Protactinocerida Temporal range:
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Superorder:	†Plectronoceratoidea
Order:	†Protactinocerida Chen et al., 1979
Genera
Protactinoceridae Benxioceras Mastoceras Physalactinoceras Protactinoceras Sinoeremoceras Wanwanoceras

Protactinocerida is a proposed order of Late Cambrian nautiloid cephalopods. Their fossils have only been found in the Late Cambrian (late Jiangshanian to early Stage 10) of North China, a diversity pattern similar to other early cephalopod orders.^[1][2]

They were very similar to members of the order Plectronocerida in most respects. Like plectronocerids, their shells were compressed and slightly curved, containing numerous closely-spaced septa. The connecting rings have a calciosiphonate structure, while the siphuncle is divided by narrow partitions known as diaphragms. Protactinocerids were originally differentiated from plectronocerids based on their proportionally wider siphuncle.^[2] For many proposed protactinocerid genera, this may be a misinterpretation of fossil cross-sections exposed along a shallower angle than in plectronocerid fossils. Nevertheless, a very broad siphuncle (more than half the width of the shell) does seem to be valid for Protactinoceras at least.^[3] Another supposed distinguishing trait is the presence of calcite deposits between the diaphragms.^[2] However, it is difficult to distinguish biological calcite deposition from later diagenetic infilling, so this trait is also dubious.^[4] Some authors have suggested merging Protactinocerida into Plectronocerida in light of the uncertainty in differentiating the two orders.^[4][5]

References