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The main structures characterizing the turtle shell are the carapace, the upper shell, and the plastron, the lower shell.

Migration of the ribs into the upper shell.
- Rib migration occurs as the result of an epithelial-mesenchymal interaction, a common kind of tissue-tissue interaction characterizing the embryonic development of many structures in vertebrates, such as limbs and feathers.

Development of the carapace in embryos of the common snapping turtle, Chelydra serpentina.
- According to normal turtle embryonic development, the key events in the development of the carapace begin during stage 14. By the end of stage 15, the embryo is fully recognizable as a turtle. The CR at stage 15 has a thickened ectoderm and presents a mesenchymal condensation composed of whorls of cells. In successive stages the CR expands both laterally and ventrally, and becomes the margin of a fully shaped carapace.

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Plastron is the lower or flat part of a turtle's shell that can be found on the 'belly' or ventral part of the turtle.
This structure of the turtle provides a bony exoskeleton for this side of the turtle. Plastron is thought to be one of the oldest feature of a turtle's shell since turtle ancestor's had plastron like ventral bones despite having only a partial carapace. (Rice R. et al., 2016)

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Carapace is the dorsal part of the turtle shell. It is composed of costal and neural bony plates that are continuous with the underlying thoracic ribs and vertebrae.
The carapace is actually a fusion of about 50 bones. Carapace usually has 38 scutes. The patterns and designs of scutes differ from each species of turtles and even differences between members of the same species. (
According to Hirasawa et al. (2013), the carapace of turtles is derived from endoskeletal ribs.

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Broad-ribbed bony structures likely evolved to provide early turtles with more stability and leverage for fossorial locomotion (burrowing). As these structures evolved and the turtle's carapace emerged in later forms, the flexibility of the spine was reduced and crawling speed declined.

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Earliest known species displaying Reptile Integument

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Earliest known species displaying Reptile Integument

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Eunotosaurus africanus, the earliest known turtle, lived 260 million years ago. Eunotosaurus was toothed, and its midsection contained nine elongated trunk vertebrae, nine pairs of broad T-shaped dorsal ribs, and five pairs of gastralia (ventrally located abdominal ribs). Collectively, these modified bones may have served as a type of intermediate shell structure from which the carapace and plastron evolved.

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As turtles are members of the reptilian order Chelonia, they possess a scaly integumentary system. The reptile epidermis  is heavily keratinized with a layer which prevents water loss. The scales present in the reptile skin, such as a turtle's skin cannot be scraped off similar to a fish scale as reptile scales are an integral part of the skin. The reptilian scale usually lacks the bony under support of any significant structural contribution from dermis (Rutland, et.al., 2018)