Intejocerida is the name given to a group of 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 speci ...

cephalopods originally found in Lower and Middle Ordovician sediments in the

Angara River The Angara ( Buryat and mn, Ангар, ''Angar'', "Cleft"; russian: Ангара́, ''Angará'') is a major river in Siberia, which traces a course through Russia's Irkutsk Oblast and Krasnoyarsk Krai. It drains out of Lake Baikal and is ...

basin in Russia; defined in the

Treatise A treatise is a formal and systematic written discourse on some subject, generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject and its conclusions." Treat ...

as an order, and combined there with the

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 Siluri ...

in the Endoceratoidea.

Diagnosis

Members of the Intejocerida are typically straight shelled with large

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

s that vary in position from ventral to central, in which septal necks from very short to holochoanitic and connecting rings from moderately thick to apparently thin. Common to all, and the character by which the order was defined, are deposits within the siphuncle that have been described as longitudinal, radially arranged, calcareous lamellae.

Taxonomy

Taxonomic relations

Flower (1976) pointed out that not only does the Intejocerida contain two groups, one with '' Intejoceras'' and '' Bajkaloceras'' with central siphuncles, the other with '' Envencoceras'', '' Padunoceras'', and a third genus '' Rossoceras'', but that combining it with the Endocerida in the Endoceratoidea, makes the latter

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

and therefore an invalid taxon.

Derivation

''Intejoceras'' and ''Bajkaloceras'' can be reasonably derived from the

Baltoceratidae Baltoceratidae is an extinct family of orthoconic cephalopods belonging to the subclass Nautiloidea endemic to what would be Asia, Australia, Europe, North America, and South America during the Ordovician living from about 480–460 mya, exis ...

or less likely from the Troedssonellidae. ''Evencoceras'', ''Rossoceras'', and ''Padunoceras'' have their probable origin in the

Proterocameroceratidae The ''Proterocameroceratidae'' were the first of the Endocerida. They began early in the Ordovician with ''Proendoceras'' or similar genus which had developed endocones, replacing the diaphragms of the ellesmerocerid ancestor. Proterocamerocerat ...

and are retained with the endocerids.

References

* Flower, R.H. 1976. Some Whiterock and Chazy Endoceroids. Part II, Mem.28; New Mexico Bureau of Mines and Mineral Resources, 1976. * Techert, C. 1964. Endoceratoidea. Treatise on Invertebrate Paleontology, Park K. Teichert & Moore (eds). Geological Society of America and Univ Kansas Press. {{Taxonbar, from=Q3799302 Prehistoric nautiloids Prehistoric cephalopod orders