Middle Ordovician – Early Carboniferous (Mississippian)
Oncocerida is an extinct order of fast-moving nektobenthic carnivores
Characteristics of the Order
- Siphuncle segments are generally slender and on the ventral side of center
- Tubular septal necks
- Typically thin connecting rings
- Conch is compressed and is curved so that venter is on or near outer convex side
- Multiple muscle scars
- As described by numerous writers, many oncocerid taxa are known to have multiple muscle scars (e.g. Foerste 1930a,b;Mutvei 1957, 1964; Sweet 1959, 1964; Zhuravleva 1972; Dzik1984; Stridsberg 1985). In many taxa, the number of the scars is up to 25 pairs around the base of the body chamber. As pointed out by Manda & Turek (2009a), the number and position of the multiple scars is highly variable in some taxa. The genus Euryrizocerina of the family Oonoceratidae has only five pairs of muscle scars that occur on the ventral side, whereas in the closely related Oonoceras numerous multiple scars occur around the entire base of the body chamber (Manda & Turek 2009a, fig. 4).Similar decrease in the number of multiple muscle scars has been observed in Doleroceras of the oncocerid superfamily Rutoceratoidea (Zhuravleva 1972; Manda & Turek 2009b).
- The siphuncular structure in oncocerids is still imperfectly studied and understood. The structure of the connecting rings is of the Nautilus type but known only in a limited number of taxa. It is possible that this order comprises several orders. The oncocerid taxa that have inflated siphuncular segments are included here in discosorids that have structurally similar but denser connecting rings. In Ordovician oncocerids, the structure of the connecting ring has been described in Richardsonoceras (Dzik 1984, fig. 16;Kro¨ger et al. 2009, figs. 5:5, 6, 8–11) and Neumatoceras (Kro¨ger et al. 2009, fig. 5:4). In Silurian and Devonian oncocerids, the structure of the connecting ring is usually not preserved. The exceptions are the Silurian Octamerella (Mutvei 2011) and the Devonian Archiacoceras (Crick & Teichert 1979). In Octamerella, the outer spherulitic-prismatic layer of the connecting ring has a porous structure (Mutvei 2011, fig. 4B, C) similar to that in Richardsonoceras described below. The inner fibrous-organic layer is not preserved (Fig. 9B). A similar connecting ring structure probably also existed in the Devonian Archiacoceras(Crick & Teichert 1979).
- Shell morphology is typically of compressed brevicones and cyrtocones.
Treatise on Invertebrate Paleontology, Part K, Vol. 3 (1964):
- Primitively compressed exogastric cyrtocones and brevicones, from which developed a number of straight, torticonic, gyroconic, and nautiliconic forms, many with depressed section and endogastric curvature. Siphuncle on the ventral side of center; septal necks tubular or suborthochoanitic in early stages and primitive forms, cyrtochoanitic in adults and probably throughout conch of derived forms. Siphuncular segments generally slender, but may be large and considerably inflated. Connecting rings typically thin, but may be thickened in adult or gerontic individuals and variously produced toform actinosiphonate deposits.