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tle's skull is unique among living amniotes (which includes reptiles, birds and mammals), it is solid and rigid with no openings for muscle attachment (temporal fenestrae). Muscles instead attach to recesses in the back of the skull. Turtle skulls vary in shape, from the elongated skulls of softshells to the broad and flattened skull of the mata mata. Some turtle species have developed large and thick heads, allowing for greater muscle mass and stronger bites. Turtles that are carnivorous or durophagous (eating hard-shelled animals), have the most powerful bites. For example, the durophagous Mesoclemmys nasuta has a bite force of 432 N. Species that are insectivorous, piscivorous (fish-eating), or omnivorous have lower bite forces. Living turtles lack teeth but have beaks made of keratin sheaths lining the edges of the jaws. These sheaths may have sharp edges for cutting meat, serrated ridges for clipping plants, or broad plates for c rushing mollusks. The necks of turtles are highly flexible, possibly to compensate for their rigid shells. Some species, like sea turtles, have short necks while others, such as snake-necked turtles, have very long ones. Despite this, all turtle species have eight neck vertebrae, a consistency not found in other reptiles but paralleled in mammals. Some snake-necked turtles have both long necks and large heads and thus have difficulty lifting them when not in water. Limbs and locomotion Due to their heavy shells, turtles are slow-moving on land. A desert tortoise moves at only 0.22–0.48 km/h (0.14–0.30 mph). By contrast, sea turtles can swim at 30 km/h (19 mph). The limbs of turtles are adapted for various means of locomotion and habits and most have five toes. Tortoises are specialized for terrestrial environments and have column-like legs with elephant-like feet and short toes. The gopher tortoise has flattened front limbs for digging in the substrate. Freshwater turtle s have more flexible legs and longer toes with webbing, giving them thrust in the water. Some of these species, such as snapping turtles and mud turtles, mainly walk along the water bottom, much as they would on land. Others, such as terrapins, swim by paddling with all four limbs with the simultan