Body position and the functional morphology of Cretaceous Heteromorph ammonites

Previous analyses of ammonite functional morphology have assumed that the animal filled the entire body chamber, and that movement of the animal (such as withdrawal of the head and arms into the shell) had little effect on orientation. An alternative anatomy is proposed here: the ammonite animal was small, mobile, and capable of moving away from the aperture when threatened. The effect of this anatomy on the hydrostatics of aspinoconic, ancycloconic, and hamiticonic heteromorph ammonites is analysed. This analysis suggests that movement of the body would significantly alter the distribution of mass and hence orientation.