''Osbornoceras'' is a genus of Lower

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 ...

cyrtoconic nautiloid cephalopods known from Ohio and possibly Manitoba, one of five general currently included in the oncocerid family Karoceratidae.

Diagnosis -morphology

''Osbornoceras'' grew to be somewhat large and is characterized by a narrow, compressed, strongly curved, exograstric shell. The venter, on the outside curvature, is narrowly rounded while the dorsum, on the opposite inside curvature, is more broadly rounded. The body chamber narrows toward the aperture, which has a deep hypomomic sinus. 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 ...

of ''Osbornoceras'' is ventral but not in contact with the ventral wall of the shell. Segments are empty and expanded ventrally but are virtually straight dorsally. Connecting rings are thin, septal necks cyrtochoanitic (outwardly flared).

Similar genera

Karoceras ''Karoceras'' is a genus of Oncocerida, oncocerid nautiloids that lived during the Silurian and possibly Early Devonian, type genus for the Karoceratidae. The shell is a compressed, exogastric cyrtocone, section sub ovoid. The siphuncle is ventral ...

'', from Europe and Russia, which followed in the Middle Silurian, is similar in overall form but is actinosiphonate. Its siphuncle in contrast contains blade-like deposits which project radially inward. Also similar is the closely related Lower

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

'' Ankyloceras'' and earlier Middle and/or Upper Ordovician oncoceratids such as '' Diagenoceras'' and '' Richardsonoceras''.

Paleoecology

''Osbornoceras'' and similar oncocerids were probably benthic predators that lived on the seafloor oriented in life as to face forward with the body chamber parallel to the seafloor and curved phragmocone arched upward. Whether they were pursuit of ambush predators is unknown. Also unknown is the number of arms. Was it the primitive 8 or 10 or the derived 90 plus found in recent

Nautilidae The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in t ...

References

* Sweet, W.C. 1964; Nautiloidea -Oncocerida; Treatise on Invertebrate Paleontology, Part K. Geological Society of America and University of Kansas Press.
''Osbornoceras''
in the Paleobiology database * Sepkoski's Online Genus Database (CEPHALOPODA

Prehistoric nautiloid genera Oncocerida {{paleo-Nautiloidea-stub