Turtle Skeleton: Labeled Diagrams and Interactive 3D Model

A turtle skeleton is a normal reptile skeleton turned inside out around a bony shell. The shell is not a separate case: its top, the carapace, is built from the turtle's own ribs and trunk vertebrae fused with plates of bone that form in the skin, and its bottom, the plastron, is joined to it at the sides by a bony bridge. The shoulder and hip girdles sit inside the rib cage, which no other animal does. In front of the shell are a toothless skull with a horny beak and eight neck vertebrae; behind it are the sacrum and a short tail.

This page uses the red-eared slider as the reference turtle. It is a hidden-necked pond turtle (suborder Cryptodira, family Emydidae) with a low, oval carapace that reaches about 18 cm in males and 25 cm in females, webbed feet and a flat plastron. Its skeleton is typical of hard-shelled turtles, so the same bones and names apply to painted turtles, box turtles and sea turtles; the sections further down list where those differ. (Animal Diversity Web: Trachemys scripta (pond slider).)

Directions used here: anterior is towards the head, posterior towards the tail, dorsal towards the carapace and ventral towards the plastron. "Left" and "right" are the turtle's own.

The labeled turtle skeleton below shows the slider from the left with the side of the shell cut away, so you can see the bones that are normally hidden: the backbone and ribs running along the inside of the carapace, and the shoulder and hip girdles tucked under them.

The bones a lab practical usually asks you to find on a turtle skeleton are:

• Nuchal bone: The single, broad bone at the front midline of the carapace, above the neck.
• Neural bones: Usually eight small, hexagonal plates down the midline of the carapace, fused to the vertebrae below.
• Pleural bones: Eight pairs of broad plates on the sides of the carapace, each a rib fused with a sheet of dermal bone.
• Peripheral bones: A ring of narrow bones (usually 11 on each side) that forms the rim of the carapace.
• Pygal bone: The single bone at the rear midline of the carapace rim, above the tail.
• Cervical vertebrae: Eight neck vertebrae, from the atlas behind the skull to the eighth, which meets the shell.
• Trunk vertebrae: Ten trunk vertebrae fused to the underside of the carapace midline.
• Ribs: Broad ribs that run outward under the carapace and are fused into the pleural bones.
• Scapula: A rod-like shoulder blade that rises from the shoulder joint to the underside of the carapace.
• Acromion process: A long process that runs from the scapula inward along the front of the shell.
• Coracoid: A flat, backward-pointing bone that completes the shoulder girdle.
• Ilium: The hip bone that rises from the hip socket to the sacral vertebrae.
• Pubis: The front, lower hip bone, with a forward-pointing process.
• Ischium: The rear, lower hip bone.
• Humerus: The curved upper-arm bone, whose head sits in the shoulder socket inside the shell.
• Femur: The thigh bone, from the hip socket inside the shell to the knee.

Yes. A turtle's shell is part of its skeleton, and most of it is bone. The carapace contains the turtle's ten trunk vertebrae and their ribs. Each rib is fused into a broad pleural (costal) plate, and the arch on top of each vertebra is fused to a neural plate on the midline. The spinal cord runs in the narrow space between the neural bones and the vertebral bodies, right under the top of the shell. Only the outer layer, the scutes, is skin: thin plates of keratin, the protein in your fingernails. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

The plastron is part of the skeleton too, although it comes from different bones. The front pair, the epiplastra, are thought to be the clavicles (collar bones), and the single entoplastron is thought to be the interclavicle. The other plastron bones are thought to come from gastralia, the belly ribs that many reptiles have in the skin of the abdomen. (Wikipedia: turtle shell (bone and scute nomenclature).)

So the common question "what type of skeleton does a turtle have?" has a simple answer: a turtle has an internal bony skeleton (an endoskeleton) like every other vertebrate. It is not an exoskeleton like a crab's, even though the shell is on the outside, because it grows from the turtle's own ribs, spine and dermal bone and is covered by living skin.

A turtle shell has two layers in two parts. The two parts are the domed carapace on top and the flatter plastron underneath, joined at the sides by the bridge. The two layers are bone on the inside and keratin scutes on the outside. The pattern of bones and the pattern of scutes are different, and their seams are offset, like the joints in a brick wall.

