Nautiloid shell taphonomy: Interpretations based on water pressure

Abstract

Nautiloid taphonomy is a difficult and widely neglected topic acting as a test of many of the assumptions made in nautiloid paleobiology, phylogenetic interpretations, and geological facies analysis. The Eocene nautiloids from the offshore London Clay Formation of England are considered from the perspective of flooding and implosion experiments on Recent Nautilus. The deeper water facies are often characterized by body chambers in approximate life position, one or two complete septa, and various types of shell debris. Intermediate depth facies show intact and often vertically embedded phragmocone walls, with zones of internally broken and spar cemented septa. Taphnomic characteristics of the former facies probably result from the implosion of shells at depths of 200 or 300 m on the seafloor, while those of intermediate depth facies result from the sinking of gas-filled shells to the seafloor and subsequent bending stress failure of septa left unsupported by sediment in the last whorl. Evidence for the removal of the body from the largely unbroken shell by octopus illustrates the potential for a biological influence on the taphonomic history of nautiloids through the Phanerozoic. Tertiary nautiloids were probably unusually prone to extensive post-mortem transport, a process which tends to remove shells from the fossil record under most geological circumstances.