Miocene Nautilus (Mollusca, Cephalopoda) from Taiwan, and a review of the Indo‐Pacific fossil record of Nautilus

Despite exhaustive investigation of present-dayNautilus,the phylogenetic relationships of the five or six recognized species within this genus remain unclear. Mitochondrial and nuclear DNA sequence data plus a suite of morphological characters are used to investigate phylogenetic relationships. Systematic analysis of the morphological variation fails to characterize described species as independent lineages. However, DNA sequence analysis indicates that there are three geographically distinct clades consisting of western Pacific, eastern Australian/Papua-New Guinean, and western Australian/Indonesian forms. The morphologically and genetically distinct speciesNautilus scrobiculatusfalls outside the three geographically recognized assemblages. Members of the genusNautilusalso exhibit low levels of sequence divergence. All these data suggest thatNautilusis currently undergoing diversification, which may have begun only several million years ago. These data also suggest that some of the morphological features used to defineNautilusspecies may simply represent fixed variations in isolated populations within the same species.

Living ectocochliate cephalopods have long been thought to be restricted to a single genus, Nautilus Linnaeus, 1758, comprising five or six extant species. The shells of two species, N. scrobiculatus Lightfoot, 1786, and N. perforatus Conrad, 1847, are quite distinct, but no soft-parts were known until 1984, when N. scrobiculatus was seen alive for the first time. Dissections show that significant anatomical differences exist between N. scrobiculatus and other Nautilus species, including differences in gill morphology and details of the male reproductive system. These differences, along with phylogenetic analysis of extant and selected fossil nautiloid species, indicate that N. scrobiculatus, and N. perforatus should be distinguished from Nautilus as a newly defined genus, Allonautilus. This analysis contradicts previous phylogenies proposed for the Nautilida, which placed Nautilus as the last-evolved member of the order. We surmise that Allonautilus is a descendent of Nautilus, that the latter is paraphyletic, and first evolved in the Mesozoic, rather than in the late Cenozoic, as has been previously suggested.