Mollusca		Cephalopoda
CYRTOSOMA		Actinocerida

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Actinocerida

CEPHALOPODA  
|--PLECTRONOCERIDA
`--+--ELLESMEROCERIDA  
   |--ENDOCERIDA  
   |--ACTINOCERIDA  
   |--PSEUDORTHOCERIDA  
   |--DISCOSORIDA  
   `--+--Nautiloidea
      `--Neocephalopoda

Introduction

The distinctive, highly specialized actinocerids are characterized animals possessing a long straight, sometimes large and nearly cylindrical shell, which, like the endoceroids, contains an unusually large and heavy siphuncle.

Diagnosis

Shell dominantly straight, siphuncle broadly expanded, usually large, occupied by annular deposits which may fill the entire siphonal cavity except for a vascular system of radial and central canals terminating in a thin cavity against the connecting ring, the perispatium.

Conch Shape

The shell is always long and straight (orthoconic), and in some forms nearly cylindrical.

Although many Actinocerids are small, the order includes many species which normally grew to a length of a meter or more, especially among the genera Actinoceras (the larger specimens of which may reach 2 meters), Armenoceras, Gonioceras, Huronia, and Lambeoceras. According to a poorly documented old report, the Carboniferous Rayonnoceras may have had shells as long as 6 meters, rivaling even the biggest endocerids in size, and making it one of the biggest animals of its time.

Siphuncle

The Actinocerid siphuncle (left, from Teichert, 1988) is very distinctive, large and heavy, and frequently the only part of the shell preserved. It is strongly cyrtochoanitic (beaded), composed of short, strongly recurved septal necks possessing a broad brim, and greatly inflated connecting rings. The siphuncle is so heavily filled with annulosiphonate deposits that only a small central and radial canals are left in the middle and rear (adapical) parts of the shell. There is also a peripheral space (perispatium) between the deposits and the connecting rings. The purpose of this canal system was presumably to allow nourishment and gas exchange to the tissues within the chambers that formed the cameral deposits.

Phylogenetic Relationships

Actinocerids do not appear to be closely related to any other groups of nautiloids, and seem to have evolved from unspecified Ellesmerocerids some time during the earlier Mid Ordovician. Although the large size of the siphuncle and the strongly calcified connecting rings would seem to relate the actinocerids with endocerids, it is now acknowledged these two groups are only distantly related. An original placement by Hyatt of actinoceroids, pseudorthoceroids, and other cyrtochoanitic nautiloids in a suborder called cyrtochoanites is also incorrect, as the cyrtochoanitic condition evolved independently a number of times, the cyrtochoanites being a polyphyletic taxon.

Evolutionary History

The earliest known actinocerids appeared during the mid-Ordovician. The Actinocerida survived with vigor the transition from the Ordovician into the Silurian that eliminated many invertebrate lineages, becoming one of the main groups of Silurian cephalopods. But by the Devonian they but became rare; perhaps unable to compete with the more compact and maneuverable coiled nautiloids and ammonoids, and the arrival of jawed fish. A few forms appear during the Mississippian subperiod, and they died out in the Carboniferous.

Principal evolutionary trends were increase in size and progressive simplification of the endosiphuncular canal system.

Families and Genera

The following list is in no way definitive, up to date, or comprehensive. It is mostly from on Flower & Kumel (1950), p.609. The dates are from The Fossil Record II

Family WUTINOCERATIDAE
The first actinocerids; they are found almost worldwide during the Darriwilian age. They are characterized by a reticular radial canal system in the siphuncle and rather thick connecting rings [Teichert, 1988, p.33)
Early to Late Ordovician (?Floian to Sandbian)
Wutinoceratidae

Family POLYDESMIIDAE Kobayashi.
The most primitive, perhaps ancestral group. Siphuncle broadly nummuloidal, annulosiphonate deposits extending inward as in Bathmoceras. May have given rise to both the Actinoceratidae and in part from Armenoceratidae. Mostly known from Manchuria.
Early to Middle Ordovician (?Floian to Darriwilian)
Polydessiia.

Family ACTINOCERATIDAE Saemann.
Necks long, recurved, not recumbent; siphuncle large, radial canals arc-like, one series.
Early Ordovician to Early Silurian (?Floian to Llandovery).
Actinoceras, Kochoceras, Saffordoceras, Troostoceras, Potydesmia. Floweroceras is probably the early stage of some member of this family.

Family ARMENOCERATIDAE Troedsson.
Siphuncle broadly expanded, brims greater than necks, often recumbent, connecting ring broadly adnate to septum at both ends.  Radial canals often complex, of several series, arc-like, becoming transverse later.
Early Ordovician to Late Silurian (?Floian to Ludlow).
Nybyoceras, Cyrtonybyoceras, A rmenoceras, Etrodoceras, Megadiscosorus, Selkirkoceras, Metarmenoceras.

Family HURONIIDAE Foerste and Teichert.
Neck recumbent, brim and neck broadly adnate to part of septum, which is bent apicad and incorporated in siphuncle; apicad end of siphuncle segments often tubular and greatly prolonged.
Middle Ordovician to Middle Silurian (Darriwilian to Wenlock).
Huronia, Huroniella, Discoadinoceras.

Family GONIOCERATIDAE Hyatt.
Siphuncles as in Armenoceratidae. Shell strongly flattened, sides acutely angled in cross section. Presumably these strange creatures were bottom dwellers.
Middle to Late Ordovician (Darriwilian to Hirnantian).
Gonioceras, Lambeoceras.

Family ORMOCERATIDAE
Siphuncle segments globular or more slender- smaller than in other Actinoceroidea. Radial canals leave central canal at right angles. derived from Actinoceratidae and in part from Armenoceratidae.
Early Ordovician to Mississippian (?Floian to Visean)
Sactoceras, Ormoceras, Deiroceras, Troedssonoceras, Treptoceras, Paractinoceras, Rayonnoceras, Leurorthoceras.

Family CARBACTINOCERATIDAE
Like Ormoceratidae, but with distinctly bilaterally symmetrical siphuncular deposits. May have reached very large size (unconfirmed report of Rayonnoceras up to 6 meters long). Possibly derived from Ormoceratidae.
Middle Carboniferous (Visean to Bashkirian)
Rayonnoceras.