Oncoceratidae
Encyclopedia
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)
Oncocerida
The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian ,in which the connecting rings are thin and siphuncle segments are variably expanded...
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
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 Spirula...
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
Graciloceratidae
The Graciloceratidae is a family of nautiloid cephalopods from the Middle and Upper Ordovician belonging to the Oncocerida, characterized by exogastric cyrtocones that expand slightly or moderately and have thin walled, orthochoantic marginal or subventral, tubular siphuncles 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
Bassleroceras
Bassleroceras is an elongate upwardly curved, exogastric, genus with the venter on the under side more sharply rouned than the dorsum on the upper...
than to Graciloceras
Graciloceratidae
The Graciloceratidae is a family of nautiloid cephalopods from the Middle and Upper Ordovician belonging to the Oncocerida, characterized by exogastric cyrtocones that expand slightly or moderately and have thin walled, orthochoantic marginal or subventral, tubular siphuncles The Graciloceratidae...
. 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
Acleistoceratidae
The Acleistoceratidae, named by Flower in Flower & Kümmel 1950, is a family of oncocerids that contains genera characterized by depressed exogastric brevicones and cyrtocones that range from the Middle Silurian to the Middle Devonian...
. 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)