Taphonomy of backshore versus deep-marine collected Nautilus macromphalus conchs (New Caledonia)

The extant Nautilus nowadays exclusively lives in the Indo-Pacific Ocean along the slopes of coral reefs, mainly in water depths of 300-400 m. It possesses a complex gas-liquid combined system to regulate its buoyancy in the water column. After death this system is lost and the shells become either positively or negatively buoyant and float for some time or respectively sink to the seafloor where taphonomic processes strongly influence the shell's condition. A major process in taphonomy is bioerosion. We present herein a detailed study of the influence of taphonomic pathways on bioerosion in backshore collected and deep-water dredged Nautilus shells from the New Caledonia region. Some bioerosion of Nautilus shells may take place during life but most occurs after death and deposition of the shell. The ichnocoenosis found in the shells collected in backshore settings indicates that these shells were positively buoyant after the death of the animals and were initially deposited in a nearshore environment (shallow euphotic zone III to deep euphotic zone) before they were transported ashore. Part of the deep-water dredged shells, in contrast, were initially deposited in the deep euphotic to dysphotic zone before being transported into aphotic depths. We demonstrate that bioerosion supports the reconstruction of taphonomic pathways of Nautilus shells and, following an actualistic approach, it can help to reconstruct taphonomic processes and depositional settings for fossil cephalopod shells.