The first sea turtles appeared 245-208 million years ago. The earliest known sea turtles appear in the fossil record from 208-144 million (Triassic period) years ago. Scientists believe that modern sea turtles are derived from marsh-inhabiting ancestors that lived during the Late Triassic period.

In addition to the saltwater crocodiles, marine snakes, and marine iquanas, sea turtles are the last surviving seawater-adapted reptiles.

Sea turtles have a streamlined shell. Unlike the land turtle, the sea turtles head and limbs do not retract.

Some of the noteworthy physical characteristics of a turtle:

• Foreflippers are long and paddlelike.
• Sea turtles have large upper eyelids that provide protection for their eyes.
• Sea turtles do not have an external ear opening.
• Sea turtles do not have teeth.
• The large, bony shell provides protection from predation and abrasion.
• Male and female sea turtles do not differ externally until they approach maturity.
• Adult males have longer, thicker tails, because the male reproductive organ is housed in the base of the tail.

The senses of the sea turtle:

• The sea turtle ear has a single bone in the middle ear that conducts vibrations to the inner ear.
• Sea turtles can see well under water but are shortsighted in the air.
• Sea turtles are sensitive to touch on the soft parts of its flippers and on its shell.
• It is believed that sea turtles have an acute sense of smell in the water
• Sea turtles open their mouths slightly and draw in water through the nose, then immediately empty it out again through the mouth.

Most females return to the same beach each year for nesting. Recent studies suggest that some females of some species will visit more than one nesting beach in a season. Females of most species usually come ashore at night. She comes alone and most often during high tide. A female sea turtle crawls above the high tide line. She uses her front flippers to dig out a pit. She then uses her hind flippers and digs an egg cavity; the depth of the cavity is the length of the hind flipper.

Sea turtles as divers:

• Leatherbacks routinely dive more than 305m (1,000 ft) and reach depths of 1,190m (3,900 ft).
• Sea turtles are ectothermic and have a slow metabolic rate.
• Like whales, sea turtles shunt blood away from tissues tolerant of low oxygen levels toward the heart, brain, and central nervous system.
• Leatherbacks have high concentrations of red blood cells and can retain more oxygen.
• Sea turtles obtain sufficient water from their diet and from metabolizing seawater and can live without a fresh water source.
• Sea turtles have a salt gland to rid their bodies of excess salt.

The sea turtles of Costa Rica icnlude five species and are as follows:

• Green sea turtle found in Tortuguero National Park nesting from July to October.
• Migrates long distances (up to 2,600-km/1,400 mi) and is named for the greenish color of its flesh and fat.
• Grows to a maximum size of about 1.2m (4 ft) and a weight of 175kg (440 lbs).
• Has a heart-shaped shell, small head, and single-clawed flippers.
• Feeds almost exclusively on sea grasses and marine algae, as an adult.
• Found in shallow waters (except when migrating) inside reefs, bays, and inlets.
• Nesting requirements include an open beach with a sloping platform and minimal disturbance.
• Is declining in population because of commercial harvest for eggs and meat.
• Crawls up onto beaches and basks in the sun, unlike other species of turtles.
• Found on the Pacific and Atlantic coast.

• Hawksbill sea turtle found in Tortuguero National Park year round but especially in August and in Gandoca-Manzanillo National Wildlife Refuge from March to August.
• Grows up to 1m (3 ft) in length and weighs up to 120kg (300 lbs).
• Has an elongated oval shell with overlapping scutes on the carapace, a relatively small head with a distinctive hawk-like beak, and flippers with two claws.
• Feeds primarily on sponges and is associated with the coral reef community.
• Is the most seriously endangered of the sea turtles and has been classified as critically endangered.
• Is declining because of harvesting and being caught in fishing nets.
• Nesting preferences are in extensive swamps or large bodies of open water which have narrow ocean connections.
• Found primarily in tropical coral reefs.
• Found on the Pacific and Atlantic coast.

• Leatherback sea turtle found on the Caribbean coast and Playa Grande Marine Turtle National Park from January to June.
• Grows from 1-2.5m (4-8 ft) in length and 195-785kg (500-2000 lbs) in weight and is the largest sea turtle which is distributed worldwide.
• Is the fourth largest reptile and is only smaller than three types of crocodiles.
• Has shell which is composed of small bones covered by firm, rubbery skin with seven longitudinal ridges or keels.
• Eats mostly jellyfish and but also eats sea urchins, squid crustacean, tunicates, fish, blue-green algae and floating seaweed.
• Nesting habits include an average of 5-7 times within a nesting season, with an observed maximum of 11 nests.
• Constructed at night, the nests hold between 70-80 yoked eggs.
• Incubating eggs need 55-75 days to hatch.
• Travels huge distances as far north as the Arctic Circle and and has been found as far south as Cape of Good Hope in Africa and New Zealand.
• Is declining dramatically and is believed to be because of humans selling the eggs and meat and accidental trapping in fish nets.
• Found on the Pacific and Atlantic coast.

• Olive Ridley sea turtle found at Ostional Wildlife Refuge,Isla del Cano Biological Reserve and Playa Grande Marine Turtle National Park from August to October.
• Grows up to 0.75m (2.5 ft) long and 40kg (100 lbs).
• Has a high and variable numbers of vertebral and costal scutes.
• Eats crabs, shrimp, rock lobsters, jellyfish, and tunicates.
• Was named for the olive color of its heart-shaped shell and is one of the smallest of the sea turtles.
• Nesting habits include 1-3 times per season, producing about 100-110 eggs.
• Incubating eggs need 50-60 days to hatch.
• Is caught in commercial trawling and dragged behind the boat where it drowns
• Found on the Guanacaste Pacific coast.

• Loggerhead sea turtle found on the Caribbean coast from December to April.
• Grows up to 364kg (800 lbs) and 1.1m (3.5 ft).
• Has a large head with blunt jaws and is reddish brown in color.
• Nesting habits include 1-7 times per season and lays about 100-126 eggs per clutch.
• Is distributed widely and can be found hundreds of miles out to sea, as well as in inshore areas such as bays, lagoons, salt marshes, creeks, ship channels, and the mouths of large rivers. It likes coral reefs, rocky places, and ship wrecks where it feeds.
• Nests on ocean beaches and occasionally on estuarine shorelines with suitable sand. Nests are made between the high tide line and the dune front.
• Hunted for its eggs and meat, its fat was used in cosmetics and for medications.
• Used in making combs, they were killed for their shells.
• Found in Tortuguero National Park, Caribbean Coast.