Protactinocerida

The Protactinocerida is an order of primitive nautiloid cephalopods from the upper Cambrian of China that contains a single family, the Protactinoceratidae, containing six named genera.

All protactinocerids have siphuncles that contain diaphragms and calcareous deposits. Septal necks may vary, even in a single specimen.

The Protactinocerida have been proposed as ancestral to the Actinocerida, which seems unlikely since the Protactinocerida seem to have gone extinct well before the end of the Cambrian and well before the earliest known actinocerids.

Protactinocerid genera
Genera included in the Protactincerida are Wanwanoceras, Sinoeremoceras, Protactinoceras, Benxioceras, Mastoceras, and Physalactinoceras. Wanwanoceras and Sinoeremoceras are found in the Upper Yenchou and Wanwankou members of the Upper Cambrian Trempealeauan Fengshan Formation of China. Protactinoceras, Benxioceras, Mastoceras, and Physalactinoceras have been found only in the Wanwankou.

Wanwanoceras has a slightly expanding shell with a slight endogastric curvature and narrow ventro-marginal siphuncle. Sinoeremoceras is similar but has a larger siphuncle and less curvature. Protactinoceras has a moderately expanded orthoconic shell with slightly expanded segments in the siphuncle; Benxioceras, a rapidly expanding endogastric shell with a narrow vertro-margial siphuncle; Mastoceras, a fairly orthoconic shell, also with a narrow ventro-marginal siphuncle. Physalactinoceras differs in that its siphuncle is broad with segments strongly expanded into the chamber.