The order Chelonia consists of about 250 species of turtles and tortoises. The term tortoise is generally reserved for the terrestrial members of the order, such as the Galápagos tortoise shown in Figure 41-13a. Turtle usually refers to chelonians that live in water, such as the green sea turtle shown in Figure 41-13b.
The earliest known turtle fossils, which are more than 200 million years old, show that ancient chelonians differed little from today's turtles and tortoises. This evolutionary stability may be the result of the continuous benefit of the basic turtle design—a body covered by a shell. The shell consists of fused bony plates. The carapace is the top, or dorsal, part of the shell, and the plastron is the lower, or ventral, portion. In most species, the vertebrae and ribs are fused to the inner surface of the carapace. Because the ribs are fused to the carapace, the pelvic and pectoral girdles lie within the ribs instead of outside the ribs, as they do in all other terrestrial vertebrates. Unlike other reptiles, turtles have a sharp beak instead of teeth.
(a) The Galápagos tortoise, Geochelone gigantops, is protected from predators by its high domed carapace. (b) The green sea turtle, Chelonia mydas, is streamlined for life in the sea.
Turtles and tortoises live in a variety of habitats. Some species are permanently aquatic, some are permanently terrestrial, and some spend time both on land and in the water. The differing demands of these habitats are reflected in the shells and limbs of turtles. For example, water-dwelling turtles usually have a streamlined, disk-shaped shell that permits rapid turning in water, and their feet are webbed for swimming. The limbs of marine turtles, which spend their entire lives in the ocean, have evolved into flippers for swimming and maneuvering. Many tortoises have a domed carapace into which they can retract their head, legs, and tail as a means of protection from predators. Their limbs are sturdy and covered with thick scales.
All turtles and tortoises lay eggs. The female selects an appropriate site on land, scoops out a hole with her hind limbs, deposits the eggs, and covers the nest. She provides no further care for the eggs or the hatchlings. Marine turtles often migrate long distances to lay their eggs on the same beach where they hatched. For example, Atlantic green sea turtles travel from their feeding grounds off the coast of Brazil to Ascension Island in the South Atlantic—a distance of more than 2,000 km (1,242 mi). These turtles probably rely on several environmental cues, possibly even the Earth's magnetic field and the direction of currents, to find this tiny island.
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