The turtle shell is a shield for turtles (order Testudines), completely enclosing all the turtle's vital organs and in some cases even the head. It comprises an upper part, the carapace, and lower part, the plastron, connected by a bony bridge. It is constructed of modified bony elements such as the ribs, parts of the pelvis, and other bones found in most reptiles. The bone of the shell consists of both skeletal and dermal bone, showing that the complete enclosure of the shell likely evolved by including dermal armor into the rib cage. The turtle's shell is important to study, not just because of the apparent protection it provides for the animal, but also as an identification tool, in particular with fossils, as the shell is one of the most likely parts of a turtle to survive fossilization. Therefore, understanding the shell structure in living species provides comparable material with fossils. The shell of the hawksbill turtle, among other species, has been used as a material for a wide range of small decorative and practical items since antiquity, including food and medicine, but is normally referred to as tortoiseshell.
Shell nomenclature
The turtle shell is made up of numerous bony elements, generally named after similar bones in other vertebrates, and a series of keratinous scutes which are also uniquely named. The ventral surface is called the plastron. These are joined by an area called the bridge. The actual suture between the bridge and the plastron is called the anterior bridge strut. In pleurodires, the posterior pelvis is also part of the carapace, fully fused with it. This is not the case in Cryptodires, which have a floating pelvis. The anterior bridge strut and posterior bridge strut are part of the plastron. On the carapace are the sutures into which they insert, known as the bridge carapace suture. In the shell, there is a turtle's epidermis layer. This layer is important to the strength of the shell surrounding it. In an international study, the layer can be as thick as two to four cells. Even with such a small thickness, the epidermis allows for the deformation the shell can experience and provides the shell more support. The epidermis layer is apparent in both sections of the shell—carapace and plastron—and is thicker in critical areas. A thicker epidermis allows a higher stress force to be experienced without permanent deformation or critical failure of the shell. The shape of the shell is from its evolutionary process, which caused many microstructures to appear to aid survival and motion. The shell shape allows the animal to escape predatory situations. Microstructures can include the scutes mentioned prior or the ribs found internally of the shell. Many ribs can be found within and throughout the shell. The rib structures provide extra structural support but allows the shell to deform elastically depending on the situation the turtle is in (i.e., predatory escape). Nonstructural mechanisms have also been in the turtle shell that aids the turtle during locomotion. A mucus film covers parts of the shell, allowing some physical protection and also reducing friction and drag. The bones of the shell are named for standard vertebrate elements. As such, the carapace is made up of eight pleurals on each side. These are a combination of the ribs and fused dermal bone. Outside of this, at the anterior of the shell, is the single nuchal bone, a series of twelve paired periphals then extend along each side. At the posterior of the shell is the pygal bone, and in front of this, nested behind the eighth pleurals, is the suprapygal.
Between each of the pleurals are a series of neural bones, which although always present are not always visible, in many species of Pleurodire they are submerged below the pleurals. Beneath the neural bone is the neural arch which forms the upper half of the encasement for the spinal cord. Below this the rest of the vertebral column. Some species of turtles have some extra bones called mesoplastra, which are located between the carapace and plastron in the bridge area. They are present in most Pelomedusid turtles. The skeletal elements of the plastron are also largely in pairs. Anteriorly there are two epiplastra, with the hyoplastra behind them. These enclose the singular entoplastron. These make up the front half of the plastron and the hyoplastron contains the anterior bridge strut. The posterior half is made up of two hypoplastra (containing the posterior bridge strut) and the rear is a pair of xiphiplastra. Overlying the boney elements are a series of scutes, which are made of keratin and are very similar to horn or nail tissue. In the center of the carapace are five vertebral scutes, and out from these are four pairs of costal scutes. Around the edge of the shell are 12 pairs of marginal scutes. All these scutes are aligned so that for the most part the sutures between the bones are in the middle of the scutes above. At the anterior of the shell there may be a cervical scute (sometimes incorrectly called a nuchal scute); however, the presence or absence of this scute is highly variable, even within species.
On the plastron there are two gular scutes at the front, followed by a pair of pectorals, then abdominals, femorals, and lastly anals. A particular variation is that the Pleurodiran turtles have an intergular scute between the gulars at the front, giving them a total of 13 plastral scutes, compared to the 12 in all Cryptodiran turtles.
Carapace
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