Mollusca		Neocephalopoda
CYRTOSOMA		Orthocerida

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Orthocerida (= Michelinocerida)

Cephalopoda
|--Nautiloidea
`--NEOCEPHALOPODA  
   |--ORTHOCERIDA  
   `--+--ASCOCERIDA  
      `--BACTRITIDA  
         |--AMMONOIDEA
         `--Coleoidea

Introduction

Ordovician to Permian or Triassic

Kionoceras as it may well have appeared in life
Silurian to Devonian, worldwide
subfamily Kionoceratinae (Ordovician - Permian) - family Orthoceratidae

Even though they are extinct today, the Orthocerida were among the most successful and long-lived of the Paleozoic Cephalopoda. With their long graceful shells they very much typified the early Paleozoic benthos and nekto-benthos. Appearing during the Middle Ordovician, they soon supplanted both the Endocerida and the Ellesmerocerida. During the later Ordovician and the Silurian they were among the common of the straight-shelled cephalopods, although generally smaller than their larger and rarer contemporaries the Actinocerids. As the most primitive of the Neocephalopoda, they were the ancestors of the Bactridida, which in turn seem to represent the stem group form which all advanced Cephalopoda evolved.  

Technical Diagnosis

Dominantly orthoceracones, circular in section, siphuncle subcentral, mostly orthochoanitic (cyrtochoanitic forms are placed in the ), Necks well developed, rings thin, homogeneous.

But see:

 The "Orthocerida" - a nightmare for taxonomists.  

Conch Shape

The Orthocerid conch is very long, slender, and either straight (orthocones) or slightly curved (cyrtocones) conchs of subcircular (slightly ovoid) cross section, Apertures are generally entire, only rarely contracted or constricted, although this statement is somewhat speculative as complete body chambers of many genera are not known. In the Ordovician and Silurian, annulated shells (e.g. Dawsonoceras) and shells with strong surface ornamentation in the form of longitudinal (family or subfamily Kionoceratinae) or transverse (family or subfamily Spyroceratinae) ridges and grooves were relatively common, but these declined after the Silurian.  

Septa

Orthocerid septa are often widely placed (especially in the family Orthoceridae), and Orthocerid shells generally tend to have endosiphuncular or cameral deposits or both. Cameral deposits are of taxonomic importance in some groups, such as the Early to Middle Devonian Lamellorthoceratidae, which have cameral deposits of closely spaced, sometimes densely packed, longitudinal lamellae

There are also genera in which the shell lack internal deposits, and these animals may have used a different kind of ballast (less likely, some sort of gas exchange mechanism.).  

Siphuncle

Orthocerids have a slender central or subcentral siphuncle with orthochoanitic (cylindrical) or slightly cyrtochoanitic (beaded) septal necks and in some cases endosiphuncular deposits (ballast).  

Evolutionary History

The order Orthocerida is perhaps the most successful of the early Paleozoic Cephalopoda. Evolving from Baltoceratid Ellesmerocerida, they first appeared during the Middle Ordovician, and increased in morphologic diversity until the Middle Silurian, when about 30 genera existed. Thereafter the order declined in abundance until the Permian, when it died out. It used to be thought (and is still often reported) that some Triassic forms belong to the family Orthoceratidae (Michelinoceratidae). However, these have a pseudorthoceratid-type protoconch, and probably could be considered to be specialized late surviving Pseudorthoceratids.  

Lifestyle and Ecology

The Orthocerids, like the Endocerids, Actinocerids, and Pseudorthocerids, with their long and graceful shells, were slow moving predators, drifting slowly through the water, mostly close to the sea bottom where they could easily catch an unwary trilobite or crustacean. Compared with the hugely elongate shell, the living chamber and the animal were relatively tiny, but the conch had perfect hydrostatic balance, and was essentially weightless as far as floatation went. This balance was achieved mainly by development of endosiphuncular or cameral deposits, or both. These were predominantly nektobenthonic and nektonic forms.  

Families and Genera

The following list is in no way definitive, up to date, or comprehensive. It is mostly from on Flower and Kummel, p.609-10, with several extra families added. The dates are from The Fossil Record II.  

Family MICHELINOCERATIDAE Flower / ORTHOCERATIDAE

Michelinoceras
Middle Silurian,
Czech Republic, Germany, Sardinia (Italy), Caucasus, 
North America, Tibet (China), China, Japan, Australia

Family GEISONOCERATIDAE

Geisonoceras rivulae Haytt, 1884
Middle Ordovician to Late Devonian,
North America, Russia, Czech Republic, Germany, Japan, Ordovician of the Russian and Siberian platforms and Bohemia; Ordovician-Devonian of Asia and North America.

Family PROTEOCERATIDAE
Early Ordovician to Late Silurian (Floian to Ludlow)

Family LAMELLORTHOCERATIDAE
Early to Middle Devonian (Pragian to Eifelian)

Family STEREOPLASMOCERATIDAE Kobayashi
Cyrtochoanitic orthoceracones, siphuncle with a continuous lining, non- segmental, heavy cameral deposits. Segments become broadly expanded at early growth stage, becoming more slender in late maturity.
Early (Floian) to Late (Sandbian) Ordovician or possibly Late Silurian (Pridoli)
Stereoplasmoceras, Stereoplasmocerina. Tofangoceras, Tofangocerina; tentatively placed here are Eskimoceras, Gorbyoceras, Striatoceras, Ephippiorthoceras.

Family CLINOCERATIDAE Flower
Shells become fusiform, slightly curved siphuncle subcentral, planoconvex to convex, free from deposits. Derived from Orthoceratidae through Centroonoceras. Clinoceras is a likely ancestor of the Ascocerida.
Middle to Late Ordovician (Darriwilian to Hirnantian).
Clinoceras, Whitfieldoceras, (?) Whiteavesites.

Family PARAPHRAGMITIDAE Flower
Annulated orthoceracones and cyrtocones, aperture tending to be modified, finally becoming breviconic forms with cyrtochoanitic siphuncles.
Silurian to Early Devonian (Wenlock to Lochkovian)
Calocyrtoceras, Cyriocyctoceras, Gaspocyrtoceras, Paraphragmites.