An embryonic ammonoid assemblage was discovered in a carbonate concretion recovered from a dysoxic, relatively offshore marine shale of Virgilian (Upper Pennsylvanian) age in Kansas, USA. The assemblage consists primarily of two species of the Goniatitina, Aristocerassp. 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 embryonic shell development starting with the formation of the initial chamber and ending with the synchronous secretion of a prismatic proseptum and nacreous swelliig (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.