On the efficiency of the buoyancy apparatus in ammonoids: evidences from sublethal shell injuries

The five greatest sublethal injuries were selected from a collection of more than 12,000 predominantly Mesozoic injured or otherwise pathological ammonoids. The loss of shell mass from these survived injuries was calculated and compared with comparable tolerances in the recent Nautilus. These ammonoids tolerated a shell loss up to four times greater than in Nautilus. The maximum tolerated shell loss indicates an unexpected buoyancy compensation mechanism. The buoyancy of the selected specimens was calculated. The results show that the buoyancy of all the observed ammonoid shells was positive. In order to maintain neutral buoyancy after injury, these ammonoids had to fill the phragmocone with a volume of mass. Nautilus compensated a maximum mass loss requiring a liquid refill of 3% of the cameral capacity, the ammonoids compensated a maximum of observed mass loss requiring a liquid refill of more than 10% of cameral capacity. The ratio of chamber volume/siphuncular surface area in the ammonoid Lithacoceras is 0.043, indicating that the relative area of the siphuncular epithelium in Lithacoceras is significantly higher when compared with a ratio of 0.12-0.14 in the adult Nautilus. The phragmocone in ammonoids offered the ability of a much more active buoyancy regulation than in Nautilus.