Nautiloids are a large and diverse group of marine cephalopods (Mollusca) belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, but only a handful of species survive to the present day.
Nautiloids are among the group of animals known as cephalopods, an advanced class of mollusks which also includes ammonoids, belemnites and modern coleoids such as octopus and squid. Other mollusks include gastropods, scaphopods and pelecypods.
Traditionally, the most common classification of the cephalopods has been a three-fold division (by Bather, 1888), into the nautiloids, ammonoids, and coleoids. This article is about nautiloids in that broad sense, sometimes called Nautiloidea ''sensu lato''.
Cladistically speaking, nautiloids are a paraphyletic assemblage united by shared primitive (plesiomorphic) features not found in derived cephalopods. In other words, they are a grade group that is thought to have given rise to both ammonoids and coleoids, and are defined by the exclusion of both those descendent groups. Both ammonoids and coleoids have traditionally been assumed to have descended from bactritids, which in turn arose from straight-shelled orthocerid nautiloids.
The ammonoids (a group which includes the ammonites and the goniatites) are extinct cousins of the nautiloids that evolved early in the Devonian period, some 400 million years ago.
Some workers apply the name Nautiloidea to a more exclusive group, called Nautiloidea ''sensu stricto''. This taxon consists only of those orders that are clearly related to the modern nautilus. The membership assigned varies somewhat from author to author, but usually includes Tarphycerida, Oncocerida, and Nautilida.
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