Exceptional examples of planispiral ammonites that were infested by epizoans during life display alterations of their normal coiling. Most commonly, the epizoan(s) settled on the venter of the ammonite and constituted an obstacle for the whorl tube grown one whorl later; this caused lateral deviation of the whorl tube and tilting of the ammonoid because of changes in the hydrostatic condition; from here on, the whorl tube periodically crossed the venter of the preceding whorl, thereby producing a zigzag coiling pattern. Some epizoans, which were particularly centered on the midventer, provoked detachment between whorls. In a few cases, lateral placement of the epizoan did not directly obstruct the normal growth path of the ammonite but induced a trochospiral coiling pattern. Both the zigzag and the trochospiral pattern were created when the ammonite tried to maintain the growth direction within the vertical plane at the same time as whorls remained in contact along a differentiated dorsal epithelium. The aperture reacted to changes in growth direction, to maintain also a permanent angle with the vertical direction. Growth direction, then, was a major morphogenetic parameter in ammonites, because it contained the necessary instructions for correct shell coiling. The model based on the observation of fabricational defects has been tested by a theoretical model by which the different situations so far observed are simulated and in which the parameters are the morphogenetic instructions inferred to have been present in the biological system.