The ontogenetic development of shell was studied in three turtle species: Testudo graeca (Testudinidae), Emys orbicularis (Emydidae), Trionyx sinensis (Trionychidae). Individual bony plates are found to form only from one source. This fact is contradictory to the hypothesis of a complex origin of shell plates due to the fusion of basal skeletal elements and osteoderms. More probably, the shell has been formed resulted from a reduction of primary trunk dorsal and ventral osteoderms. This process was accompanied with expansion of basal bones (ribs, neural arches, bones of a shoulder girdle, and gastrals) in dermis. Hence, carapace and plastron appeared. Osteodermal peripheral plates connected them to the unique armored structure of turtle shells, Paedomorphosis as an instrument of evolutionary development is not characteristic of turtles. The morphogenetic data confirm phylogenetic hypotheses about the saltation origin of turtles and their close relation with Pareiasauria.