Plectronocerida is a primitive order from which subsequent

cephalopod 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 ...

orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic Pres Inc.

Occurrence

Plectronoceratids are known from the Upper

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 ...

(upper Franconian – middle Trempealeauan) of China and North America (Minnesota, Wisconsin). Two families are recognized (Flower, 1964),Flower, Rousseau H. 1964. The Nautiloid Order Ellesmeroceratida (Cephalopoda); Memoir 12, New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico the generally straight to endogastric

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

and the slightly exogastric Balkoceratidae.

Diagnostic characters

Members of the Plectronocerida are characterized as follows. Shells are generally small, some even tiny, laterally compressed, curved (cyrtoconic) or straight (orthoconic). Most cyrtoconic forms are endogastric, with the ventral side longitudinally concave, or the dorsal side more longitudinally convex. A few, the two known genera in Balkoceratidae are exogastrically curved, with the ventral side convex and dorsal side concave. Septa are close spaced, in some less than a millimeter. Siphuncles are ventral, and in most, proportionally large. Connecting rings are in general poorly calcified and may expand as siphonal bulbs into the chambers where not restricted by septal necks. As with all shelled cephalopods, plectronocerids had a tube called a

siphuncle The siphuncle is a strand of biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

, which let them fill the chambers of their

phragmocone The phragmocone is the chambered portion of the shell of a cephalopod. It is divided by septa into camerae. In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camerae are linked by ...

with gas instead of water, thus controlling their buoyancy. They were not, however, adapted for jet-powered swimming.

Phylogeny

The Plectronoceratida gave rise in the

Late Cambrian The Furongian is the fourth and final Epoch (geology), epoch and Series (stratigraphy), series of the Cambrian. It lasted from to million years ago. It succeeds the Miaolingian series of the Cambrian and precedes the Lower Ordovician Tremadocia ...

(early and middle Trempealeauan) to the other three plectronoceratoid orders, the

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 ...

and 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 ...

. The Plectronoceratidae gave rise to the other ellesmerocerid families, including the Upper Cambrian exogastric Balkoceratidea, and with remote possibility to the

Discosorida Discosorida are an 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 co ...

. The Balkoceratidae are unrelated to later exogastric forms that first appeared in the

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 ...

Ecology

Plectronocerids were probably

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

animals that crawled along the bottom in search of food or safety, facing downwards, with the shell carried above. Nothing is known of their specific soft-part anatomy or to what extent tentacles, if any, had developed.

Occurrence

Diagnostic characters

Phylogeny

Ecology

References

Further reading