Evolution of the coiled ammonoid conch from the uncoiled bactritid conch was probably coupled with changes in ma-
 noeuvrability and swimming velocity. The gradual transformation of uncoiled to coiled ammonoid conchs has essential
 functional consequences. The radical change in conch geometry during phylogeny but also in ontogeny of early
 ammonoids implies a shift of the aperture from an original roughly downward, via a downward oblique and an upward
 oblique to an upword orientation, presuming a neutrally buoyant condition of the ammonoid animal.  Similar trends were
 reconstructed for the three main ammonoid lineages in the Middle Devonian, the agoniatitid, the anarcestid, and the
 tornoceratid lineages. This allowed an increase in manoeuvrability and in the maximum horizontal swimming speed.

 Key words: Bactritida, Ammonoidea, ontogeny, phylogeny, locomotion, coiling, Devonian.