TAPHONOMY AND SEDIMENTOLOGY OF A MODERN NAUTILUS DEPOSIT IN A NEARSHORE ENVIRONMENT

Nautilus is the only living externally shelled cephalopod and is a model for the interpretation of fossil cephalopods. Today Nautilus inhabits only the Indopacific region. Lifou of the Loyalty Islands in the South Pacific has an abundance of Nautilus macromphalus inhabiting the island’s reefs. Nautilus is exclusively a marine animal and is commonly found adjacent to reefs in water depths of 200-400 m. When Nautilus dies, the gas-filled chambers in the shell cause the shell to rise and sometimes float on the water surface. Shells that float on the surface can float for more than 1600 km and can be deposited on beaches far from their living sites. If enough chambers in the shell become water-filled, however, the shell sinks to the bottom. Little is known about the taphonomy of shells that sink. This is partly due to the fact that most occur in deep water and are inaccessible for detailed study. A submerged assemblage of 34 N. macromphalus shells was collected in 2008 from a shallow bay on Lifou. The specimens were collected from a carbonate sand substrate in 1-3 meters of water and were resting on the sediment surface, partially buried in loose sand, or lodged in small karst holes in the underlying limestone. While 66% of the specimens were collected in a horizontal orientation, 34% were in a vertical orientation. The shells display a variety of wear and abrasion features from those that were complete and clean to those with large breaks and faded color patterns. The exposed surfaces of most of the worn shells were encrusted with a diverse assemblage of algae and epizoans. Sediment samples were collected on the beach along four transects starting at the high tide line and extending 20-30 m into the bay. Additional sediment samples were collected with the in situ Nautilus shells. The sediment samples were analyzed in the laboratory to determine the weight percent of each grain size and the general composition. These data were collected to produce a model of the facies relationships expected in a similar depositional environment in the rock record. The results of this study represent the first taphonomic analysis of a modern, externally shelled cephalopod deposit from a shallow water marine environment. The data collected can be used to improve the interpretation of ancient nearshore deposits containing the fossils of externally shelled cephalopods.