Initiation of a reversal to uniseriality in the polymorphinid Nodocantabricus duplexmurus n. gen., n. sp.: A double-layered Foraminifera from the lower-middle Cenomanian of Cantabria, N-Spain

The new benthic foraminifer Nodocantabricus duplexmurus n. gen., n. sp. is introduced from the lower-lower middle Cenomanian of the Bielba and Altamira formations (North Cantabrian Basin, Spain). This distinctive and atypical Polymorphinidae combines primitive and advanced test characteristics. On the one hand, it fully develops uniserially arranged chambers from an initial spiral coiling, a tendency usually observed in advanced polymorphinids. On the other hand, it possesses a composite, double-layered calcitic wall, made of inner dark microgranular and outer fibro-hyaline layers, a structure seldom documented in post-Paleozoic foraminifers.In the Cretaceous, various polymorphinid groups record significant diversification periods, which taxonomic, phylogenetic and stratigraphic significances have not been evaluated yet. We here discuss the long term evolution of the family Polymorphinidae and its phylogenetic relationship(s) with the family Nodosariidae. A tendency to reversal in the chamber arrangement, from spiral to uniserial, is initiated in the Cenomanian. This evolutive inclination may later have originated a false “nodosariid” lineage, casting doubt upon the assumed monophyly of the family Nodosariidae.