Hydrodynamic and structural constraints on ammonoid shell shape

This work examines the structural role of the ammonoid shell in supporting hydrostatic load and the role of shell shape in facilitating swimming. The history of studies of the role of the shell in supporting hydrostatic load is discussed first, and is followed by an analysis of the function of the septal suture. The discussion of swimming consists of a critique of the previous work which emphasized the use of Nautilus as a modem analogue for swimming in ammonoids. This is followed by a discussion of flow tank studies that I conducted to examine the role of shell shape in drag production and power consumption of swimming in ammonoids.

It had long been thought that the the shells of chambered cephalopods served as buoyancy compensation devices. However, it was not until the 1960's that the internal pressure of the shell was demonstrated to be less than one atmosphere, and that the shell structure supports hydrostatic load. It is remarkable that the internal pressure of cephalopod shells was not ascertained until the 1960s. As early as 1832 the great anatomist Richard Owen called for an empirical investigation of this subject. Functional and adaptive explanations were in vogue in Owen's time as a consequence of William Paley's argument from design. During the late 19th and early 20th century adaptation and functional explanations were no longer emphasized in the interpretation of morphology; non-adaptive evolutionary ideas in vogue in this time period stressed the role of development as a guiding force. It was not until the mid 20th century that there was renewed interest in cephalopod functional morphology and the role of the shell in resisting implosion was ascertained.