Mollusca		Cephalopoda
CYRTOSOMA		Endocerida

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Endocerida

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

Introduction 

an endocerid cephalopod captures a trilobite

The Endocerida were among the most important groups of Ordovician cephalopods.  They were a diverse group, including both short and straight orthoconic (long-shelled) sometimes nearly cylindrical shelled forms.  During the middle Ordovician some long-shelled forms like Endoceras and Cameroceras attained lengths of 4 to as much as 10 meters - among the largest molluscan shells ever.

The relatively small living chamber and capacious siphuncle indicates that much of the visceral mass of the animal may have been housed within the siphuncle itself, unlike other nautiloids in which the body is located in the living chamber.  Many endocerids are breviconic - having very short, wide shells, but long cylindrical shells of nearly uniform diameter also are known. In some, the siphuncle is located at the ventral margin and fills the entire apical portion of the shell.

All endocerids are characterized by their unusually large siphuncle, which was usually located towards the ventral (bottom) side of the shell.  Unlike most other nautiloids, in which the siphuncle is a delicate structure, that of Endocerids is the strongest portion of the shell, and commonly is the only part preserved.

Not only is the siphuncle much larger than in other groups of nautiloids, but it has a distinct structure as well.  It also contains calcareous hollow conical deposits, open toward the mouth of the shell, called endocones, arranged like paper cups within a drink dispenser, which presumably served to counterbalance the weight of the long shell. A thin tube, the endosiphotube, extends through the apices of all the endocones. The existence of these endocones does away with the need for cameral deposits, and no endocerid has chambers containing deposits.

Evolutionary History

The endoceroids were among some half a dozen cephalopod orders that appeared in the earliest Ordovician. They reached their greatest diversity during the early and earlier middle Ordovician, were already in decline by the middle of this period, and became extinct at the end Ordovician. Some rare forms from the mid-Silurian have an Endocerid-like structure and are attributed to the endocerids. It is perhaps also conceivably possible that these were unrelated forms that had evolved an endocerid-like structure. In any case, the Endocerids, these giants of the early Ordovician seas, were among the first of the major cephalopod orders to become extinct.

Lifestyle and Ecology

Without doubt these large animals were the superpredators of the Ordovician, drifting slowly through the water column, probably close to the sea bottom where they could easily snatch an unwary trilobite or crustacean. The short squat breviconic forms were probably faster and more maneuverable than the long-shelled genera like Endoceras. But even the brevicones were weighed downed by the heavy siphuncle and endoconic ballast, and were unlikely to have been able to move very rapidly. Adult endocerids, like all large Ordovician and Silurian cephalopods, had no natural predators.

Families and Genera

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

Family PROTEROCAMEROCERATIDAE Flower

Primitive endoceroids, similar to the Ellesmeroceratidae in structure of the siphuncle, but differing in the presence of endocones. Shell slender, siphuncle ventral, necks ranging from vestigial to nearly the length of the camera. Siphuncle not enlarged apically. Septal necks still tend to elongate in some forms. Endocones simple to complex in structure. Siphuncle enclosed in phragmocone to tip of shell. Ancestral to other Endocerid groups.
Early to Middle Ordovician (Tremadoc to Darriwilian)
Proterocameroceras, Cotteroceras, Clitendoceras, Cyptendoceras (?)Manitouoceras (may not be a distinct genus; known from isolated siphuncles), Paraendoceras, Utoceras, Oderoceras, Oeiandoceras, Meniscoceras.

Family PILOCERATIDAE Miller

Cassinoceras grande Ulrich and Foerste Cross section of large pilocerid, length 25 cm Cassin Limestone (?Floian), Vermont image from Moore, et al. (1952)	Piloceras life position image from Flower (1955).

Short shell rapidly expanding from initial part, siphuncle tending to enlarge very rapidly, mature living chambers may be faintly breviconic; mostly compressed in cross-section. Evolved from the Proterocameroceratidae, holochoanitic. Evolved from Proterocameroceratidae and probably ancestral to Endoceratidae
Early to Middle Ordovician (Tremadoc to Dapingian)
Piloceras, Allopiloceras, Cassinoceras.

Family MANCHUROCERATIDAE Kobayashi
Asian forms, probably descended from Proterocameroceratidae. Shells grading from slender to rapidly expanding, cross-section generally depressed or circular. Siphuncle wall as in Proterocameroceratidae. Endocones may show elaborate specializations in cross section and in development of blades.
Early to Middle Ordovician (?Floian to Darriwilian)
Manchuroceras, Coreanoceras, ?Penhioceras, ?Liaoutungoceras.

Family CHIHLIOCERATIDAE Grabau
Like piloceridae but with complex endocones with several concavities.
Early Ordovician (?Floian)
Chihlioceras

Family CYRTENDOCERATIDAE

Exogastrically curved forms
Early to Middle Ordovician (?Floian to Darriwilian)
Cyrtendoceras

Family ENDOCERATIDAE Hyatt

Suecoceras barrande Dewitz
Cross section of small Endocerid, length 15 cm
?Dapingian of Sweden
image from Moore, et al. (1952).
Orthoconic shells, often very large; septal necks extending apically at least to the next septum, connecting rings usually retained. In earlier forms the siphuncle is inflated and may occupy the entire initial part of the shell (Nanno), or enclosed in the phragmocone (Suecoceras, above). Generic groupings based on early stages are hard to correlate with those based on mature shells. Probably evolved from the Piloceratidae. These are the last and also the largest of the Endocerida, mature shells reaching 4 or even (in the case of Cameroceras) 10 meters in length
Early to Late Ordovician (?Floian to Hirnantian)
Cameroceras, Endoceras, Nanno, Suecoceras, Vaginoceras Cydendoceras, Cyrtendoceras, Cyrtovaginoceras, Paravaginoceras, Dideroceras. Inadequately known genera include Colpoceras, Sidemina and Conoceras.