| 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
| |
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]