Middle Ordovician – Late Silurian
Oncoceratidae is an extinct family of fast-moving nektobenthic carnivores
Description of the Family
- Siphuncle lacks deposits
- Sutures have broad lateral lobes
- Body chamber is relatively short
- Thin connecting rings
- Hyponomic sinus usually well-developed
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Manda and Turek (2009):
- Oncocerids with an exogastrically cyrtoconic shell; shell curvature varies only slightly within a species; siphuncle without deposits, marginal, in later ontogenetic stages slightly sub-marginal; phragmocone chambers are low; suture with broad lateral lobes; hyponomic sinus usually well-developed; body-chamber relatively short, slightly longer than wide; cross-section laterally compressed. Embryonic shell is cup-like, early shell is less curved than adult shell (after Manda & Turek 2009).
- The vast majority of Silurian Oonoceratidae from Bohemia possess shell with compressed cross-section as is seen in Euryizocerina. Cyrtoceras obesum Barrande, 1866 has a slightly curved exogastric shell with a circular cross section. The length of the body-chamber is approximately equal to, or slightly greater than the shell height at the aperture. Shell morphology is similar to that of the type species of Euryrizoceras Foerste, 1930 from the middle Silurian of Illnois (Sweet 1964). Barrande (1866, 1877) described this long-ranging species from the early Wenlock, Monograptus belophorus Zone (St. Iwan, pl. 521, refigured here in Fig. 6), early Ludlow, Gorstian, Colonograptus colonus Zone (Butowitz el, pl. 114), late Ludlow, Ludfordian, Monograptus fragmentalis Zone (Kozorz, pl. 508) and probably late Pridoli (Novy Mlyn, pl. 114). Euryizoceras obesum strongly resembles Euryrizocerina and Euryrizoceras are not considered to be closely related. It seems likely that there are many convergences in shell form in the Oncocerida. Muscle scars may be useful in recognition of such convergences.
Treatise on Invertebrate Paleontology, Part K, Vol. 3 (1964):
- Exogastric, ventromyarian brevicones and cyrtocones with empty siphuncles on ventral side of center. Siphuncles suborthochoanitic and tubular in early stages of early forms, but cyrtochoanitic with expanded segments in later stages and throughout conch of later forms. Connecting rings thin. Conch typically compressed , but a few species of circular or depressed section are known Actinosiphonate deposits developed in a dew advanced forms.