Field experiments with Nautilus pompilius in the Philippines have uncovered two previously unknown postmortem phenomena: (1) waterlogging of the phragmocone does not occur until the mantle tissue detaches from the shell due to decomposition, and (2) the phragmocones of empty shells flood with seawater rapidly due to low internal gas pressure (<0.9 atm). The latter is more significant for small shells, because of the lower total gas pressure in their phragmocones compared to larger shells, and the smaller limit of seawater volume that still allows flotation. The limiting diameter for shells to sink from only the latter mechanism (lower internal gas pressure) is ∼200 mm. Ammonoids generally have body chambers longer than N. pompilius, but the ratio of body chamber volume to phragmocone volume for most ammonoids is the same as for N. pompilius. This ratio is the determining factor that allowed ammonoid shells either to float or sink from pressure compensation alone. This strongly suggests that (1) their limiting shell diameters approximate that of N. pompilius (i.e., ∼200 mm), (2) such shells sank rapidly close to their habitats, even if they initially floated, and (3) only large shells had the potential to drift postmortem over long distances. These findings are crucial to interpreting the early taphonomic history of extinct ammonoids.