Intejocerida is the name given to a group of generally straight shelled nautiloid cephalopods originally found in Lower and Middle Ordovician sediments in the Angara River

Angara River

The Angara River is a long river in Irkutsk Oblast and Krasnoyarsk Krai, south-east Siberia, Russia. It is the only river flowing out of Lake Baikal, and is the headwater tributary of the Yenisei River....

 basin in Russia; defined in the Treatise

Treatise on Invertebrate Paleontology

The Treatise on Invertebrate Paleontology 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 covering every phylum, class, order, family, and genus of fossil and...

 as an order, and combined there with the Endocerida in the Endoceratoidea.

Diagnosis

Members of the Intejocerida are typically straight shelled with large siphuncle

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 Spirula...

s that vary in position from ventral to central, in which septal necks from very short to holochoantic 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.

Taxonomic relations

Flower (1976) pointed out that not only does the Intejocerida contain two groups, one with Intejoceras and Bajkaloceras

Bajkaloceras

Bajkaloceras is a straight-shelled orthoceroid, and possibly a member of the Intejocerida, from the Angara River basin in central Russia, named by Balashov in 1962...

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 and therefore an invalid taxon.

Derivation

Intejoceras and Bajkaloceras can be reasonably derived from the Baltoceratidae

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, existing for approximately .-Taxonomy:Baltoceratidae was...

 or less likely from the Troedssonellidae

Troedssonellidae

Troedssonellidae is a family of orthoceroid cephalopods from the Ordovician, derived from rod-bearing Baltoceratidae, that have a continuous lining within the siphuncle that resembles very thin and slender endocones. Shells are generally slender and orthoconic. The siphuncle is central or...

. Evencoceras, Rossoceras, and Padunoceras have their probable origin in the Proterocameroceratidae

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....

and are retained with the endocerids.