Turtle Shell

 The shell of a turtle is composed of a carapace and a plastron. The carapace is located dorsally, and the plastron is located ventrally. They are connected by a bridge on both sides (Schoch & Sues., 2020).

Figure 1: (a) dorsal view of carapace.
(b) ventral view of carapace showing nine thoracic vertebrae.
(c) stained specimen of the carapace in a young turtle
(d) increased magnification of costal plates
(Hirasawa et al., 2013)

The carapace is made up of bony elements: the neural plates, costal plates, and peripheral plates. The neural plates are in the center line of the carapace, and they are attached to the vertebrae on the underside (Schoch & Sues., 2020). The costal plates surround the neural plates and support the ribs (Schoch & Sues., 2020). The peripheral plates are located surrounding the costal plates. There is also a nuchal which is the anterior portion of the carapace (Figure 1).

Figure 2: (a) Dissection of the surface layer of keratinized scutes.
 Cancellous bone lies between the dorsal cortex and ventral cortex.
(b) Close up of a suture
(Achrai & Wagner., 2013)

The plastron is also made-up bony elements: the epiplastron which is the most anterior followed by the entoplastron, hyoplastron, hypoplastron, and the xiphiplastron (Schoch & Sues., 2020). Keratinized scutes are located on the outer most surface of the shell. This keratinous scute layer is waterproof. They connect the plates of the carapace (Achrai & Wagner, 2013). The dorsal carapace bone lies directly underneath the keratinous surface layer (Figure 2). The costal plates are composed of two types of bone tissue: the cortical bone and the cancellous (spongy) bone. The spongy bone lies between two layers of cortical bone (Achrai & Wagner, 2013).

The function of the shell is mainly protection from predators. It can also be protection from environmental stressors such as collisions against rocks (Achrai & Wagner, 2013). In addition, the shell encompasses the organs which serves as protection.

References:

Achrai, B., & Wagner, H. D. (2013). Micro-structure and mechanical properties of the turtle carapace as a biological composite shield. Acta biomaterialia, 9(4), 5890–5902. https://doi.org/10.1016/j.actbio.2012.12.023

Hirasawa, T., Nagashima, H. & Kuratani, S. (2013). The endoskeletal origin of the turtle carapace. Nature Communications 4, 2107. https://doi.org/10.1038/ncomms3107

Schoch, R.R., & Sues, H.-D. (2020). The origin of the turtle body plan: evidence from fossils and embryos. Palaeontology, 63: 375-393. https://doi.org/10.1111/pala.12460