Development of Skin and Dermal Bone of the Shell in <i>Pelodiscus maackii</i> (Testudines: Trionychidae)

The article is devoted to the study of the ontogenetic development of the skin and the structure of the skin-bone shell of the trionychid turtle Pelodiscus maackii (Brandt, 1858). The process of formation of the epidermis and dermis, as well as their derivatives (epidermal scales and dermal ossifications), is described. The development of epidermal scales in Pelodiscus maackii has been shown to be similar to the development of asymmetric scales in other reptiles. Five stages of formation of the definitive structure of the dermal bones have been identified. Structural differences between the internal and external parts of the bony plates (rod-shaped elements and bony callosities) are due to the previous stratification of the dermis into histologically distinct strata: hypodermal, fibrous and papillary. In particular, the fibrous stratum, consisting of several layers of orthogonally oriented collagen bundles, is a precursor to the plywood-like structure of the trionychid bony plates. The growth of bone tissue around the vascular network in the papillary layer is the reason for the formation of surface ornamentation of the trionychid callosities. Some questions of the evolutionary significance of the unique features of the trionychid shell are discussed.