Plectronoceratoidea is a superorder or subclass containing primitive

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

from the Late

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

and Early

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

. This group is best considered a

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

grade Grade most commonly refers to: * Grading in education, a measurement of a student's performance by educational assessment (e.g. A, pass, etc.) * A designation for students, classes and curricula indicating the number of the year a student has reach ...

of early

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

, 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. Oc ...

, 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 siphun ...

. Under some classification schemes, the plectronoceratoid grade may also contain the paraphyletic Cambro-Ordovician

Ellesmerocerida The Ellesmerocerida is an extinct order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician. Morphology The Ellesmerocerida are characterized by shell ...

. 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 ''Clarkoceras'' is a genus of breviconic ellesmerocerid cephalopods, one of only two genera known to have crossed from the Late Cambrian, Trempealeauan, into the Early Ordovician, Gasconadian. (Flower 1964, Teichert 1988); the other being ''Ect ...

''. Ordovician ellesmerocerids in turn likely gave rise to several new nautiloid superorders:

Endoceratoidea Endocerida, from Ancient Greek ἔνδον (''éndon''), meaning "inside", and κέρας (''kéras''), meaning "horn", is an extinct Nautiloidea, nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their s ...

Multiceratoidea Multiceratoidea is a major subclass or superorder of Paleozoic nautiloid cephalopods. Members of this group can be characterized by nautilosiphonate connecting rings, with an organic inner layer and outer layer of calcitic spherules and blades, ...

, and

Orthoceratoidea Orthoceratoidea, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", is a major subclass of nautiloid cephalopods. Members of this subclass usually have orthoconic (straight) to slightly ...

. 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