Carapace bones. On the midline, from front to back, are the broad nuchal over the neck, usually eight neurals, one or two suprapygals and the pygal over the tail. On each side are eight pleurals, and around the rim is a ring of peripherals, usually eleven on each side. Extra or missing neurals are fairly common, and many side-necked turtles have neurals that are reduced or hidden under the pleurals. (Wikipedia: turtle shell (bone and scute nomenclature).)

Carapace scutes. A slider has a small cervical (nuchal) scute at the front, five vertebral scutes down the middle, four pairs of pleural (costal) scutes on the sides and twelve pairs of marginal scutes around the edge, the last pair of which are called the supracaudals. Sea turtle guides call the pleurals "lateral scutes" and also count inframarginal scutes along the bridge. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

Plastron. The plastron has nine bones: paired epiplastra, the unpaired entoplastron, and paired hyoplastra, hypoplastra and xiphiplastra. The hyo- and hypoplastra extend up the sides as the bridge. On top of them lie twelve scutes in six pairs, named from front to back gular, humeral, pectoral, abdominal, femoral and anal. Side-necked turtles have a thirteenth, the intergular, between the gulars. (Wikipedia: Pleurodira (side-necked turtles).)

No. A turtle cannot leave its shell or live without it, because the shell contains its backbone and rib cage. Removing the shell would mean removing the spine. Cartoons of turtles climbing out of their shells, and searches for a "turtle skeleton without shell", imagine an animal that does not exist: there is no naked turtle body inside.

What a turtle can lose are its scutes. Aquatic turtles shed the outer layer of each scute from time to time, and a shell injury can strip scutes and expose bone. A cracked shell is a broken bone and needs a vet. If you look at a turtle skeleton in a museum with the plastron removed, you are seeing a preparation in which the bridge has been cut, not a turtle that ever lived that way.

The softshells and the leatherback sea turtle come closest to "shell-less". They have lost their scutes and much of the bone around the edge of the carapace, and are covered by leathery skin instead. Even so, their spine and ribs still form the core of the shell. (Wikipedia: Trionychidae (softshell turtles).)

The axial skeleton of a turtle is the skull, the vertebral column and the ribs. The column has four regions. All turtles have eight cervical (neck) vertebrae. Behind them come ten trunk (dorsal, or thoracic) vertebrae, fused to the carapace, then two or three sacral vertebrae whose side processes support the hips, and a series of caudal vertebrae, 12 or more in sea turtles. Each trunk vertebra carries a pair of ribs, and each rib head sits at the junction of two vertebral bodies. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

Because the trunk vertebrae are fused into the shell, a turtle's back cannot bend at all. All its flexibility is in the neck and tail. In sea turtles the eighth neck vertebra is also fused to the carapace, so only seven move; in pond turtles the last neck vertebra meets the first trunk vertebra under the nuchal bone. The first and tenth ribs are small and only partly join the shell.

Turtle ribs are unusual in the direction they grow. In other reptiles the ribs curve down around the body cavity. In a turtle embryo the ribs stop short and grow outward and forward instead, inside a fold of the body wall called the carapacial ridge, which becomes the edge of the shell. Bone then spreads from each rib to make the pleural plates. (The Scientist: embryonic twist yields turtle shell.)

A turtle's shoulder blades and hip bones are inside its rib cage, under the carapace. This is the most famous oddity of the turtle skeleton. In you, the shoulder blade lies on the outside of the ribs; in a turtle, the ribs have grown out over it, so the whole girdle hangs inside the shell. (National Geographic on Nagashima et al. (2009, Science): how the turtle got its shell.)

The shoulder girdle of each side has three prongs (it is triradiate). The rod-like scapula runs up from the shoulder joint and attaches to the carapace near the first trunk vertebra. The acromion process runs inward from the scapula and is tied by ligaments to the entoplastron. The flat coracoid points back over the plastron. The shoulder socket (glenoid) is formed by the scapula and coracoid together. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

The pelvis has the usual three bones on each side: the ilium rises to the sacral vertebrae and carapace, and the pubis and ischium form a floor over the plastron, with a large opening, the thyroid fenestra, between them. All three meet at the hip socket (acetabulum). In hidden-necked turtles like the slider the pelvis is tied to the shell only by ligaments; in side-necked turtles it is sutured firmly to both carapace and plastron. (Wikipedia: turtle (anatomy and evolution).)

