''Endoceras'' (

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

for "inner horn") is an extinct genus of large, straight shelled cephalopods from the Middle and Upper

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

that gives its name to the Nautiloid order

Endocerida Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silu ...

. 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, 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'' ("chambered horn") is a genus of extinct, giant orthoconic cephalopod that lived mainly during the Ordovician period. It first appears during the middle Ordovician, around 470 million years ago, and was a fairly common componen ...

'', the two may be synonymous, but differs from the genus '' Nanno'' 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 A museum ( ; plural museums or, rarely, musea) is a building or institution that cares for and displays a collection of artifacts and other objects of artistic, cultural, historical, or scientific importance. Many public museums make thes ...

measures 3 meters (9.8 ft) as preserved. The most recent estimate puts its complete size at 5.7 meters (18.7 ft). This would make it the largest cephalopod by length in the fossil record. There is additionally an unconfirmed report of a 9.1 meter (30 ft) shell that was destroyed.

References

*Teichert, C. 1964. Endoceratoidea;

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

, 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