O. covingtonense Flower 1946P68 |--O. exile Flower 1946P68 |--O. fossatum Flower 1946P68 |--O. heteroclitum [=Cyrtoceras heteroclitum]H84 |--O. mumiaformeH84 |--O. parvum Foerste 1935P68 |--O. pupaeforme Ruedemann 1921P68 | |--O. p. pupaeformeP68 | `--O. p. femina Ruedemann 1926P68 `--O. tumidum Schuchert 1900P68
Zittelloceras hallianum (d’Orbigny 1850)S64 [=Cyrtoceras hallianumS64; incl. C. lamellosum Hall 1847 non de Verneuil 1842S64, *Zittelloceras lamellosumH84]
*Type species of generic name indicated
References
[H84] Hyatt, A. 1883–1884. Genera of fossil cephalopods. Boston Soc. Nat. History, Proc. 22: 253–338.
[KE19] King, A. H., & D. H. Evans. 2019. High-level classification of the nautiloid cephalopods: a proposal for the revision of the Treatise Part K. Swiss Journal of Palaeontology 138: 65–85.
[P68] Purnell, L. R. 1968. Catalog of the type specimens of invertebrate fossils. Part I: Paleozoic Cephalopoda. United States National Museum Bulletin 262: 1–198.
[S64] Sweet, W. C. 1964. Nautiloidea—Oncocerida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K277–K319. The Geological Society of America and the University of Kansas Press.