The erroneous distinction between tetrabranchiate and dibranchiate cephalopods

Abstract

The informal subclass name Dibranchiata is still attributed to extant coleoids, referring to the possible taxonomic significance of the gill number in cephalopods. Ammonoids and the dibranchiate octopods exhibit a remarkable similarity in breeding strategies and an ammonite shape of argonautid egg cases, suggesting close phylogenetic relationships in which octopods are nude ammonoids, and accordingly reflect the dibranchiate anatomy of ammonoids. All these cephalopods descended from the Paleozoic nautiloids, which are represented today by two genera with a tetrabranchiate gill structure and other anatomical features, which differ from those in extant coleoids. The physiology of extant nautiloids enables them to survive in waters with low oxygen content at a few hundred meters depth, toward where the nautiloids withdrew. The two pairs of gills, which occur in extant nautilids only, are suggested to reflect a minor anatomical modification to improve respiration in low oxygen settings by the duplication of the cephalopod initial single pair of gills, and are thus of no taxonomic significance.