Turtle shell
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, but is normally referred to as tortoiseshell.
A thicker epidermis allows a higher stress force to be experienced without permanent deformation or critical failure of the shell.
A mucus film covers parts of the shell, allowing some physical protection and also reducing friction and drag.
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.
[3] Some species of turtles have some extra bones called mesoplastra, which are located between the carapace and plastron in the bridge area.
[3][4] Overlying the boney elements are a series of scutes, which are made of keratin and are a lot like horn or nail tissue.
On the plastron there are two gular scutes at the front, followed by a pair of pectorals, then abdominals, femorals and lastly anals.
[3][11]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.
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 of them.
In most turtles the shell is relatively uniform in structure, species variation in general shape and color being the main differences.
[16] However, analysis of the transitional fossil, Eunotosaurus africanus shows that early ancestors of turtles lost that intercostal muscle usually found between the ribs.
[20] 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.
[22] The discovery of an ancestral turtle fossil, Pappochelys rosinae, provides additional clues as to how the plastron formed.
For example, for the eastern box turtle, the plastral formula is: an > abd > gul > pect > hum >< fem.
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,[25] though there are exceptions.
[27] The appearance of scutes correlates to the transition from aquatic to terrestrial mode of life in tetrapods during the Carboniferous period (340 Ma).
[28] 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.
[36] Odontochelys semitestacea presents evidence of axial arrest that is observed in embryos but lacks fan-shaped ribs and a carapace.
Genetic observations of Pax1 and Shh further provide an understanding in key gene expression that could potentially be responsible for changing turtle morphology.
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".
[41] 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.
[39] Recent stem-turtle fossil discoveries provide a "comprehensive scenario" of the evolution of the turtle's shell.
A fossil that may be a stem-turtle from the Permian of South Africa, Eunotosaurus, some 260 million years ago, had a short broad trunk, and a body-case of broadened and somewhat overlapping ribs, suggesting an early stage in the acquisition of a shell.
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.
[44][45] 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.
[39] Also in the Late Triassic, some 220 million years ago, the freshwater Odontochelys semitestacea of Guangling in southwest China has a partial shell, consisting of a complete bony plastron and an incomplete carapace.
