Oncoceratidae are characterized by generally compressed, cyrtoconic, and breviconic shells with an exogastric curvature such that the ventralprofile is convex or more so than dorsal, and in which the siphuncle 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 suborthochoaniticseptal 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.
Evolution
The Oncoceratidae first appeared early in the Middle Ordovician simultaneously with the Graciloceratidae, Tripteroceratidae, and Valcouroceratidae, derived from the Gracilocertidae. 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 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 and through the Early SilurianAmphycertoceras, 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.
Subgroups
The Oncoceratidae can be subdivided into arbitrary and convenient subgroups based on morphologic similarity, based on illustrations in the Treatise K1964. One comprising elongate slender forms is referred to as the Slender Oncoceratidae. The other, comprising generally short, breviconic forms is referred to as the Breviconic Oncoceratidae.
Breviconic Oncoceratidae
Breviconic Oncoceratidae comprise genera included in the family Oncoceratidae known from generally short, i.e. breviconic and often curved shells. They are considered oncoceratids on the basis of having typically empty siphuncles ventral of the center that may be suborthochoanitic and tubular in the early growth stage and cyrtochoantic with expanded segments in later growth stages or entirely. Connecting rings are thin. Breviconic oncoceratids differ from slender oncoeratids only in the matter of relative length. Inclusion is based on descriptions and illustrations in Sweet 1964 on the Oncerida in the Treatise on Invertebrate Paleontology, Park K.
Included genera
Breviconic oncoceratid genera include: Oncoceras; compressed, curved brevicones, M-U Ord, N Am., Eu.
Beloitoceras; compressed, curved brevicones, like Oncoceras, M-U Ord, N Am., Eu.
Metarizoceras; slightly curved, compressed, rapidly expanding brevicons, M Sil. N Am.
Neumatoceras; compressed brevicones with maximum heightbehind posterior end of body chamber, M-U Ord., N Am., Eu.
?Vaupella; depressed, cyrtoconic brevicones with large ventral cytrochoanitic siphuncle, otherwise with characters of the family.
