Ontogenetic Development of the Shell in Trionyx sinensis (Trionychidae, Testudinata) and Some Questions on the Nomenclature of Bony Plates

The embryonic and postembryonic development of the bony shell in Trionyx sinensis (Trionychidae) is observed. There are two main types of ossifications in trionychid armor, like in other turtles. Ossifications of the first type (neurals and costals) are modified elements of axial skeleton. Ossifications of the second type (nuchal, preneural, plastrals) are dermal bones developing independently from the internal skeleton. Hypotheses on the special epithecal origin of bony callosities as well as on the unique nature of anterior neural and plastral plates in Trionychidae do not confirm by morphogenetic data.