Mollusca		Nautiloidea
CYRTOSOMA		Oncocerida

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Oncocerida

Cephalopoda
|--NAUTILOIDEA
|  |--TARPHYCERIDA
|  |  |--Tarphycerina  
|  |  `--BARRANDEOCERINA
|  `--ONCOCERIDA 
|     |--Oncoceratidae  
|     `--NAUTILIDA    
|        |--NAUTILIDA i.s.
|        `--NAUTILINA
`--Neocephalopoda
   |--Ammonoidea
   `--Coleoidea

Introduction

The Oncocerida include a large number of early and middle Paleozoic nautiloids with short, curved shells and actinosiphonate deposits. Although a large number of genera have been described, their taxonomic placement is often made difficult by poor preservation and in many cases by lack of clearly recognizable distinctive morphological features. Jerzy Dzik has also plausibly suggested (p.7) that what have been described as different genera are merely growth stages of a single species. Although Dzik probably goes to the other extreme in lumping what were probably distinct species together, it is quite likely that nautiloid taxonomy is greatly over-inflated as far as number of genera goes.  

Conch Shape

The Oncocerids are a rather homogeneous group, in which short straight blunt shells (brevicones) and short curved shells (breviconic cyrtocones) predominate, although long and straight or slightly curved, "nautilus"-like planispirally coiled (Stereotoceras), and "snail like" torticonic (Mitroceras, Foersteoceras) shells also occur.

There is a definite trend toward strongly contracted and visored apertures, especially in the endogastric Hemiphragmoceratidae and the exogastric Trimeroceratidae. Many oncoceroids at maturity also have but greatly constricted apertures, bulbous living chambers. Both these attributes also occur in the unrelated discosorid cephalopods. Such shells almost certainly were oriented with the apex up, the living chamber down.  

Septa

The septa are usually very thin and often very closely spaced. There are no cameral deposits.  

Siphuncle

The oncocerid siphuncle is of moderate size. The siphuncle is tubular or nearly so in the early stages of life history but then generally form inflated (cyrtochoanitic) segments in the adult. A distinctive feature of the Oncocerida is the occurrence of "actinosphunculate deposits" - longitudinal ridges or lamellae or blades of shell matter in the siphuncles of many forms. These project inward from the inner surface of the siphuncle wall toward the central axis of the siphuncle. Several types of such deposits exist but basically they are thickenings or projections of the connecting ring or of a lining in the septal foramen. They may be continuous either through many segments or discontinuous from one segment to the next.  

Ontogenic Growth

The shell of a typical oncocerid, like Valcouroceras from Middle Ordovician (Darriwilian) of North America , begins as a slightly curved cone, containing an empty suborthochoanitic siphuncle near the ventral margin. With advance toward maturity, curvature of the shell increases, the siphuncle gradually becomes cyrtochoanitic, and actinosiphonate deposits are formed. Due to changes in form with growth, it is not unlikely that in some cases what have been thought to be separate species and genera are actually simply growth stages of a single species, as Dzik has pointed out.  

Evolutionary History

Oncocerids were most common during the Ordovician; Silurian, and Devonian, with only a few stragglers continuing into the Early Carboniferous. Five oncocerid families appeared in the Middle Ordovician, but only one, the Oncoceratidae, survived the end of the Ordovician. Seven new families originated in the Silurian. Six of these continued on through the Devonian period, when they were joined by three new families. Oncocerids are the commonest cephalopods in the Early and Middle Devonian but decline in the Late Devonian, when about 18 genera are present. Only two Devonian families, and no Silurian families, survived into the Early Carboniferous.  

Lifestyle

Some oncocerids - especially slender shelled forms such as representatives of the Oncoceratidae, Tripteroceratidae, and others - used the crowding the septa for buoyancy control. This is the most primitive form of buoyancy control and is found in the earliest nautiloids. In other oncocerids the size of the phragmacone relative to the body chamber was greatly reduced, as, for example, in the Hemiphragmoceratidae, which superficially were very like the Mandaloceratid Discosorids. These forms most certainly would drift head-down close to the sea bottom, snatching prey with their long arms, and with limited movement only. However other types, like the coiled Stereotoceras and Nothoceras, were more probably active swimmers (nektonic) in the water column, like the modern-day Nautilus.  

Families and Genera

The following list is in no way definitive, up to date, or comprehensive. The descriptions are mostly from on Flower and Kummel, p.611-612, while the genus list is from Sweet 1964 (Treatise). The dates are from The Fossil Record II. Sometimes the dates and the genera attributed don't match up, so there has been some revision since the Treatise was published, but unfortunately I don't have access to the papers in question

Family GRACILOCERATIDAE Flower in Flower & Kummel 1950
Exogastric cyrtocones, probably derived from the Bassleroceratidae, from which they differ mainly in the thin homogeneous connecting ring. Probably ancestral to the Oncoceratidae
Middle to Late Ordovician (Darriwilian to Hirnantian).
Graciloceras, Eorizoceras, Piersaloceras, Ringoceras.

Family ONCOCERATIDAE Hyatt 1884
Compressed brevicones and cyrtocones, including secondarily forms of broader section, siphuncle ventral, generally free from deposits, relatively slender in older forms. This stock includes most of the Ordovician cyrtocones, apparently persisting as Oonoceras into the Silurian, giving rise to a second great expansion including straight and endogastric forms. Many Ordovician and Silurian genera would seem to distinct though strongly homeomorphic. Conversely, the group is probably over-split as some forms may be growth stages. The family would seem to be the stem group from which most other Oncoceratida developed
Middle Ordovician to Late Silurian (Darriwilian to Pridoli)
Ordovician: Digenuoceras, Dunleithoceras, Oncoceras, Beloitoceras, Ehlersoceras, Kentlandoceras, Loganoceras, Maelonoceras, Miamoceras, Neumatoceras, Richardsonoceras, Romingeroceras, Scofieldoceras, Vaupellia, Zitteloceras.
Ordovician to Silurian: Oonoceras, Rizoceras,
Silurian: Centrorizoceras, Cyrtorizoceras, Ekwanoceras, Metarizoceras, Oocerina, Paroocerina .

