| Ophioceras Temporal range: Upper Silurian | |
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| 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]
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon.
The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician to the Middle Devonian. Some, such as Aphetoceras and Estonioceras, are loosely coiled and gyroconic; others, such as Campbelloceras, Tarphyceras, and Trocholites, are tightly coiled, but evolute with all whorls showing. The body chamber of tarphycerids is typically long and tubular, as much as half the length of the containing whorl in most, greater than in the Silurian Ophidioceratidae. The Tarphycerida evolved from the elongated, compressed, exogastric Bassleroceratidae, probably Bassleroceras, around the end of the Gasconadian through forms like Aphetoceras. Close coiling developed rather quickly, and both gyroconic and evolute forms are found in the early middle Canadian.
The Lituitida are the Lituitidae of the Treatise, reranked as an order and combined with other orthoceratoids. They are considered to be more closely related to the Orthocerida than to the Ascocerida or Pseudorthocerida which are also included.
Ophioceras seem to have its origin in the Trocholitidae, possibly in Graftonoceras . [2]
The Trocholitidae are Tarphycerida with whorls in close contact as with the Tarphyceratidae, but in which the siphuncle, similar in structure, becomes dorsal. The Trocholitidae are derived from the Tarphyceratidae, perhaps from different tarphyceratids.
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]
Lituites is an extinct nautiloid genus from the Middle Ordovician and type for the Lituitidae that in some more recent taxonomies has been classified with the orthocerids and listed under the order Lituitida. it's now considered widespread, Fossils have been found in New York, Argentina, Norway, Sweden, Estonia, and China.
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]
