An embryonic ammonoid assemblage was discovered in a carbonate concretion recovered from
  a dysoxic, relatively offshore marine shale Virgilian (Upper Pennsylvanian) age in Kansas, USA.
  T'he assemblage consists primarily of two species of the Goniatitina, Aristoceras sp. and Vidrioceras
  sp., whose initial chambers (protoconchs) differ in size and shape.  Microscopic observations of
  serial thin sections of specimens at different growth stages reveal the sequence of embryonionic shell
  development starting with the formation of the initial chamber and ending with the synchronous
  secretion of a prismatic proseptum md nacreous swelling (primary varix) at the aperture.  The
  mode of occurrence of the embryonic shells of the two species in the concretion suggests that these
  ammonoids produced numerous small offspring, a reproductive strategy similar to that in many
  extant coleoids. [Ammonoids, embryonic shells, development, Carboniferous, Kansas.