A turtle skull is solid and box-like, with large eye sockets, no teeth and a sharp keratin beak. The beak is called the rhamphotheca: an upper sheath over the premaxillae and maxillae and a lower sheath over the dentaries. It grows continuously, like a fingernail, and its shape matches the diet. In sliders it has a sharp cutting edge; green sea turtles have finely serrated jaws for cropping seagrass; loggerheads have broad, powerful crushing jaws for hard-shelled prey. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

The skull roof is made of paired prefrontals, frontals, parietals, postorbitals and squamosals. The cheek has the jugal and quadratojugal, and at the back the quadrate carries the jaw joint and supports the eardrum. A blade-like supraoccipital crest sticks out backward from the braincase, anchoring the jaw muscles. The lower jaw is a composite of the dentary, angular, surangular, prearticular, splenial and articular bones. Under the jaws hangs the hyoid apparatus, which supports the tongue and glottis but is usually lost from museum skeletons. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

A living turtle's skull has no true temporal openings (fenestrae) behind the eyes. For more than a century this 'anapsid' skull was read as a sign that turtles were a very ancient branch of reptiles that split off before the diapsids, the group with two openings on each side that includes lizards, snakes, crocodiles and birds. The skull is still often taught that way. (Bever et al. (2015), Evolutionary origin of the turtle skull, Nature.)

Genomic studies contradict that view: they place turtles inside the diapsid radiation, usually closest to the archosaurs (crocodilians and birds). If that is right, the closed skull of turtles is a secondary condition, built by closing over openings that their ancestors had. The fossils that tested this were found in 2015. Bever and colleagues used CT scans to show that Eunotosaurus, a 260-million-year-old stem turtle from the Karoo Basin of South Africa, had a lower temporal opening and a small upper one that was being covered over by an expanded supratemporal bone. They described it as 'a diapsid reptile in the process of becoming secondarily anapsid'. (Bever et al. (2015), Evolutionary origin of the turtle skull, Nature.)

The same year, Pappochelys, a 240-million-year-old stem turtle from Germany, was described with two pairs of temporal openings, a full diapsid skull. Together the two fossils support the idea that turtles began as diapsids and lost the openings later. The debate is not completely closed, because some researchers still question whether Eunotosaurus belongs on the turtle line, but the diapsid-origin view now has both genetic and fossil support. (Wikipedia: Pappochelys (Schoch & Sues 2015, Nature 523:584).)

Do not confuse true fenestrae with emarginations. Many modern turtles, including the red-eared slider, have a deep notch cut into the back of the skull roof (temporal emargination) and often another from below the cheek. These notches give the jaw-closing muscles room to bulge, but they are open edges, not holes ringed by bone. Their jaw muscles also run over a bony pulley: the otic capsule in hidden-necked turtles and the pterygoid in side-necked turtles. (Wikipedia: turtle (anatomy and evolution).)

Turtles pull their heads in in one of two ways, and this splits living turtles into two suborders. Hidden-necked turtles (Cryptodira), including the slider, snapping turtles, box turtles, tortoises, softshells and sea turtles, bend the neck into a vertical S-shape and draw the head straight back. Double joints between the sixth, seventh and eighth neck vertebrae let the neck fold in that one plane. (Wikipedia: Cryptodira (hidden-necked turtles).)

Side-necked turtles (Pleurodira), such as the mata mata and the African helmeted turtles, fold the neck sideways and tuck the head under the front edge of the carapace beside a front leg. Today they live only in fresh water in South America, Africa, Madagascar and Australasia. (Wikipedia: Pleurodira (side-necked turtles).)

Sea turtles are cryptodires that have mostly lost the ability to retract the head, and snapping turtles cannot hide fully because their plastron is too small. (Wikipedia: turtle (anatomy and evolution).)

Turtle legs have the standard four-limbed vertebrate bones, but the upper bones start inside the shell. The humerus runs from the shoulder joint under the carapace out through the front opening to the elbow; the forearm has a radius and ulna; the wrist has several carpals (radiale, intermedium, ulnare, centrale, pisiform and distal carpals); and each hand has five metacarpals and their phalanges. The hind leg has a femur, a tibia and fibula, a short ankle (calcaneum, astragalus and distal tarsals), five metatarsals and the toe phalanges. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

The elbow and knee bend so that the lower leg can swing inward when the limb is pulled into the shell. In a slider the feet are broad and webbed and carry long claws; males have very long front claws used in courtship. Tortoises have short, stumpy, unwebbed feet, and sea turtles have flippers. (Animal Diversity Web: Trachemys scripta (pond slider).)

