The size of the siphuncle in cephalopod evolution

Abstract

The relative siphuncle surfaces of 250 specimens of the most important clades of shelled cephalopods from the Early Palaeozoic to Cenozoic are compared. An index (si) was calculated, which gives the relative inner surface of the connecting ring relative to the volume of the phragmocone chambers. It is shown that cephalopods with very high si developed strong de-coupling spaces between the outer surface of the siphuncular epithelium and the open space of the phragmocone chambers. Additionally these cephalopods often show depressed cross sections or flattened venters. Conversely, cephalopods with low si show simple connecting rings but very different shell shapes. It is shown that fast buoyancy changes were possible in cephalopods both with high si and low si. The size of the siphuncle is therefore considered not to be a simple measure of the performance of the buoyancy apparatus. Instead it refers to the level of metabolic energy of the cephalopods. It is shown that during their entire evolution shelled cephalopods can be divided into taxa which show evidence of a high energy level metabolism and taxa which show evidence of a low energy level metabolism for buoyancy regulation. The general trend since the end of the Palaeozoic is clearly towards a more energy efficient buoyancy regulation. This trend is considered to be an effect of increasing constraints of selection through time.