REVISITING THE NOTION OF POSTMORTEM DRIFT IN FOSSIL CEPHALOPODS

Biogeographic distributions are important factors in both evolution and extinction. Paleobiogeographical analyses must make the assumption that fossil localities provide an accurate estimate of a taxon's living geographic range. While this assumption is reasonable for many benthic marine invertebrate groups, it has been the subject of dispute among fossil cephalopod workers because of cephalopods' chambered shell, which may remain buoyant after death, resulting in postmortem transport or drift by ocean currents. In the last decade, arguments in the literature have ranged from the idea that postmortem drift is the dominant fate of cephalopod shells to the claim that transport is relatively rare for most cephalopod groups, at least for those shells surviving into the fossil record. I used data on fossil cephalopod shell forms and geographic distributions to assess the potential impact of postmortem transport on observed paleobiogeographic ranges. The results of classical experimental studies on postmortem shell buoyancy were used to predict which planispiral shell forms within Westermann morphospace have more or less potential for postmortem drift. Late Cenomanian and Early Turonian (Late Cretaceous) ammonoid genera were then categorized into "drifters" and "non-drifters" in order to test the hypothesis that fossil taxa with shell forms likely to drift have larger observed geographic ranges than other fossil cephalopods. Overall, shell forms with relatively large whorl widths are more likely to be buoyant postmortem. Most ammonoids have shell forms less likely than modern or fossil nautilids to drift after death. There is no correlation between shell form and observed geographic ranges for Cenomanian and Turonian cephalopod genera and no significant difference in the distribution of geographic ranges between predicted drifters and non-drifters. These results support the contention that, while postmortem drift can indeed happen (especially for shells with wide whorl sections), it is relatively rare among fossil cephalopods and is not likely to distort the observed geographic ranges of cephalopod taxa.