Several examples of progenetic species among Mesozoic ammonites are investigated. The ammonites are Middle Jurassic kosmoceratids, Upper Jurassic oppeliids, and Upper Cretaceous scaphites. As assessed through outgroup comparison, the progenetic species in each of these examples is characterized by small size at maturity and the presence of morphological features typical of those of the juveniles of more primitive species. However, in none of these examples is the progenetic species a duplicate form of the juvenile stage of more primitive species. There are at least three kinds of features of progenetic species that differ from those of the juveniles of more primitive species: (1) symplesiomorphous features associated with maturation, (2) unique features whose presence is not clearly related to progenetic processes, and (3) unique features that may represent epigenetic outgrowths of juvenile morphology. This novel combination of juvenile. adult. and unique features may endow progenetic species with the evolutionary potential to play a role in the origin of higher taxa. □Progenesis, heterochrony, Ammon-oidea. Jurassic, Cretaceous.