From INTRODUCTION:

  In this chapter we discuss the mode and rate of growth in ammonoids,
  focusing primarily on postembryonic growth.  We first discuss the general
  mode of growth and then describe the ontogenetic sequence of growth stages.
  These stages are recognized on the basis of changes in morphology.- For
  example, a graph of the increase in size of whorl width versus shell diameter
  in an individual reveals changes through ontogeny that pinpoint the end of
  one growth stage and the beginning of another.  We next discuss the overall
  rate of growth through ontogeny and establish a generalized growth curve.  In
  this discussion, we refer to other cephalopods whose rate of growth is known.
  Fluctuations in the rate of growth that are superimpose on this growth curve
  are indicated in ammonoids by the presence of such shell features as varices
  and constrictions.
      The absolute rate of growth in ammonoids depended on a variety of factors,
  including temperature, food availability, and injuries to the individual animal.
  In addition, ambient pressure and the permeability of the siphuncle governed
  the rate at which cameral liquid was removed and, hence, the growth rate.
  Most methods to determine the actual rate of growth in ammonoids assume
  that particular morphological features were secreted at a known rate or
  periodicity.  Other methods attempt to identify an environmental signal that
  was captured in the morphology or chemistry of the shell.  It is also possible
  to study epizoans that grew on the shell of an ammonoid while the ammonoid
  was alive.  In this method, the epizoans are used as chronometers to measure
  the rate of ammonoid growth.  We compile the data from these various methods
  to arrive at soine general estimates of the age at maturity, especially for
  shallow-water taxa.