The Actinoceriatidae are a family of actinocerids named by Saemann in 1853 for those that grew to have large shells with blunt apices and large siphuncles with widely expanded segments and a generally arcuate endosiphucular canal system.Teichert 1964. Actinoceratoidea,

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.(Nautiloidea) Their range is from the upper Middle

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 ...

to the Lower Silurian. Actinocerids are generally straight-shelled

nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...

cephalopods with a siphuncle composed of expanded segments, typically with thin connecting rings, in which the internal deposits are penetrated by a system of canals Actinoceratids are derived from ''

Wutinoceras ''Wutinoceras'' is a genus of now extinct nautiloid cephalopods of the Wutinoceratidae family. It exhibits orthoconic actinocerids with ventral siphuncles composed of broadly expanded segments.Memoir 2, Studies of the Actinocerida, New Mexic ...

'', possibly through an early '' Armenoceras'' or through '' Nybyoceras''Flower 1957.Studies of the Actinoceratida; Memoir 2; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM and give rise to ''

Lambeoceras ''Lambeoceras'' is a genus of large actinocerids with a convexly lenticular cross section from the Upper Ordovician of North America and the sole representative of the family Lambeoceratidae. Morphological description ''Lambeoceras'' is of mediu ...

'' and to the Huroniidae. Seven genera are included in the Actinoceratidae, ''

Actinoceras ''Actinoceras'' is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician. It is an extinct genus of nautiloid cephalopod that thrived in the warm waters of the U ...

'', '' Floweroceras'', '' Kochoceras'', '' Leurorthoceras'', '' Paractinoceras'', '' Saffordoceras'', and '' Troostoceras''. ''Actinoceras'' is the earliest but with a range that carries it into the Lower Silurian. ''Troostoceras'' followed by ''Saffordoceras'' are later Middle Ordovician genera related to early ''Actinoceras''. ''Kochoceras'' followed by ''Floweroceras'' are Upper Ordovian genera related to later ''Actinoceras''. ''Leurothoceras'' and ''Paractinoceras'', both from the Upper Ordovician, are shown to have their source in Middle Ordovician actinoceratids. ''Paractinoceras'' is probably the most distinct actinoceratid from ''Actinoceras''. ''Paractinoceras'' has a long, straight, slender shell and a siphuncle that starts off like that in ''Actinoceras'', but becomes narrow like that in '' Ormoceras'' in the anterior part of the phragmocone. ''Kochoceras'' has a large blunt shell that expands more rapidly than Actinoceras and is strongly flattened on the ventral side causing possible confusion with ''Lambeoceras'' to casual observation. ''Floweroceras'' is doubtfully distinct from ''Kochoceras''. ''Leurothoceras'' is synonymous with ''

''; ''Saffordoceras'' and ''Troostoceras'' are closely related.

References

{{Taxonbar, from=Q15728112 Prehistoric nautiloid families Ordovician cephalopods Silurian cephalopods Middle Ordovician first appearances Silurian extinctions