Family TRIPTEROCERATIDAE Flower 1941
Shells straight or faintly exogastric, depressed, venter flattened, siphuncle ventral, cyrtochoanitic or suborthochoanitic siphuncle.
Ordovician
Allumettoceras, Hadeoceras, Rasmussenoceras, Tripteroceras, Tripterocerina.

Family VALCOUROCERATIDAE Flower 1945

Augustoceras
Late Ordovician (Sandbian) of Laurentia; length about 12 cm.

Family DIESTOCERATIDAE Foerste 1926
Straight to faintly endogastric brevicones, compressed, marginal actinosiphonate siphuncles.
Middle to Late Ordovician (Darriwilian to Hirnantian).
Danoceras, Diestoceras, Suttonoceras.

Family HEMIPHRAGMOCERATIDAE Foerste 1926
Endogastric brevicones essentially straight adorally, siphuncle nummuloidal, actinosiphonate, aperture with hyponomic sinus and paired lateral sinuses.
Middle to Late Silurian (Wenlock to Pridoli).
Conradoceras, Hemiphragmoceras, Hexameroceras, Mandatoceras, Octamerella, Tetrameroceras.

Family TRIMEROCERATIDAE Hyatt in Zittel 1900
Superficially similar to the above, but exogastric, siphuncle slender, with no deposits, usually with a median dorsal sinus.
Silurian.
Clathroceras, Eotrimeroceras, Trimeroceras, Inversoceras, Pentameroceras, Stenogomphoceras.

Family BREVICOCERATIDAE Flower 1941
Exogastric trochoceroids, gyrocones and brevicones stemming from Oonoceras in the Silurian, tending to develop vestigial actinosiphonate deposits and a subtriangular section. Possibly ancestral to the Nautilida (but see also Acleistoceratidae)
Devonian: Lochkovian to Famennian
Silurian: Foersteoceras, Mitroceras, Oxygonioceras (not Brevicoceratid?)
Devonian: Aletoceras, Anglicornus, Blastocerina, Eleusoceras, Exocyrtoceras, Gonionaedyceras, Gyronaedyceras, Micronoceras, Naedyceras, Ovoceras, Steveotoceras, Verticoceras, Wissenbachia.

Family JOVELLANIIDAE Foord 1888
Actinosiphonate, developing into orthoceracones and cyrtocones of triangular to depressed section. Probably derived from Valcouroceratidae
Early Silurian to Late Devonian (Landovery - Famennian).
Silurian: Mixosiphonoceras, Projovellania, Herkimeroceras.
Devonian: Jovellania, Laumontoceras

Family NOTHOCERATIDAE Fischer 1882

Nothoceratid evolution. Active swimming evolute (nautilid)-shaped forms like Nothoceras (bottom) probably gave rise to sedentary snail-like types like Lorieroceras (top and above left)	Lorieroceras lorieri (Barrande), a snail-shaped nautiloid from the Early Devonian of France. The cut-away section shows the location of the siphuncle Height : image from Moore, Lalicker and Fischer, Invertebrate Fossils, p.357

Breviconic to nautiliconic shells, depressed in cross-section, aperture contracted or visored, siphuncle ventral, generally concavosiphonate, actinosiphonate.
Early to Late Devonian (Lochkovian to Givetian)
Silurian: Perimecoceras (not Nothoceratidae?)
Sil. - Dev: Blakeoceras (not Nothoceratidae?)
Devonian: Bolloceras, Conostichoceras, Cyrthoceratites, Lorieroceras, Metaphragmoceras, Paraconradoceras, Nothoceras, Turnoceras.

Family ACLEISTOCERATIDAE Flower, 1950
Depressed exogastric brevicones, siphuncle broadly expanded, often actinosiphonate. Derived from Amphicyrtoceras and other contemporary forms which are intermediate between the Silurian Oncoceratidae and this family; may originate from Ordovician Oncoceras independent of the Oonoceras line. Probably ancestral to the Poterioceratidae, and possibly ancestral to the Nautilida (but see also Brevicoceratidae).
Middle Silurian to Middle Devonian (Wenlock to Givetian)
Silurian: Amphicyrtoceras, Anomeioceras, Byronoceras, Chadwickoceras, ?Clionyssiceras, Crateroceras, Euryrizoceras, Galtoceras, Hercytoceras, Perioidanoceras, Rhomboceras, Slocomoceras, Streptoceras
Devonian: Acleistoceras, ?Gonatocyrtoceras, Grimsbyoceras, Paracleistoceras, Potenocerina, Paradeistoceras, ?Tumidoceras, Worthenoceras.

Family ARCHIACOCERATIDAE Teichert 1939
Exogastric shells with a broadly expanded actinosiphonate dorsal siphuncle (see above).
Early to Middle Devonian (Pragian to Givetian)
Archiacoceras, ?Cyrtoceratites, Devoncheilus

Family KAROCERATIDAE Teichert 1939
Compressed exogastric cyrtocones and orthocones with slender ventral siphuncle; possibly an artificial grouping.
Early Silurian to Late Devonian (Landovery to Famennian)
Karoceras, Osbornoceras, ?Shuranoceras