Plectronoceratoidea

Plectronoceratoidea Temporal range: Upper Cambrian–Ordovician PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Superorder:	†Plectronoceratoidea Wade, 1988

Plectronoceratoidea is a superorder of primitive nautiloids from the Late Cambrian and Early Ordovician that include the ancestors of subsequent cephalopod orders. Included within are the exclusively Cambrian Plectronocerida, Protactinocerida, and Yanhecentida, and the Cambro-Ordovician Ellesmerocerida.

The Plectronocerida is the earliest group of the four and gave rise to the other three. Of these four orders, only the Ellesmerocerida crossed (barely) into the Ordovician, with two genera, Ectenolites and Clarkoceras. The Ellesmerocerida in turn gave rise to the superorders Endoceratoidea, Actinoceratoidea, Discosoratoidea, Orthoceratoidea, and Nautilitoidea. The Ellesmerocerida, Endoceratoidea, Actinoceratoidea, and Discosoratoidea are restricted to the Paleozoic. The Orthoceratoidea, which extend at least into the Triassic, gave rise to the Ammonoidea which became extinct by the end of the Cretaceous. Nautilitoidea exist living today as the Nautilidae, represented by Nautilus and Allonautilus.

Plectronoceratoids are generally small to tiny forms with orthoconic or endogastric shells, a few being exogastric, with proportionally large ventral siphuncles that contain numerous diaphragms. Orders are determined principally by differences in siphuncle detail.

References

• Chen, J. Y.; Teichert, C. (1983). "Cambrian cephalopods". Geology. 11 (11): 647–650. Bibcode:1983Geo....11..647J. doi:10.1130/0091-7613(1983)11<647:CC>2.0.CO;2. ISSN 0091-7613. 
• Flower, R.H. 1964, The Nautiloid Order Ellesmeroceratida (Cephalopoda). Memoir 12; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.
• Wade, M. 1988. Nautiloids and their descendants: cephalopod classification in 1986. Memoir 44, pp 15–25; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.