Turtle shell

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. 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. 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. 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. 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 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. 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 The plastral formula is used to compare the sizes of the individual plastral scutes (measured along the midseam). The following plastral scutes are often distinguished (with their abbreviation): : Comparison of the plastral formulas provides distinction between the two species. For example, for the eastern box turtle, the plastral formula is: . Turtle plastrons were used by the ancient Chinese in a type of divination called plastromancy. == Scutes ==

The turtle's shell is covered in scutes that are made of keratin. The individual scutes (as shown above) have specific names and are generally consistent across the various species of turtles. Terrestrial tortoises do not shed their scutes. New scutes grow by the addition of keratin layers to the base of each scute. Aquatic chelonii shed individual scutes. The scute effectively forms the skin over the underlying bony structures; there is a very thin layer of subcutaneous tissue between the scute and the skeleton. The scutes can be brightly colored in some species, and turtle shells often follow Thayer's law with carapace usually being a darker patterning than the plastron, though there are exceptions. Moustakas-Verho and Cherepanov's embryological study reveals that the patterning of the plastral scutes appear independent from the patterning of carapacial scutes, suggesting that the carapace and plastron evolved separately. The appearance of scutes correlates to the transition from aquatic to terrestrial mode of life in tetrapods during the Carboniferous period (340 Ma). In the evolution from amphibians to terrestrial amniotes, transition in a wide variety of skin structures occurred. Ancestors of turtles likely diverged from amphibians to develop a horny cover in their early terrestrial ancestral forms. == Ontogeny ==

of the shell: seen in the egg at stage 16/17, the carapace is developing. In section, the ribs are growing sideways not downwards, into the carapacial ridge, seen here as a bud, to support the carapace. It causes axial arrest which causes the ribs to be dorsalized, the shoulder girdle to be rearranged and encapsulated in the rib cage, and the carapace to develop. Odontochelys semitestacea presents evidence of axial arrest that is observed in embryos but lacks fan-shaped ribs and a carapace. This suggests that the primitive carapacial ridge functioned differently and must have gained the function of mediating the ribs and carapace development later. During the development of the turtle embryo, the ribs grow sideways into the carapacial ridge, unique to turtles, entering the dermis of the back to support the carapace. The development is signalled locally by fibroblast growth factors including FGF10. == Evolutionary origin ==

Bony dermal plates theory: the "Polka Dot Ancestor" Zoologists have sought to explain the evolutionary origin of the turtles, and in particular of their unique carapace. In 1914, J. Versluys proposed that bony plates in the dermis, osteoderms, fused first to each other and then to the ribs beneath them. The theory persisted into the 21st century, when Olivier Rieppel proposed a hypothetical turtle precursor, its back covered by bony armour plates in the dermis, which he called the "Polka Dot Ancestor". Michael Lee proposed that the transformation of the carapace began with an unarmoured parareptile and then an armoured pareiasaur, and ended with modern turtles with a fully developed carapace and a relocated rib cage. The theory accounted for the evolution of fossil pareisaurs from Bradysaurus to Anthodon, but not for how the ribs could have become attached to the bony dermal plates. Broadened ribs theory origin for the turtles. During the Permian, the broadened ribs may have provided great stability in burrowing, giving a body shape resembling the extant fossorial gopher tortoise, with strong shoulders and forelimbs, and increased muscle attachment structures such as their tubercle on the posterior coracoid and their large and wide terminal phalanges creating shovel-like "hands". Fossoriality may have helped Eunotosaurus survive the global mass extinction at the end of the Permian period, and could have played an essential role in the early evolution of shelled turtles. Triassic: evolution of complete shell A stem-turtle from the Middle Triassic of Germany, some 240 million years ago, Pappochelys, has more distinctly broadened ribs, T-shaped in cross-section. A Late Triassic stem-turtle from Guizhou, China, Eorhynchochelys, is a much larger animal, up to long, with a long tail and broadened but not overlapping ribs. Like the earlier fossils, it has small teeth. The fossil showed that the plastron evolved before the carapace. Like crown turtles, it lacked intercostal muscles, so rib mobility was limited. The ribs were laterally expanded and broadened without ossification, like the embryos of modern turtles. It lacked the ability to pull its head into its shell, and had a long neck and a long, spiked tail ending in a club, somewhat like an ankylosaur. ==Diseases==

Shell rot Septicemic cutaneous ulcerative disease (SCUD) or "shell rot" causes ulceration of the shell. Pyramiding Pyramiding is a shell deformity of captive tortoises in which the shell grows unevenly, resulting in a pyramid shape underlying each scute. Factors which may contribute to pyramiding include inadequate water supply; the consumption of excessive animal or vegetable protein; inadequate calcium, UVB, and/or vitamin D3; and poor nutrition. Tortoise breeder Richard Fife documents that his wife raised two groups of red-foot tortoise hatchlings, with identical diets over a number of months, but with different environmental moisture. The group raised in low humidity showed pyramiding, whereas the group raised in high humidity did not and had shells identical to wild tortoises. They found the same results with several other species. File:Hermannswithshellrot.jpeg|Plastron of wild male Hermann's tortoise with ongoing shell rot (circled in red) and scars from previous shell rot (circled in black) File:Gopherus agassizii - Buffalo Zoo.jpg|A gopher tortoise with severe pyramiding == See also ==