Locomotion in ancient seas: Constraint and opportunity in Cephalopod adaptive design

Abstract

The locomotor system of shelled cephalopods (ectocochleates) is typically characterized by volumetrically large buoyancy devices, small mantle cavities, and little propulsive muscle. Equilibrium control is based on high hydrostatic stability. Drag is high and maximum swimming velocities are generally low. Ectocochleates having coiled shells were better designed in the locomotor sense, and out-performed apically ballasted non-coiled types. These in turn were propulsively superior to unballasted non-coiled forms. Locomotor design appears to be a key factor influencing the course of cephalopod evolutionary history. Shelled cephalopods dominated active predatory marine lifestyles only until the late Devonian diversification of jawed, paired-fin fishes of greatly superior locomotor design. After the appearance of such animals, ectocochleate evolution was canalized by locomotor disparities into paths that separated them ecologically from such advanced design swimmers. Ammonoids appear to have emphasized locomotion at very slow, but very efficient speeds, while nautiloids appear to have retained some slight capacity for higher velocities.