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.Romer, A.S. (1956) Osteology of the Reptiles. Univ. of Chicago Press.Zangerl, R. 1969. The turtle shell. In: Gans, C., Bellairs, D. d'A. and Parsons, T. A. (Eds). Biology of the Reptilia, Vol 1, Morphology A. London: Academic Press. pp. 311–340 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.Solomon, S. E., J. R. Hendrickson, and L. P. Hendrickson. "The structure of the carapace and plastron of juvenile turtles, Chelonia mydas (the green turtle) and Caretta caretta (the loggerhead turtle)." Journal of anatomy 145 (1986): 123.
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).Wei Zhang, Chengwei Wu, Chenzhao Zhang, Zhen Chen, “Microstructure and mechanical property of turtle shell.” Theoretical and Applied Mechanics Letters, Volume 2, Issue 1, (2012): 014009, ISSN 2095-0349. 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.Pritchard, P.C.H., and P. Trebbau. 1984. The Turtles of Venezuela. SSAR Contributions to Herpetology 2:.
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
The carapace is the dorsal (back), convex part of the shell structure of a turtle, consisting of the animal's ossified ribs fused with the dermal bone. The spine and expanded ribs are fused through ossification to dermal plates beneath the skin to form a hard shell. Exterior to the skin, the shell is covered by scutes, which are horny plates made of keratin that protect the shell from scrapes and bruises. A keel, a ridge that runs from front to the back of the animal, is present in some species. These may be single, paired, or even three rows. In most turtles, the shell is relatively uniform in structure, species variation in general shape and color being the main differences. However, the soft shell turtles, pig-nose turtles, and the leatherback sea turtle have lost the scutes and reduced the ossification of the shell. This leaves the shell covered only by skin. These are all highly aquatic forms.
The evolution of the turtle's shell is unique because of how the carapace represents transformed vertebrae and ribs. While other tetrapods have their scapula, or shoulder blades, found outside of the ribcage, the scapula for turtles is found inside the ribcage.Wang, Z., J. Pascual-Anaya, A. Zadissa, W. Q. Li, Y. Niimura, Z. Y. Huang, C. Y. Li et al. 2013. The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan " Nature Genetics 45:701-+. The shells of other tetrapods, such as armadillos, are not linked directly to the vertebral column or rib cage, allowing the ribs to move freely with the surrounding intercostal muscle.Hirasawa, T., H. Nagashima, and S. Kuratani. 2013. The endoskeletal origin of the turtle carapace . Nature Communications 4. However, analysis of the transitional fossil Eunotosaurus africanus shows that early ancestors of turtles lost that intercostal muscle usually found between the ribs.
Plastron
The plastron (plural: plastrons or plastra) is the nearly flat part of the shell structure of a turtle, what one would call the belly or ventral surface of the shell. It also includes within its structure the anterior and posterior bridge struts and the bridge of the shell. The plastron is made up of nine bones, and the two epiplastra at the anterior border of the plastron are homologous to the clavicles of other tetrapods. The rest of the plastral bones are homologous to the gastralia of other tetrapods. The plastron has been described as an exoskeleton, like osteoderms of other reptilians; but unlike osteoderms, the plastron also possesses osteoblasts, the osteoid, and the periosteum.
The evolution of the plastron has remained more mysterious, though Georges Cuvier, a French naturalist and zoologist in the 19th century, wrote that the plastron developed primarily from the sternum of the turtle. This fits well with the knowledge obtained through embryological studies, showing that changes in the pathways of rib development often result in malformation or loss of the plastron. This phenomenon occurs in turtle development, but instead of experiencing complete loss of the sternum, the turtle body plan repurposes the bone into the form of the plastron. However, other analyses find that the endochondral sternum is absent and replaced by the exoskeletal plastron. The ventral ribs are effectively not present, replaced by the plastron, unless the gastralia from which the plastron evolved were once floating ventral ribs. During turtle evolution, there was probably a division of labor between the ribs, which specialized to stabilize the trunk, and the abdominal muscles, which specialized for respiration; these changes took place 50 million years before the shell was fully ossified.
The discovery of an ancestral turtle fossil, Pappochelys rosinae, provides additional clues as to how the plastron formed. Pappochelys serves as an intermediate form between two early stem-turtles, E. africanus and Odontochelys, the latter of which possesses a fully formed plastron. In place of a modern plastron, Pappochelys has paired gastralia, like those found in E. africanus. Pappochelys is different from its ancestor because the gastralia show signs of having once been fused, as indicated by the fossil specimens which show forked ends. This evidence shows a gradual change from paired gastralia, to paired and fused gastralia, and finally to the modern plastron across these three specimens.
In certain families there is a hinge between the pectoral and abdominal scutes allowing the turtle to almost completely enclose itself. In certain species the sex of a testudine can be told by whether the plastron is concave (male) or convex (female). This is because of the mating position; the male's concave plastron allows it to more easily mount the female during copulation.
The plastral scutes join along a central seam down the middle of the plastron. The relative lengths of the seam segments can be used to help identify a species of turtle. There are six laterally symmetric pairs of scutes on the plastron: gular, humeral, pectoral, abdominal, femoral, and anal (going from the head to the tail down the seam); the abdominal and gular scute seams are approximately the same length, and the femoral and pectoral seams are approximately the same length.
The gular scute or gular projection on a turtle is the most anterior part of the plastron, the underside of the shell. Some tortoises have paired gular scutes, while others have a single undivided gular scute. The gular scutes may be referred to as a gular projection if they stick out like a trowel.
Plastral formula