Editing Nautiloid (section)

==Taxonomic relationships==
Nautiloids are among the group of animals known as [[cephalopoda|cephalopods]], an advanced class of [[Mollusca|mollusks]] which also includes [[Ammonoidea|ammonoids]], [[Belemnoidea|belemnites]] and modern [[coleoidea|coleoids]] such as octopus and squid. Other mollusks include [[Gastropoda|gastropods]], [[Scaphopoda|scaphopods]] and [[Bivalvia|bivalves]].

Traditionally, the most common classification of the cephalopods has been a four-fold division (by Bather, 1888), into the [[Orthoceratoidea|orthoceratoids]], nautiloids, [[ammonoid]]s, and [[coleoid]]s. This article is about nautiloids in that broad sense, sometimes called Nautiloidea ''sensu lato''.

[[Cladistics|Cladistically]] speaking, nautiloids are a [[paraphyletic]] assemblage united by shared primitive ([[Plesiomorphy|plesiomorphic]]) features not found in derived cephalopods. In other words, they are a [[Evolutionary grade|grade]] group that is thought to have given rise to orthoceratoids, ammonoids and coleoids, and are defined by the exclusion of those descendent groups. Both ammonoids and coleoids have traditionally been assumed to have descended from [[Bactritida|bactritids]], which in turn arose from [[orthocone|straight-shelled]] [[Orthoceratoidea|orthoceratoids]]. The ammonoids appeared early in the [[Devonian]] period (some 400 million years ago) and became abundant in the [[Mesozoic]] [[Geologic time scale#Divisions of geologic time|era]], before their extinction at the end of the Cretaceous.

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 to the exclusion of other modern cephalopods. In this restricted definition, membership is somewhat variable between authors, but it usually includes Tarphycerida, Oncocerida, and Nautilida.