The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species. All told, between 22 and 34 families and 165 to 184 genera have been recognised, making this the largest order of the subclass Nautiloidea.
Classification and phylogeny
Current classification
The current classification of the Nautilida, in prevalent use, is that of Bernhard Kummel (Kummel 1964) in the Treatise which divides the Nautilida into five superfamilies, the Aipoceratoidea, Clydonautiloidea, Tainoceratoidea, and Trigonoceratoidea, mostly of the Paleozoic, and the later Nautilaceae. These include 22 families and some 165 or so genera (Teichert and Moore 1964)
Other concepts
Shimansky 1962 (in Kummel 1964) divided the Nautilida into five suborders, the mostly Paleozoic Centroceratina, Liroceratina, Rutoceratina, and Tainoceratina, and the Mesozoic to recent Nautilina. These include superfamilies which are different from those of Kummel (1964) and of less extent. The Centroceratina are comparable to the Trigonoceratoidea, the Liroceratina to the Clydonautiloidea, and the Nautilina to the Nautilaceae. The main difference is that the Rutoceratidae are included with the Aipoceratoidea of Kummel (1964) in the Rutoceratina. The remaining Tainoceratoidea are the Tainoceratina.
Rousseau Flower (1950) distinguished the Solenochilida, Rutoceratida, and Centroceratida, as separate orders, from the Nautilida, derived from the Barrandeocerida, which are now abandoned. Within the Nautilida, he placed 10 families, included in the Nautilaceae and the no longer considered ancestral Clydonautiloidea. Teichert's 1988 classification is an abridged version of Shimansky's and Flower's early schemes.
Derivation and evolution
Both Shimansky and Kummel derive the Nautilida from the Oncocerida with either the Acleistoceratidae or Brevicoceratidae which share some similarities with the Rutoceratidae as the source. The Rutoceratidae are the ancestral family of the Tainoceratoidea and of the Nautilida (Kummel 1964) and of Shimansky's and Teichert's Rutoceratina.
The Tainoceratoidea gave rise, probably through the ancestral Rutoceratidae, to the Trigonoceratoidea and Clydonautiloidea in the Devonian and to the Aipoceratoidea early in the Carboniferous. The Trigonoceratoidea, in turn, gave rise late in the Triassic through the Syringonautilidae to the Nautilaceae, which include the Nautilidae, with Nautilus. (Kummel 1964)
Diversity and evolutionary history
thumb|left|[[Procymatoceras|Procymatoceras subtruncatus fossil]]
The Nautilida are thought to be derived from either of the oncocerid families, Acleistoceratidae or Brevicoceratidae (Kummel 1964), From the Oligocene onward, the appearance of pinnipeds in the geological record of a region coincides with the disappearance of nautilids from that region. As a result, nautilids are now limited to their current distribution in the tropical Indo-Pacific ocean, where pinnipeds are absent.