Turtles breathe with their abdominal (flank) muscles, not their ribs. In lizards, snakes and mammals the ribs swing outwards to enlarge the chest and suck air in. A turtle cannot do this: its ribs are locked into the carapace, and the small intercostal muscles that move ribs in other reptiles are lost while the embryo develops. Instead, two pairs of antagonistic muscles do the work. (Lyson et al. (2014), Origin of the unique ventilatory apparatus of turtles, Nature Communications 5:5211.)

To breathe out, the transversus abdominis contracts. It is a sling-shaped muscle that runs from the front and back of the shell around the belly organs, so when it tightens it presses the viscera up against the lungs, which lie on top of everything under the carapace. To breathe in, the obliquus abdominis contracts. At rest it bulges like a dome in the soft skin between the hind leg and the bridge; when it flattens it pulls the gut and its lining back and down, the pressure in the lungs falls and air flows in. You can see this as a rhythmic pumping of the skin in front of the hind legs of a resting turtle. (Lyson et al. (2014), Origin of the unique ventilatory apparatus of turtles, Nature Communications 5:5211.)

The lungs are large, spongy and elastic, and they are attached directly to the underside of the carapace and the vertebral column. Because they sit dorsally, above the liver, stomach and intestines, they also help an aquatic turtle float level in the water. Movements of the limbs and neck in and out of the shell change the space inside the body cavity, so they add to breathing, especially when the turtle is active. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

Some aquatic turtles have an extra trick. They can take up dissolved oxygen across the lining of the cloaca and its sacs, which helps during long dives or when they spend the winter under ice. This cloacal respiration supplements the lungs; it does not replace them. (Wikipedia: turtle (anatomy and evolution).)

The fossil record suggests the abdominal breathing system evolved before the shell. The 260-million-year-old stem turtle Eunotosaurus already had a rigid, broad-ribbed trunk, no sign of intercostal muscles and bony attachment marks for the main breathing-out muscle. Lyson and colleagues concluded that the ribs and the abdominal muscles divided their jobs, the broad ribs stiffening the trunk and the muscles taking over breathing, about 50 million years before a fully bony shell appeared. (Lyson et al. (2014), Origin of the unique ventilatory apparatus of turtles, Nature Communications 5:5211.)

Under the carapace, the organs are stacked in layers. The two big lungs lie on top, pressed against the underside of the carapace and the backbone. Below them, the large, dark liver spreads across the body cavity just above the plastron, with the three-chambered heart on the midline between its lobes, close behind the shoulder girdles. The stomach curves along the left side, and the coiled intestines fill the back half, leading to the cloaca at the base of the tail. The kidneys lie behind the lungs against the carapace, and the gonads sit near the base of the lungs. Unlike snakes, turtles have a urinary bladder on the midline in front of the pelvis; urine reaches it from the cloaca, not directly from the ureters. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

In the 3D model, hide the "Carapace bones" system to see the lungs and ribs from above, or hide "Plastron & bridge" to see the heart, liver and gut from below, which is how turtles are opened in a veterinary or museum dissection.

• Lungs: Two large, spongy lungs pressed against the underside of the carapace.
• Trachea & bronchi: The windpipe running down the neck, splitting into two unbranched bronchi that enter the lungs.
• Heart: A three-chambered heart (two atria, one ventricle) on the midline, just above the plastron.
• Liver: A large, dark, two-lobed liver lying across the body cavity just above the plastron.
• Stomach: A J-shaped muscular stomach on the turtle's left side.
• Intestines: The coiled small intestine and the shorter, wider large intestine (colon).
• Kidneys and ureters: Two flattened, lobed kidneys pressed against the inside of the carapace at the back of the body cavity, each drained by a ureter.
• Urinary bladder: A thin-walled sac that stores urine; turtles have one, unlike snakes.
• Cloaca: The shared chamber at the end of the gut, urinary and reproductive tracts.

