Plectronocerida

The Plectronocerida, confined to the Upper Cambrian, is the oldest order within Nautiloidea and therefore the Cephalopoda. Shells are minute, camerae (chambers) very short, siphuncle bulbous. It is divided into two families, the Plectronoceratidae which contains straight and slightly endogastric forms, and the Balkoceratidae which contains exogastric forms.

The Plectronocerida, and therefore the entire Cephalopoda, starts of Plectronoceras (Plectronoceratidae) which is Franconian and early Trempealeauan (Late Cambrian) in age. Plectronoceras gave rise to Paraplectronoceras and possibly Lunanoceras in the Trempealeauan along with three related genera, all from eastern China. Straight shelled Paleoceras from the Upper Cambrian San Saba Limestone of central Texas is contemporary.

Narrow and moderately exogastric Balkoceras, type for the Balkoceratidae, also comes from the San Saba of Texas while rapidly expanding Theskeloceras of the same family comes from China.

The Plectronoceratidae gave rise to the Ellesmerocerida, Protactinocerida and Yanhecerida, all in the Late Cambrian. Of these four orders, only the Ellesmerocerida persisted into the Ordovician and reaching the Early Silurian.