Micronautilus n. gen., a new dwarf Bathonian (Middle Jurassic) nautilid from Western France

The new genus Micronautilus n. gen. (type species Micronautilus evolutus n. gen., n. sp.) is described and assigned to the family Paracenoceratidae Spath, 1927. Known from only two species with a very small shell, this taxon is regarded to represent a natural lineage within the family. As currently known, the occurrence of Micronautilus n. gen. is limited to the Bathonian of Western France, extending from the Zigzag Zone to the Retrocostatum Zone. The origin of Micro­nautilus n. gen. is likely to lie in the upper Bajocian group of nautilids assigned to ‘Cenoceras’ fuscum (Crick, 1898). Paleoenvironmental data suggest that the dwarfism of this new taxon cannot be interpreted as a consequence of an isolated population or any specific environmental conditions.