Subclass Nautiloidea - Nautiloids

Of related interest:
Nautiloid Fossils
About Ammonoidea
Ammonite Fossils
Large Ammonite Pictures

The Nautiloids are a group of marine animals of Phylum Mollusca in the subclass Nautiloidea, and are thus closely related to ammonites. Nautiloids were particularly abundant during the early Paleozoic era when they were primary predators and evolved a large diversity of shell morphologies. More than 2,500 species of nautiloids are known from the fossil record, though extant nautiloids species are few in number. All extant cephalopods are believed descended from Paleozoic nautiloids.

Three key shell features, internal chambers, the siphuncle and the sutures, are shared by all nautiloids. Thin walls between the internal chambers (i.e., camerae) of the shell are called septa. As the nautiloids grew, they would detach their bodies from the walls of the shell, slide forward, and secrete a new septum to the rear. Each new septum added a new camera in the shell. The body of the animal itself occupied only the last added chamber of the shell. The septa were perforated by the siphuncle, a tissue tube running through all of the internal chambers of the shell. Surrounding the were structures made of Aragonite, a polymorph of Calcium Carbonate that during fossilization was readily converted to Calcite.

Nautiloids are considered to be a paraphyletic group united by shared primitive features absent in other cephalopods. They are believed to be ancestors of both ammonoids and coleoids, specifically, nautiloids of Order Bactritida, that descended from straight-shelled nautiloids of Order Orthocerida. The earliest Nautiloids in the fossil record are from the late Cambrian, Fengshan Formation in northeast China. These early forms appeared to have died out, leaving but a single family, the Ellesmeroceratidae that survived to the early Ordovician and gave rise to all subsequent cephalopods. Nautiloids apparently diversified during the Early and Middle Ordovician, possibly due to new ecological niches opened by the extinction of the predatory anomalocarids at the end of the Cambrian. Nautiloids remained diverse and widespread through the Ordovician, Silurian, and Devonian periods, exhibiting a wide variety of straight, curved and coiled shell morphologies. Nautiloids began to decline during the Devonian, possibly because of competition with their Ammonoids and Coleoids descendants and relatives the, with only Order Nautilida with tightly coiled shells apparently able to flourish. Nautilids were much less affected by the Permian-Triassic extinction event than the ammonoids. Nonetheless, only a single genus, Cenoceras, with a shell resembling the modern nautilus, survived the extinction event ending the Triassic Period. Nautiloids radiated again during the Mesozoic, as evidenced by 24 genera that are known from the Cretaceous, and were not as impacted by the extinction event at the end of the Cretaceous as the ammonoids which had no survivors. Three families and at least five genera of nautilids are known to have survived the extinction, enabling resurgence during the Paleocene, Eocene, and Miocene.