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]
| 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 | |
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 edit
- ^ Fang, Xiang; Kröger, Björn; Zhang, Yuan-Dong; Zhang, Yun-Bai; Chen, Ting-En (2019-03-01). "Palaeogeographic distribution and diversity of cephalopods during the Cambrian–Ordovician transition". Palaeoworld. Onset of the GOBE. 28 (1): 51–57. doi:10.1016/j.palwor.2018.08.007. hdl:10138/308817. ISSN 1871-174X. S2CID 134586179.
- ^ a b c Jun-Yuan, Chen; Teichert, Curt (1983). "Cambrian cephalopods". Geology. 11 (11): 647. Bibcode:1983Geo....11..647J. doi:10.1130/0091-7613(1983)11<647:CC>2.0.CO;2.
- ^ Mutvei, Harry (2020-04-02). "Restudy of some plectronocerid nautiloids (Cephalopoda) from the late Cambrian of China; discussion on nautiloid evolution and origin of the siphuncle" (PDF). GFF. 142 (2): 115–124. doi:10.1080/11035897.2020.1739742. ISSN 1103-5897. S2CID 219047374.
- ^ a b Mutvei, H.; Zhang, Y.; Dunca, E. (2007). "Late Cambrian Plectronocerid Nautiloids and Their Role in Cephalopod Evolution". Palaeontology. 50 (6): 1327–1333. doi:10.1111/j.1475-4983.2007.00708.x.
- ^ Pohle A, Kröger B, Warnock RC, King AH, Evans DH, Aubrechtová M, et al. (April 2022). "Early cephalopod evolution clarified through Bayesian phylogenetic inference". BMC Biology. 20 (1): 88. doi:10.1186/s12915-022-01284-5. PMC 9008929. PMID 35421982.