The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

; one possible member is known from the Early Permian), in which the connecting rings are thin and siphuncle segments are variably expanded (Flower, 1950). At present the order consists of some 16 families, a few of which, such as the Oncoceratidae, Brevicoceratidae, and Acleistoceratidae contain a fair number of genera each while others like the Trimeroceratidae and Archiacoceratidae are represented by only two or three (Sweet, 1964). Cyrtoceras NT small

Physical characteristics

The shells of oncocerids are primarily somewhat compressed cyrtoconic brevicones. More advanced forms include gyrocones, serpenticones, torticones, and elongate orthocones and cyrtocones, reflective of the different families and genera (Flower, 1950; Sweet, 1964). The siphuncle in the Oncocerida is commonly located at or near the ventral margin. Connecting rings are most commonly thin and structureless but in certain derived forms may become actinosiphonate with inwardly projecting radial lamellae. The juvenile segments in early genera are straight and tubular, with short orthochoanitic septal necks inherited from the Bassleroceratidae. Later in the mature stages of early forms and throughout in the more advanced the connecting rings are inflated with cyrtochoanitic septal necks, giving what can be described as a "beaded" or "ellipsoidal" appearance (Sweet, 1964).

Evolution and diversity

The Oncocerida are thought to be derived from the Bassleroceratidae through '' Graciloceras'' as a result of a thinning of the connecting rings in the siphuncle (Flower, 1976). Oncocerids reached their greatest generic diversity in the Middle Silurian with some 43 genera representing nine families (Sweet, 1964), the most at any time. Of these 43 or so genera, about 38 were new, a recovery from a precipitous decline in the Late Ordovician and Early Silurian. A second period of greater diversity occurred in the Middle Devonian with eight families represented by some 37 genera, following a second decline after the Middle Silurian. After this the order declined until its extinction in the Early Carboniferous ( Mississippian). Near the beginning of the Devonian and well before its end, the Oncocerida gave rise to the

Rutoceratidae Rutoceratidae is a family of prototypical nautilids, derived probably from either Brevicoceratidae or Acleistoceratidae of the order Oncocerida early in the Devonian. Rutoceratidae comprise a family within the oncocerid superfamily Tainoceratac ...

(Flower, 1976; Kümmel, 1964), which form the root stock of the Nautilida, which among its members includes the modern '' Nautilus'' and '' Allonautilus''.

Distribution and range

Oncocerids are well known as fossils from the later Ordovician,

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

, and

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

in North America, Europe, and Australia, and to a lesser extent from parts of Asia, after which the order declined into the Mississippian and reached its end by the Pennsylvanian (late Carboniferous) (Flower, 1976; Sweet, 1964).

Taxonomy

Families in the Oncocerida, according to the Treatise on Invertebrate Paleontology, follow with the number of genera in each shown in parentheses, along with the stratigraphic range. *

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, orthochoanitic marginal ...

(4) M-U Ord *

Tripteroceratidae The Tripteroceratidae is a family of depressed, straight to slightly curved nautiloid cephalopods from the middle and upper Ordovician with generally flattened venters and empty siphuncles with straight to inflated segments included in the Oncoce ...

(5) M-U Ord *

Valcouroceratidae The Valcouroceratidae is a family within the Oncocerida, nautiloid cephalopods from the middle and upper Ordovician, established by Rousseau Flower in 1945. Diagnosis Valcouroceratids are characterized by exogastric cyrtocones and brevicones tha ...

(8) M-U Ord * Diestoceratidae (5) M-U Ord *

Oncoceratidae Oncoceratidae is a family of nauatiloid cephalopods in the order Oncocerida established by Hyatt, 1884, that range from the Middle Ordovician to the Upper Silurian. Diagnosis Oncoceratidae are characterized by generally compressed, cyrtocon ...

(24) M Ord - U Sil * Jovellaniidae (5) ? U Ord, M Sil - L Dev. *

Nothoceratidae Nothoceratidae is a family of nautiloid cephalopods in the orthoceratoid order Oncocerida in which shells are exogastrically or endogastrically breviconic, planospiral, or torticonic; often with a constricted or visored aperture; and a siphun ...

(10) L Sil - U Dev *

Karoceratidae Karoceratidae is a family of nautiloids within the order Oncocerida The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous; one possible memb ...

(3) L - M Sil, ?L Dev *

Hemiphragmoceratidae Hemiphragmoceratidae is a family of endogastrically brevconic oncocerids characterized by elaborately visored apertures in which the hyponomic sinus in mature specimens is on a spout-like process and there may be lateral and dorsal salients. (S ...

(5) M-U Sil, ?M Dev *

Acleistoceratidae The Acleistoceratidae is a family of oncocerids that contains genera characterized by depressed (or rarely compressed) exogastric brevicones and cyrtocones (Sweet, 1964 K398) that range from the Middle Silurian to the Middle Devonian. The siphu ...

(22) M Sil - M Dev *

Polyelasmoceratidae Polyelasmoceratidae is a family of oncocerid nautiloids characterized by rapidly expanding endogastrically curved shells, curved such that the ventral side is longitudinally concave. In cross section shells are typically tear-drop in shape to su ...

(13) M Sil - U Dev *

Brevicoceratidae The Brevicoceratidae is a family of oncocerids that contains genera characterized by exogastric (or rarely endogastric) gyrocones, brevicones, and torticones. that tend to develop vestigial actinosiphonate deposits and subtriangular transverse ...

(17) M Sil - U Dev *

Poterioceratidae Poterioceratidae is a family of nautiloid cephalopods included in the Oncocerida that lived during the period from the Early Devonian to the Early Carboniferous ( Mississippian). Members of the Poterioceratidae are distinguished by a subcircular ...

(7) L Dev - L Carb * Tripleuroceratidae (2) ?L Dev, M Dev - L Carb * Archiacoceratidae (3) M Dev/

Descendants

According to more current thinking, e.g. Flower, Teichert, and Kümmel, the Oncocerida gave rise to the

which form the root stock of the Nautilida, which after a number of iterations, ends up with the modern '' Nautilus'' and '' Allonautilus''.

References

*Flower, R.H. in Flower and Kümmel Jr 1950; A Classification of the Nautiloidea; Journal of Paleontology, Vol 24, no 5, pp 604–616, Sept 1950 *Flower, R.H. 1976; Ordovician Cephalopod Faunas and Their Role in Correlation, pp 523–552 in The Ordovician System: proceedings of a Paleontological Association symposium; Bassett, M.G. Ed. *Kümmel,B. 1964; Nautiloidea -Nautilida; Treatise on Invertebrate Paleontology, PartK, R.C. Moore Ed. *Sweet, W.C. 1964; Nautiloidea -Oncocerida; Treatise on Invertebrate Paleontology, PartK, R.C. Moore Ed. {{Taxonbar, from=Q3882417 Nautiloids Mollusc orders Middle Ordovician first appearances Mississippian extinctions