Allonautilus

Allonautilus is the second of known living genera of ectocochliate (shell bearing) cephalopods, the other being the genus Nautilus. Two species are recognized, Allonautilus scrobiculatus described by Lightfoot in 1786 and Allonautilus perforatus described by Conrad in 1847, both originally included in Nautilus, described from empty shells alone.

Living animals, and soft parts, became known in 1984 by Drs Ward of Bryn Mar and Saunders of the University of Washington, which prompted their determination of a separate genus which differs from Nautilus not only in shell form but in details such as of the gill structure and male reproductive system. Allonautilus shells are quadrate in cross section as opposed to those of Nautilus which are ovoid. Also the umbilucus of the shell of Allonautilus is much larger than that of Nautilus. Live Allonautilus have a fuzzy coating on their shells.

Allonautilus scrobiculatus is native to the waters around New Guinea, New Britain, and the Solomon Islands; Allonautilus perforatus from the waters around Bali, Indonesia. Both are easily recognized by the large open umbilicus, which is around 20% of their shell diameter at its widest point.

Genetic study (DNA analysus) indicates that Allonautilus is most likely derived from Nautilus. But from which now extinct species, which itself would be paraphyletic, not the genus.