Plectronoceratoidea is a superorder of primitive nautiloids from the Late Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 and Early Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 that include the ancestors of subsequent cephalopod orders. Included within are the exclusively Cambrian Plectronocerida

Plectronocerida

Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.-Occurrence:Plectronoceratids are known from the Upper Cambrian of China and North America...

, Protactinocerida, and Yanhecentida, and the Cambro-Ordovician Ellesmerocerida

Ellesmerocerida

The Ellesmerocerida is a order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.-Morphology:...

.

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

Ectenolites

Small, slender, cylindrial members of the Ellesmeroceratidae that resemble small but proportionally narrower Ellesmeroceras. Septa, as typical for ellesmerocerids, are close spaced with shallow lobes on either flank. The body chamber is proportionally long, the shell itself slightly compressed...

and Clarkoceras

Clarkoceras

Clarkoceras is a breviconic ellesmerocerid cephalopod, one of only two genera known to have crossed from the Late Cambrian, Trempealeauan, into the Early Ordovician, Gasconadian...

. The Ellesmerocerida in turn gave rise to the superorders Endoceratoidea, Actinoceratoidea

Actinocerida

The Actinocerida comprise an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial...

, Discosoratoidea

Discosorida

Discosorida is a unique order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and connects the chambers in cephalopods, which unlike...

, Orthoceratoidea

Orthoceratoidea

The Orthoceratoidea is a superorder , sometimes considered an infraclass or even a subclass, that comprizes nautiloid orders that have orthoconic to slightly cyrtoconic shells and central to subcentral siphuncles in which there may be internal deposits...

, and Nautilitoidea

Nautilitoidea

The Nautilitoidea is a superoder within the subclass Nautiloidea, comprizing the phylogenetically related Nautilida, Oncocerida, and Tarphycerida....

. The Ellesmerocerida, Endoceratoidea, Actinoceratoidea, and Discosoratoidea are restricted to the Paleozoic. The Orthoceratoidea, which extend at least into the Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

, gave rise to the Ammonoidea which became extinct by the end of the Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

. Nautilitoidea exist living today as the Nautilidae, represented by Nautilus

Nautilus

Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...

and Allonautilus

Allonautilus

The genus Allonautilus contains two species of nautiluses, which differ significantly in terms of morphology from those placed in the sister taxon Nautilus. Allonautilus is now thought to be a descendant of Nautilus and the latter paraphyletic.-External links:*...

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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.