''Endoceras'', from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

ἔνδον (''éndon''), meaning "inside", and κέρας (''kéras''), meaning "horn", is an extinct genus of large, straight-shelled

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s that gives its name to the Nautiloid order

Endocerida Endocerida, from Ancient Greek ἔνδον (''éndon''), meaning "inside", and κέρας (''kéras''), meaning "horn", is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. End ...

. The genus lived during the middle and upper

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

470 to 443 million years ago. The cross section in the mature portion is slightly wider than high, but is narrower laterally in the young. Sutures are straight and transverse. ''Endoceras'' has a large

siphuncle The siphuncle is a strand of biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

, located close to the ventral margin, composed of concave segments, especially in the young but which may be tubular in the adult stage. Endocones are simple, subcircular in cross section, and penetrated by a narrow tube which may contain diaphragms reminiscent of the Ellesmerocerid ancestor. ''Endoceras'' was named by Hall in 1847. Distribution is widespread, especially in North America and Europe; and fossils have been found in Australia. ''Endoceras'' is similar to ''

Cameroceras ''Cameroceras'', from Ancient Greek καμάρα (''kamára''), meaning "chamber", and κέρας (''kéras''), meaning "horn", is an extinct genus of endocerid cephalopod which lived in equatorial oceans during the entire Ordovician period. Lik ...

'', the two may be synonymous, but differs from the genus ''

Nanno Nanno () was a ''comune'' (municipality) in Trentino in the northern Italian region Trentino-Alto Adige/Südtirol, located about north of Trento. As of 31 December 2004, it had a population of 623 and an area of .All demographics and other stati ...

'' in that the siphuncle in ''Nanno'' fills the entire apical portion of the shell while in ''Endoceras'' the siphuncle is ventral even there with septa formed at the onset. Mature, full grown, ''Endoceras'' were most likely ambush predators that lay in wait on the sea floor, moving when necessary to gain the advantage. Younger individuals with compressed cross sections may have been more actively mobile.

Size

A specimen of ''Endoceras giganteum'' at the

Museum of Comparative Zoology The Museum of Comparative Zoology (formally the Agassiz Museum of Comparative Zoology and often abbreviated to MCZ) is a zoology museum located on the grounds of Harvard University in Cambridge, Massachusetts. It is one of three natural-history r ...

measures as preserved. The most recent estimate puts its complete size at approximately . This would make it the largest cephalopod by length in the fossil record. There is additionally an unconfirmed report of a shell that was destroyed.

References

*Teichert, C. 1964. Endoceratoidea;

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

, Part K; Geol Soc of America and University of Kansas Press {{Taxonbar, from=Q135494 Prehistoric cephalopod genera Middle Ordovician first appearances Late Ordovician extinctions Paleozoic cephalopods of North America Paleozoic life of Ontario Verulam Formation Paleozoic life of Manitoba Paleozoic life of Newfoundland and Labrador Paleozoic life of the Northwest Territories Paleozoic life of Nunavut Paleozoic life of Quebec Endoceratidae Fossil taxa described in 1847