Plectronoceratoidea is a superorder or subclass containing primitive nautiloids from the Late

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

and Early Ordovician. This group is best considered a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

grade of early

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...

, as it contains the ancestors of subsequent post-Cambrian cephalopod orders. Plectronoceratoidea contains several exclusively Cambrian cephalopod orders:

Plectronocerida Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic Pres Inc. O ...

, Protactinocerida, and

Yanhecerida Yanhecerida is a small order of Late Cambrian (mid- Stage 10) nautiloid cephalopods. They were similar to the more diverse Plectronocerida and Ellesmerocerida, with short shells, closely-spaced septa, and diaphragms (partitions) within the sip ...

. Under some classification schemes, the plectronoceratoid grade may also contain the paraphyletic Cambro-Ordovician Ellesmerocerida. A few older studies consider Plectronocerida to be descended from Ellesmerocerida (rather than ancestral), but this is no longer believed to be the case. The Plectronocerida is the earliest group of the four, and may have given rise to the other three. Of these four orders, only the Ellesmerocerida crossed (barely) into the Ordovician, with two genera, ''

Ectenolites ''Ectenolites'' is a genus of small, slender, cylindrical Ellesmeroceratids that resemble '' Ellesmeroceras'' but are smaller and proportionally narrower. Septa, as typical for ellesmerocerids, are close spaced with shallow lobes on either flank ...

'' and '' Clarkoceras''. Ordovician ellesmerocerids in turn likely gave rise to several new nautiloid superorders: Endoceratoidea, Multiceratoidea, and Orthoceratoidea. The Ordovician-Triassic Orthoceratoids are ancestral to post-Paleozoic groups such as ammonoids (ammonites) and coeloids (modern cephalopods without external shells). The origin of modern nautilids is less certain, though they may be descended from coiled multiceratoids or orthoceratoids. Ellesmerocerida proper are restricted to the Paleozoic, though their indirect descendants survive to the present. 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

*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. {{Taxonbar, from=Q7204415 Protostome superorders Nautiloids Paraphyletic groups