Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.

Occurrence

Plectronoceratids are known from the Upper 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...

 (upper Frankonian – middle Trempealeauan

Trempealeauan

The Trempealeauan is the upper or latest stage of the Upper or Late Cambrian in North America, spanning about 4 million years from about 492.5 to 488.3 m.y.a., equivalent to the Fengshanian of China...

 )of China and North America (Texas, New Mexico?). Two families are recognized (Flower, 1964), the generally straight to endogastric Plectronoceratidae 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 (cyrtochonic) or straight (orthconic). Most cyrtoconic forms are endogastric, with the ventral side longitudinally concave, or the dorsal side more longitudinally convex. A few, the two know genera in Balkoceratidae are exogastrically curved, with the ventral side convex and dorsal side concave. Septa are close spaced, in some less than a milimeter. 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 cephalopds, plectronocerids possessed a tube called a siphuncle

Siphuncle

The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

, which allowed them to fill the chambers of their phragmocone

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 camarae are linked by a siphuncle which determines buoyancy by means of gas...

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

, Protactinocerida, and Yanhecerida. The Plectronoceratidae gave rise to the other ellesmerocerid families, including the Upper Cambrian exograstric Balkoceratidea, and with remote possibility to the Discosorida

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...

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

Ecology

Plectronocerids were probably benthic 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.