From INTRODUCTION:

     Ammonoids retain a record of growth in their shells, and, therefore, material
     is readily available for studies of early ontogeny.  Such studies were performed
     first in the mid-19th century and have been pursued with vigor ever since.
     Using optical and scanning electron microscopy, ammonoid workers have
     described the morphology of the early whorls and have attempted to recon-
     struct the sequence of early ontogenetic development and to identify the
     embryonic shell.
        Studies of early ontogeny are obviously crucial in understand ing the
     ecology and mode of life of adults.  Such factors as Population structure and
     biogeographic distribution grow out of the constraints of early ontogeny.  For
     example, differences in early life history may explain why some ammonoid
     species are more restricted in their biogeographic distribution than are others.
     These relationships may bear, in turn, on broader evolutionary issues such as
     species longevity and extinction.
        Studies of early ontogeny are also helpful in trying to reconstruct phylo-
     geny.  In the studies of Hyatt (1866, 1883, 1889, 1894), Smith (1898, 1914) and
     Buckman (1887-1907, 1909, 1918), ontogeny and phylogeny were closely
     linked together in a theory of recapitulation.  According to these authors, the
     early ontogenetic stages of an individual represented a recapitulation of the
     adult stages of its ancestors.  Although this view no longer is considered valid,
     there are, nevertheless, numerous characters in early ontogeny that are useful
     in reconstructing phylogeny.
       Much of the information presented in this chapter, especially with respect
     to the size of the embryonic shell, is new.  However, the morphological
     descriptions and interpretations of ontogenetic development rely heavily on
     previously published data.  Many of these data are based on Mesozoic rather
     than Paleozoic ammonoids because the former are generally better preserved.
     Specimens cited in this chapter are reposited in the American Museum of
     Natural History (AMNH), the University of  Iowa (SUI), the University Mu-
     seum of the University of Tokyo (UMUT), the New York State Museurn
     (NYSM), and the Yale Peabody Museum of Natural History (YPM).