Allonautilus in the context of "Cephalopod"

Nautiloids are a group of cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living Nautilus and Allonautilus. Fossil nautiloids are diverse and species rich, with over 2,500 recorded species. They flourished during the early Paleozoic era, when they constituted the main predatory animals. Early in their evolution, nautiloids developed an extraordinary diversity of shell shapes, including coiled morphologies and giant straight-shelled forms (orthocones). No orthoconic and only a handful of coiled species, the nautiluses, survive to the present day.

In a broad sense, "nautiloid" refers to a major cephalopod subclass or collection of subclasses (Nautiloidea sensu lato). Nautiloids are typically considered one of three main groups of cephalopods, along with the extinct ammonoids (ammonites) and living coleoids (such as squid, octopus, and kin). While ammonoids and coleoids are monophyletic clades with exclusive ancestor-descendant relationships, this is not the case for nautiloids. Instead, nautiloids are a paraphyletic grade of various early-diverging cephalopod lineages, including the ancestors of ammonoids and coleoids. Some authors prefer a narrower definition of Nautiloidea (Nautiloidea sensu stricto), as a singular subclass including only those cephalopods which are closer to living nautiluses than they are to either ammonoids or coleoids.

Nautilus is a marine cephalopod genus in the mollusk family Nautilidae. Species in this genus differ significantly, morphologically, from the two nautilus species in the adjacent sister-taxon Allonautilus. The oldest fossils of the genus are known from the Late Eocene Hoko River Formation, in Washington State and from Late-Eocene to Early Oligocene sediments in Kazakhstan. The oldest fossils of the modern species Nautilus pompilius are from Early Pleistocene sediments off the coast of Luzon in the Philippines.

The commonly used term 'nautilus' usually refers to any of the surviving members of Nautilidae, and more specifically to the Nautilus pompilius species. The entire family of Nautilidae, including all species in the genera Nautilus and Allonautilus, is listed on Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).

The Nautilaceae form one of five superfamilies that make up the Nautilida according to Bernard Kummel (1964), and the only one that survived past the Triassic. The Nautilaceae comprise six families: Nautilidae, Paracenoceratidae, Pseudonautilidae, Cymatoceratidae, Hercoglossidae, and Aturiidae. Shimanskiy (1957) separated the Paracenoceratidae and Pseudonautilidae from his near equivalent Nautilina and added them to the Lyroceratina, expanding the equivalent Clydonautiloidea and bringing it into the Jurassic. The Nautilaceae are represented by Nautilus and Allonautilus genera of the Nautilidae, the only extant family in this group.

Species in the Nautilaceae are generally smooth and involute with straight to strongly sinuous sutures and a small siphuncle. Some groups have sinuous plications or ribs.