Oncoceratidae

Oncoceratidae Temporal range: MOrd- U Sil
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Order:	Oncocerida
Family:	Oncoceratidae Hyatt, 1884

Oncoceratidae, a family in the Oncocerida established by Hyatt, 1884, which is characterized by generally compressed, cyrtoconic and breviconic shells with an exogastric curvature such that the ventral profile is convex or more so than dorsal, and in which the siphucle is generally empty and located ventral of the center. In primitive forms the siphuncle in early growth stages is composed of tubular segments with almost straight, suborthochoanitic septal necks but becomes cyrtochoanitic with expanded segments in the later growth stages, and is expanded and cyrtochoanitic throughout in advanced forms. In a few advanced forms the siphuncle is actinosiphonate (Flower 1950, Sweet 1964)

The Oncoceratidae are generally thought to have first appeared early in the Middle Ordovician (Sweet 1964) simultaneously with the Graciloceratidae, Tripteroceratidae, and Valcouroceratidae and derived from the Gracilocertidae. (Flower 1950, 1976). Some Middle Ordovician oncoceratids such as Richardsonoceras and Oonoceras are however externally more similar to Bassleroceras than to Graciloceras. Moreover Richardsonoceras has been reported from the upper Lower Ordovician (Arenigian) of China, bringing its first appearance before the earliest known graciloceratids. Other oncoceratids such as Rhizoceras and Miamoceras show a greater affinity to the Graciloceratidae

The Oncoceratidae through Oncoceras is thought to be the most likely source for the slightly endogastric Ordovician Diestoceratidae (Sweet 1964) and through the Early Silurian Amphycertoceras, for the Acleistoceratidae. The Oncoceratidae also gave rise to the dominantly Devonian Brevicoceratidae and Siluro-devonian Nothoceratidae though Oonoceras and to the largely Devonian Polyelasmoceratidae through either Oonoceras or Oocerina. (Sweet 1964)

See also

• List of nautiloids

References

• Flower 1950, in Flower & Kummel, A Classification of the Nautiloidea, Journal of Paleontology 24(3) Sept 1950
• Flower 1976, Ordovician Cephalopod Faunas and Their Role in Correlation; The Ordovician System; proceedingas of a palaeonotlogical Association symposium, Birmingham U.K. 1974.
• Sweet, W.C. 1964. Nautiloidea -Oncocerida. Treatise on Invertebrate Paleontology Part K Nautiloidea, Teichert & Moore (eds)