Nautiloid

Nautiloids are a large and varied group of marine cephalopods (Mollusca) in the subclass Nautiloidea. They began in the later Cambrian. "Nautilus" is the only surviving genus. 
Nautiloids flourished during the early Palaeozoic era, when they were the main predatory animals. They developed an extraordinary range of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, though only a few species survive today.
Taxonomic relationships.
Nautiloids are among the group of animals known as cephalopods. The cephalopods are an advanced class of molluscs. The cephalopods also include ammonoids, Belemnites and modern coleoids such as octopus and squid. 
Traditionally, the most common classification of the cephalopods has been a three-fold division 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 group united by shared primitive (basal) features not found in derived cephalopods. In other words, they are a evolutionary grade that is thought to have given rise to both ammonoids and coleoids. They are defined by the exclusion of both those descendent groups. Both ammonoids and coleoids probably 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 which evolved early in the Devonian, some 400 million years ago.