Ophioceras

Ophioceras; Temporal range: Upper Silurian
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Order:	Tarphycerida
Family:	Ophidioceratidae; Hyaatt, 1894

Ophioceras is a genus of closely coiled tarphycerid nautiloid cephalopods, the sole representatives of the family Ophidioceratidae, characterized by an evolute shell with narrow, subrounded, annulated whorls and a subcentral siphuncle composed of thin connecting rings that show no evidence of layering. The mature body chamber is strongly divergent and is the longest proportionally of any tarphycerid. The aperture has a deep hyponomic sinus and ocular sinuses, and so resembles some lituitids.[1]

Ophioceras seem to have its origin in the Trocholitidae, possibly in Graftonoceras. [2]

Ophioceras has a narrow, evolutely wound, closely coiled shell that diverges in the adult stage. The shell is more or less circular in cross section, strongly annulate with close spaced, rounded, transverse ribs. The venter is bicarinate, with keels along either side separated by a flattened surface. The siphuncle is tubular and thin walled, and may be slightly ventral or dorsal of the center. The adult aperture has deep hyponomic and ocular sinuses, not too dissimilar from the Ordovician Lituites, to which this genus is possibly allied.[2][3]

Some 12 species have been described which have been reduced to two, Ophioceras rudens Barrande, 1865 and O. simplex Barrande, 1865 with two subspecies, O. simplex simplex and O. s. proximum. Minor evolutionary changes can be observed within both species, which In spite of their widespread distribution do not demonstrate any major geographical variations.[4]

References

Furnish, W. M, and Glenister, Brian F. 1964. Nautiloidea - Tarphycerida, Treatise on Invertebrate Paleontology, Part K, Geo Soc. of America. Teichert and Moore (eds.)
W.M Furnish and Brian F Glenister 1964. Nautiloidea-Tarphyhceria. Treatise on Invertebrate Paleontology Part K. Geological Soc of America and Univ Kansas Press.
Charles Hepworth Holland 2010. Coiled nautiloid cephalopods from the British Silurian. Proceedings of the Geologists’ Association 121 (2010) 13–23
Stridsberg, S. & Turek, V., 1997: A revision of the Silurian nautiloid genus Ophioceras Barrande. GFF, Vol. 119 (Pt. 1, March), pp. 21–36. Stockholm. ISSN 1103-5897; Abstract.

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[1] Furnish, W. M, and Glenister, Brian F. 1964. Nautiloidea - Tarphycerida, Treatise on Invertebrate Paleontology, Part K, Geo Soc. of America. Teichert and Moore (eds.)

[Furnish-2] W.M Furnish and Brian F Glenister 1964. Nautiloidea-Tarphyhceria. Treatise on Invertebrate Paleontology Part K. Geological Soc of America and Univ Kansas Press.

[3] Charles Hepworth Holland 2010. Coiled nautiloid cephalopods from the British Silurian. Proceedings of the Geologists’ Association 121 (2010) 13–23

[4] Stridsberg, S. & Turek, V., 1997: A revision of the Silurian nautiloid genus Ophioceras Barrande. GFF, Vol. 119 (Pt. 1, March), pp. 21–36. Stockholm. ISSN 1103-5897; Abstract.