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ecies of turtle (and fourth-largest reptile) is the leatherback turtle which can reach over 2.7 m (8 ft 10 in) in length and weigh over 500 kg (1,100 lb). The largest known turtle was Archelon ischyros, a Late Cretaceous sea turtle up to 4.5 m (15 ft) long, 5.25 m (17 ft) wide between the tips of the front flippers, and estimated to have weighed over 2,200 kg (4,900 lb). The smallest living turtle is Chersobius signatus of South Africa, measuring no more than 10 cm (3.9 in) in length and weighing 172 g (6.1 oz). Shell Main article: Turtle shell Photograph of one half of a tortoise skeleton, cut in half vertically showing the vertebrae following curving along the carapace Sagittal section of a tortoise skeleton The shell of a turtle is unique among vertebrates and serves to protect the animal and provide shelter from the elements. It is primarily made of bone, and consists of two parts: the carapace which usually contains 50–60 bones a nd covers the back of the animal while the plastron has only 7–11 bones and covers the belly. They are connected by lateral extensions of the plastron. The carapace is fused with the vertebrae and ribs while the plastron is formed from bones of the shoulder girdle, sternum, and gastralia (abdominal ribs). During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering the dermis (inner skin) of the back to support the carapace. The development is signaled locally by proteins known as fibroblast growth factors which include FGF10. The shoulder girdle in turtles is made up of two bones, the scapula and the coracoid. Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development. drawing of a section through a turtle embryo showing formation of the shell, the ribs are growing sideways Development of the shell. The ribs are gr owing sideways into the carapacial ridge, seen here as a bud, to support the carapace. The outer surface of the shell is covered in epidermal (outer skin) scales know