Postmortem transport in fossil and modern shelled cephalopods

Abstract

The chambered shells of cephalopod mollusks, such as modern Nautilus and fossil ammonoids, have the potential to float after death, which could result in significant postmortem transport of shells away from living habitats. Such transport would call into question these clades’ documented biogeographic distributions and therefore the many (paleo)biological interpretations based on them. It is therefore imperative to better constrain the likelihood and extent of postmortem transport in modern and fossil cephalopods. Here, I combine the results of classic experiments on postmortem buoyancy with datasets on cephalopod shell form to determine that only those shells with relatively high inflation are likely to float for a significant interval after death and therefore potentially experience postmortem transport. Most ammonoid cephalopods have shell forms making postmortem transport unlikely. Data on shell forms and geographic ranges of early Late Cretaceous cephalopod genera demonstrate that even genera with shell forms conducive to postmortem buoyancy do not, in fact, show artificially inflated biogeographic ranges relative to genera with non-buoyant morphologies. Finally, georeferenced locality data for living nautilid specimens and dead drift shells indicate that most species have relatively small geographic ranges and experience limited drift. Nautilus pompilius is the exception, with a broad Indo-Pacific range and drift shells found far from known living populations. Given the similarity of N. pompilius to other nautilids in its morphology and ecology, it seems unlikely that this species would have a significantly different postmortem fate than its close relatives. Rather, it is suggested that drift shells along the east African coast may indicate the existence of modern (or recently extirpated) living populations of nautilus in the western Indian Ocean, which has implications for the conservation of these cephalopods.

Toward an understanding of cosmopolitanism in deep time: a case study of ammonoids from the middle Permian to the Middle Triassic

“…Modern observations and experiments on cephalopods document that only highly inflated forms may float after death during long time intervals (Yacobucci 2018). However, most ammonoids have conch morphologies, making postmortem transport unlikely (Yacobucci 2018).…”

Lower Jaw of Spathites (Ammonoidea: Acanthoceratoidea) from the Upper Cretaceous (Turonian) of New Mexico

“…One of the issues in interpreting deposits containing ammonites is whether the ammo¬ because of its buoyant phragmocone. This interpretation was predicated on observations that dead shells of Nautilus are broadly distributed in the Indo-Pacific far from their origi¬ nal habitats (for a recent reinterpretation, see Yacobucci, 2018). However, Chamberlain et al (1981) investigated this hypothesis using a mathematical approach and determined that the probability of postmortem drift depended on the size of the shell and the depth at which it died.…”

Spatial distribution of oncocerid cephalopods on a Middle Devonian bedding plane suggests semelparous life cycle

“…Post-mortem transportation. Cephalopod shells may drift considerably after death, as evident in the extant Nautilus and Spirula although recent studies show that extended periods of drift are probably more the exception than the rule 46 . This point might be less relevant in our case, as already short-distance transport could confound the results.…”

“…Before analysing the spatial distribution of the specimens, we first considered first-order effects, such as post-mortem transportation. This was at first done qualitatively by comparing the preservation of the specimens to the existing literature on post-mortem transportation of cephalopod shells 46,51,74 .…”