Sutural inversion in a heteromorph ammonite and its implication for septal formation

The first known sutural inversion in ammonoids occurred in the adolescent stage of a late Cretaceous Glyptonoceras subcompressum (Forbes). Inversion has affected all folioles and lobules which are convex adapically instead of adorally, but not the tie-points from which they are 'suspended' and which shape the principal saddle and lobes. The ventral median saddle is also normal due to its proximity to the siphuncle. The partially inverted sutures are also strongly approximated. This suggests that, in this instance, body advance was mainly by muscular pull against a negative pressure differential of cameral liquid to 'ambient' body pressure across the septal mantle, owing to insufficient liquid in the newly forming chamber. Conversely, a slightly positive pressure differential is inferred for normal ammonitic septum formation. In spite of reversal, the length of folioles and lobules remains constant, indicating the existence of a 'permanent' sinuous attachment band resembling the posterior aponeurosis of Nautilus , with tie-points for primary wall attachment.