The so-called ‘cosmopolitan palaeobiogeographic distribution’ of Tertiary Nautilida of the genus Aturia Bronn 1838: the result of post-mortem transport by oceanic palaeocurrents

Abstract

The common view among palaeontologists that nautiloids are of little palaeobiogeographic usefulness is not justified, at least for Palaeozoic and Mesozoic genera. Post-mortem transport of their shells seems to have a small impact on the fossil record, and the original distribution of these cephalopods appears to be preserved in the majority of cases. However, the genus Aturia Bronn 1838 (Nautilida known from the Palaeocene to the Miocene) constitutes an exception. Like modern Nautilus, Aturia was in all likelihood a nektobenthic cephalopod at all stages of its post-embryonic development. Its ‘cosmopolitan distribution’ is therefore surprising because many deposits bearing it are located in areas separated since the Palaeocene by large oceans acting as palaeobiogeographic barriers. This widespread distribution results mainly from post-mortem transport of shells by oceanic palaeocurrents. Many sedimentological and taphonomical clues indicate that Aturia shells in various Neogene deposits of the Aquitaine Basin, like many other shells of this genus in the world, indisputably represent floated shells. The frequency of occurrence of this transport can be explained by the singular morphology of the siphuncle of Aturia. This siphuncle, in a dorsal position, exhibits very long septal necks, which represent a unique morphological feature among the post-Triassic Nautilida. There is a very small surface for the connecting ring (permeable part of the siphuncle), and so the post-mortem filling rate of the chambers during the ascent of the shell in the water column was very limited. Thus, shells would have reached the surface very frequently and floated long distances, transported by oceanic palaeocurrents. Aturia shells had certainly the greatest potential for post-mortem drift among the post-Triassic Nautilida. Post-mortem transport tends to destroy shells in sedimentary contexts where they are most likely to be deposited, that is high-energy shallow littoral environments. However, at the global scale, these environments are clearly more common in the Cenozoic sedimentary record (visible in outcrop) than in the Jurassic and the Cretaceous. Therefore, the probability of these drifted Tertiary shells being preserved was correspondingly greater. A main distribution area of Aturia populations is suggested to have been the eastern and western coasts of Central, North and South America. This could explain the occurrence of many drifted shells in the Indo-Pacific and on the western coasts of the Atlantic ocean. Shells of the fossil Nautilida that are indisputably nekroplanktonic provide evidence for palaeoceanographic reconstructions. Some Aturia shells, often isolated, fragmentary and badly preserved, have been specifically identified according to typologic criteria. In addition to their stratigraphic distribution, the geographical distance between Aturia-bearing deposits has sometimes implicitly constituted a taxonomic criterion. Post-mortem transport of shells over long distances refutes the geographical ‘argument’. A systematic revision of Aturia is called for.