Endocerid

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iendocerid
Fossil range: Ordovician - ?Silurian

Scientific classification

Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Order:	Endocerida

families

†Proterocameroceratidae
†Piloceratidae
†Endoceratidae

The endocerids were a diverse group of cephalopods that lived during the Middle Ordovician to Late Silurian periods. Their shells had both short and straight orthoconic (long-shelled) and sometimes nearly cylindrical-shelled forms. Some long-shelled forms like Endoceras and Cameroceras attained lengths as much as 3.5m (11 ft 8 in) - among the largest mollusc shells known. These giant orthoconic nautiloids are rare in the fossil record. The overwhelming majority of nautiloids are much smaller, usually less than a meter long fully grown.

The endocerids have a relatively small living chamber and particularly wide siphuncle that indicates 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. They are also distinguished by the presence of endocones, or calcareous (calcite) deposits that formed within the siphuncle. As the animal grew, chambers at the end were successively filled with calcite instead of gas, and it is assumed this was used to counterweight the animal's body.

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

Endocerids may have been 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. They filled the shark (predator) niche of their day. The short, squat breviconic (short-shelled) forms were probably faster and more maneuverable than the long-shelled genera like Endoceras. But even the brevicones were weighed down 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.