Among the 800+ species of modern cephalopods, only the five Nautilus species have external shells. But, in the distant geological past, most cephalopods had heavy, large, chambered shells that, like the Nautilus shells, allowed them to regulate their buoyancy and orientation in the water. David J. Peterman of Wright State University and his collaborators recently published a comprehensive article in Palaeontologia Electronica suggesting that distinct groups of long-extinct cephalopods differed in the way they controlled buoyancy, static orientation (position of the animal at rest), ability to swim forward, and skills in turning and moving quickly to avoid predators or catch prey. The authors used complex three-dimensional modelling techniques to analyze and interpret volumes, shell shapes, and shell thicknesses. They concluded, for instance, that the open-coiled shells of Phragmoceras nautiloids (photo), which lived around 430 million years ago, let them achieve neutral buoyancy quickly and allowed for forward swimming. This latter was probably an advantage to the mollusk, giving their bottom-feeding habits. Read more about their research here.