A sea turtle skeleton has the same bones as a pond turtle's, reshaped for swimming. The shell is flatter and more streamlined, the head cannot be pulled in, and the front legs are long, stiff flippers. The flipper blade is formed by widening and flattening the wrist bones and by greatly lengthening the metacarpals and phalanges. The humerus is short and flat with a large medial process for the flight-like swimming muscles, and in adults the radius and ulna are bound together by fibrous tissue. Most hard-shelled species have two claws on each flipper. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

In young sea turtles the carapace is still open: the ribs reach out towards the peripherals with gaps, called fontanelles, between them. These close with bone in some adult ridleys and loggerheads but stay open towards the back in green turtles and hawksbills. The shape of the entoplastron also differs between species: arrow-shaped in green turtles, cross-shaped in loggerheads and T-shaped in hawksbills. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

The leatherback is the exception. It has no scutes and no hard shell. Its carapace is smooth, black, rubbery skin with seven ridges running its length, lying over a mosaic of small bony plates (dermal ossicles), a layer of oily connective tissue and fat, and the ribs and vertebrae. Its bony shell is reduced to an expanded nuchal, the ribs and the vertebrae, its plastron is only a ring of slender bones with no entoplastron, and the bones of its long flippers keep thick cartilage at their ends. Adults are typically 1.5 to 1.8 m long and weigh 340 to 450 kg, and they dive deeper than any other turtle. (NOAA Fisheries: leatherback turtle.)

Sizes from NOAA Fisheries species pages, converted from feet and pounds and rounded. (NOAA Fisheries: green turtle.) (NOAA Fisheries: loggerhead turtle.)

A snapping turtle skeleton is built like a slider's but with a big head, a long neck and tail. The common snapping turtle (Chelydra serpentina) has a very small plastron that leaves most of its limbs exposed, so it cannot close up inside its shell; it relies on its powerful beak-like jaws and a neck that can reach back across the carapace to bite. Its tail is nearly as long as its shell and has saw-toothed keels along the top. (Animal Diversity Web: Chelydra serpentina (common snapping turtle).)

The alligator snapping turtle (Macrochelys temminckii) is much larger, commonly with a carapace of 35 to 80 cm and a weight of 8 to 80 kg. Its carapace carries three ridges of large, pointed scutes that flatten with age. Inside its mouth a worm-shaped pink appendage on the tongue lures fish within reach of its jaws. (Wikipedia: alligator snapping turtle.)

A box turtle can shut itself in completely because its plastron has a hinge. In the eastern box turtle (Terrapene carolina) a well-developed hinge runs across the plastron along the seam between the pectoral and abdominal scutes, dividing it into two movable lobes. The front lobe closes over the head and forelegs, the rear lobe over the hind legs and tail. (Illinois Natural History Survey: Terrapene carolina.)

To make this work the box turtle's plastron is "bridgeless": it has no rigid bony bridge locking it to the carapace, and the high, domed carapace leaves room for the folded limbs. Males have a concave rear plastral lobe and, usually, red eyes; females have a flat lobe and brown eyes. Like tortoises, box turtles keep their old scute layers, so growth rings build up on the shell. (Animal Diversity Web: Terrapene carolina (eastern box turtle).)

Softshell turtles (family Trionychidae) have a light, flexible shell with no horny scutes. A solid layer of bone lies under the middle of the carapace, but the edges have no bone and are soft, leathery and flexible. Some species have plastral bones that are not attached to the bones of the carapace. They also have a long, soft, snorkel-like snout. Despite the name, their shell is still built on their ribs and spine. (Wikipedia: Trionychidae (softshell turtles).)

The mata mata (Chelus fimbriatus) of South America is a side-necked turtle with a flat, triangular head, a long fringed neck, a snorkel-tipped snout and a rough, knobby carapace up to about 45 cm long. It cannot chew. It lies still on the bottom, then thrusts its head out and opens its mouth wide to suck in fish. (Wikipedia: mata mata.)

A tortoise skeleton and a turtle skeleton have the same plan, because tortoises are turtles: the family Testudinidae is one branch of the hidden-necked turtles. Both have a shell of fused ribs and vertebrae, girdles inside the rib cage, eight neck vertebrae, a toothless beak and ten trunk vertebrae. The differences are in shape. In everyday North American English, "turtle" covers the whole order and "tortoise" means a land-living testudinid. (Wikipedia: turtle (anatomy and evolution).)

