The most common trends observed in ammonoid evolution during
ecologically stable periods are characterized by an increase of shell
curvature (e.g. evolute to involute), by the development of more complex
ornamentation (flexuosity of ribbing, appearance of nodes and spines)
and by a long term increase of the suture line's fractal dimension.
Major evolutionary jumps in ammonoids occur during severe extinction
events, and are characterized by the sudden appearance of simple,
primitive-looking forms which are similar to remote ancestors of their
more complex immediate progenitors. Such forms are interpreted as
atavistic. According to this hypothesis, homeomorphic species generated
during such sublethal stress events can be separated by several millions
of years.