HURRICANE IMPACT ON NAUTILUS (CEPHALOPODA) SHELL PRESERVATION IN RECENT TROPICAL INTERTIDAL AND SHALLOW MARINE ENVIRONMENTS

Shells of Nautilus macromphalus, an extant externally shelled cephalopod deposited in three bays on Lifou, The Loyalty Islands in the South Pacific, were collected in 2011 for taphonomic examination. The distribution and completeness of the Nautilus shells in two of these bays were previously studied in 2008 after at least 6 years without tropical cyclones impacting the area. Approximately 6 months prior to the 2011 field excursion, a Category 1 tropical cyclone passed over the island significantly impacting the Nautilus shell taphonomy.

Both shallow water (~2 m), intertidal, and storm tidal beach erosion cuts were examined. More than 25 shells were recovered from each of the previously studied bays near the southern tip of the island. In the 2008 survey, only 2 shells were recovered from one bay and 32 shells were recovered from the other; all were below the low tide level in 1–3 m of water. The Nautilus shells collected in 2008 were more complete than those collected in 2011. The majority (55.5% of the 2011 samples) were phragmocone cores that could not float, whereas in the 2008 samples, there were no phragmocone cores recovered. In a newly explored bay near the center of the island, 25 more shells were recovered in approximately 4–6 m of water. All of these shells were relatively complete and had extensive algal growths on exposed surfaces. Based on local reports, cyclone erosion removed approximately 10 m of beach sediment from the bay’s north end and added 3 m of beach sediment to the bay’s south end. In the eroded north end where well-developed storm tidal beach erosion cuts were present, five Nautilus shells were recovered from a newly exposed pumice bed. All of the shells from the pumice bed were extremely fragile and were only partly complete.

Using these deposits as a modern analog, it appears unlikely that similarly constructed fossil cephalopod shells (ammonoids and nautiloids) would survive beach conditions and become part of the fossil record unless they were completely buried and not periodically re-exhumed by storm conditions. Additionally, fossil cephalopod shells deposited in shallow water near beaches and where intense storms are periodically present will have extensive breakage. Fossil specimens deposited in ancient beach deposits above the tide line are unlikely to be preserved.