Gopher tortoise details from Animal Diversity Web; males of that species also have longer gular projections on the front of the plastron. (Animal Diversity Web: Gopherus polyphemus (gopher tortoise).) (Wikipedia: tortoise.)

A turtle's shell grows with the turtle, from the bones and the scutes at the same time. The bones grow at their sutures, the seams where neighbouring plates meet. In a hatchling the carapace is mostly ribs and vertebrae; sheets of dermal bone then spread out along each rib until the pleural plates meet each other and the peripherals. Spaces left between rib tips and the rim are called fontanelles, and in some species they stay open for life. (Wyneken (2001), The Anatomy of Sea Turtles, NOAA Tech. Memo. NMFS-SEFSC-470.)

The keratin scutes grow in two ways: they widen at their margins and they thicken. Each growth season adds a new layer of keratin at the base of every scute. Terrestrial tortoises and box turtles keep all the layers, so their scutes build up raised rings (annuli). Aquatic turtles such as sliders shed the outer layer of individual scutes as thin, clear flakes while the new layer underneath takes over. (Wikipedia: turtle shell (bone and scute nomenclature).)

Counting annuli is only a rough guide to age. Growth depends on food and water, a tortoise can add more than one ring in a season, and old rings wear smooth. In aquatic turtles that shed, the rings are largely lost. (Wikipedia: tortoise.)

Normal shedding comes off in whole, thin, translucent pieces shaped like the scute, with healthy hard scute underneath. Soft spots, pits, smells, or bone showing through are not normal shedding and need a reptile vet. Because the shell is living bone with nerves and blood vessels just under the keratin, a turtle can feel touch and pain through it.

The turtle shell evolved in steps over tens of millions of years, and the ribs came first. Eunotosaurus, about 260 million years old, had very broad trunk ribs that made its body stiff, but no shell. It was a land animal, which moves the ecological setting of the origin of turtles onto land rather than into water. (Bever et al. (2015), Evolutionary origin of the turtle skull, Nature.)

Pappochelys, from the Middle Triassic of Germany about 240 million years ago, adds the next clue. It was small, up to about 20 cm long with a tail making up half of that, and had broad, blade-like ribs and a belly covered with tightly packed, partly fused gastralia (belly ribs). Gastralia are thought to be the source of most of the plastron bones. (Wikipedia: Pappochelys (Schoch & Sues 2015, Nature 523:584).)

Odontochelys semitestacea, about 220 million years old, from Guizhou in southwest China, had a complete bony plastron but no bony carapace, only broadened ribs. It still had teeth in both jaws. The fossil showed that the plastron evolved before the carapace. (Wikipedia: Odontochelys (Li et al. 2008, Nature 456:497).)

Odontochelys also shows the shoulder blade still in front of the rib cage, as in other reptiles. Studies of turtle embryos by Nagashima and colleagues in 2009 explain how modern turtles moved it inside. The ribs stop growing downwards around the body and instead grow outwards and forwards, and the body wall folds inwards along the carapacial ridge (the future edge of the shell). The ribs then cover the shoulder blade, which ends up inside the rib cage. The muscles linking scapula and ribs keep their old connections and simply twist into the new arrangement. (National Geographic on Nagashima et al. (2009, Science): how the turtle got its shell.)

The fully shelled Proganochelys of the Late Triassic of Germany and Thailand had a complete carapace and plastron. It could not pull its head into the shell; it had a long neck and a long, spiked tail ending in a club. The oldest known side-necked and hidden-necked turtles, the two living groups that can pull the head in, date from the Late Jurassic. (Wikipedia: turtle shell (bone and scute nomenclature).)

The largest turtle that ever lived was Archelon, a sea turtle of the Late Cretaceous Western Interior Seaway of North America, about 80 to 74 million years ago. The biggest specimen, nicknamed Brigitta and now in the Natural History Museum Vienna, is about 4.6 m long and 4 m across the flippers. Its carapace had thin bony plates and was probably leathery, like a leatherback's, but Archelon belongs to an extinct group, the protostegids, that is not closely related to living leatherbacks; the leathery shells evolved separately. (Wikipedia: Archelon.)

Every bone, scute and organ in the 3D model, grouped by system. Click any name in the viewer's list to jump to it in 3D.