History and Evolution 

The reptile Eunotosaurus is the earliest known ancestor to the turtles we know today (Bennet). But unlike turtles, the Eunotosaurus lacked a shell and spent its life exclusively on land (Bennet). Odontochelys semitestacea is believed to be the first species of turtle to live in the water (Bennet). Although Odontochelys semitestacea was still without a shell, it did have an expanded rib cage that later evolved into a shell (Bennet). Proganochelys quenstedti is one of the first recorded turtle species to possess a fully developed shell (Bennet). Turtles split into 2 different groups during the Jurassic period; Cryptodira (can retract their necks) and Pleurodira (can pull their heads to the side) (Bennet). Although they no longer can retract their necks, modern sea turtles are a part of the Cryptodira (Bennet). The most recent marine turtle ancestor to resemble modern sea turtles lived during the cretaceous period and is called Desmatochelys padillai (Bennet). 

Classification 

Sea turtles are marine reptiles. All reptiles have lungs to breathe, are cold blooded, lay eggs, and have scaly skin without feathers or hair (“Reptiles”). There are many land reptiles, but only four types of marine reptiles; sea snakes, crocodiles, marine iguanas, and sea turtles. 

There used to be four families of sea turtles; Cheloniidae, Dermochelyidae, Protostegidae, and Toxochelyidae (Bennet). Protostegidae and Toxochelyidae went extinct due to the same asteroid event that killed the dinosaurs (Bennet). Now, only Cheloniidae and Dermochelydae exist (Bennet). Cheloniidae possess scutes while Dermochelydae’s shells are made of skin with ridges (Bennet). The leatherback sea turtle is the only remaining member of the Dermochelydae today, due to competition with other creatures (Bennet). 

Anatomy and Adaptation 

All species of sea turtle have a carapace, or shell. It protects them from predators and the high pressure increases as they dive deeper into the water. The leatherback sea turtle, who has rigged skin covering its carapace, is different from the other species of sea turtles. All the other species have scutes. Scutes are evolved scales that are large and cover their carapace. There are three different regions of scutes; medial/central (in the center column of scutes), lateral (on the sides), and marginal (lining the outer edge). 

Land turtles have legs, but for sea turtles, their legs evolved into flippers. The turtle’s “belly” is called its plastron. Some species also have a claw or multiple claws on their flippers. 

Due to their reptilian classification, sea turtles have lungs and not gills like other marine animals. This prevents them from breathing underwater. Sea turtles have adapted the ability to hold their breath for long periods of time, making their trips to the surface for oxygen more minimal. 

Another adaptation that sea turtles have undergone is the development of their salt gland. The salt gland is located near their eyes. It filters the salt out of the ocean water when they drink it. The filtered out salt exits the sea turtle’s bodies through their tears. This makes it so that they don’t have to find freshwater in order to stay hydrated.

The Seven Different Species 

There are seven species of sea turtles. There is the previously mentioned leatherback sea turtle. The other six species are the green sea turtle, the loggerhead, hawksbill, flatback, the Kemp’s Ridley, and Olive Ridley. Different species can be identified by a number of characteristics; their beak or mouth, the number of medial and lateral scutes that they possess, the amount of claws they have, where they live, their size, carapace shape, diet, and color.

Green Sea Turtle 

Carapace Shape: oval

Medial Scutes: 5

Lateral Scutes: 4

Claws: one on front flippers 

Size: 2nd largest turtle 

Diet: seagrass 

Color: brown/yellow

Location: along coasts 

Beak/Mouth: sharp and serrated to aid in chewing plants

Reason for name: The seagrass they eat causes their fat to be green.

Fun Fact: They are the only sea turtles that are herbivores.

Loggerhead Sea Turtle 

Carapace Shape: rounded end

Medial Scutes: 5

Lateral Scutes: 5

Claws: 2 on front flippers 

Size: 3rd largest sea turtle 

Diet: Crabs, snails, and clams 

Color: reddish-brown

Location: ranging from California, to Florida, to Australia 

Beak/Mouth: Crushing jaw

Reason for name: They have a large head, similar to a log.

Fun Fact: They are the most common sea turtles.

Kemp’s Ridley Sea Turtle 

Carapace Shape: rounded end 

Medial Scutes: 5

Lateral Scutes: 5

Claws: 1 on front flippers 

Size: smallest (and rarest) sea turtle 

Diet: crustaceans 

Color: pale-green 

Location: Mexico 

Beak/Mouth: strong jaw 

Reason for name: Discovered by Richard Kemp, a fisherman

Fun Fact: The females nest in groups during the day 

Olive Ridley Sea Turtle 

Carapace Shape: rounded end 

Medial Scutes: 6 

Lateral Scutes: 7

Claws: 1 on front flippers 

Size: 2nd smallest sea turtle 

Diet: Crustaceans, seagrass, and squid 

Color: olive-green 

Location: Atlantic, Pacific, and Indian Oceans

Reason for name: The color of their carapace 

Fun Fact: They also nest in groups

Leatherback Sea Turtle 

Carapace Shape: Leathery and rigged 

Medial Scutes: none

Lateral Scutes: none

Claws: none

Size: Largest (and oldest) sea turtle 

Diet: Jellyfish 

Color: dark grey or black with white spots

Location: colder waters 

Beak/Mouth shape: Downward curving spine located in their mouth that allows them to capture their prey and aid in swallowing. 

Reason for name: Skin like leather covering their back

Fun Fact: They are the only remaining species of sea turtle that has no scutes. 

Flatback Sea Turtle 

Carapace Shape: flat

Medial Scutes: 5

Lateral Scutes: 4 

Claws: none

Size: smaller than green sea turtles 

Diet: crustaceans 

Color: pale-green 

Location: Australia 

Beak/Mouth shape: rounded

Reason for name: have a flat shell 

Fun Fact: they live in murky water so they are not often seen

Hawksbill Sea Turtle

Carapace Shape: Pointed near the tail and overlapping scutes. 

Medial Scutes: 5

Lateral Scutes: 4

Claws: 2 

Size: one of the smaller species 

Diet: Sea sponges 

Color: multicolored; mostly brown and yellow

Location: coral reefs

Beak/Mouth shape: hawk-like 

Reason for name: the shape of their beak 

Fun Fact: They are poached because of their beautiful carapace, which people use to make jewelry.

Life Cycle and Reproduction 

All reptiles lay eggs. Even though they are marine reptiles, sea turtles lay their eggs on land. The majority of female sea turtles will return to the place they were born to nest. This is called their natal beach. The females find their natal beach in a variety of different ways; geomagnetic imprinting, chemical cues, ocean currents & wave cues, and celestial & sunlight orientation. Nesting occurs at night because it is easier to avoid predators or be disrupted. They lay their eggs at least twice a season, but not every season. The number of eggs per nest depends on the species of sea turtle. Nest must be high up on the beach to avoid getting drowned in water during high tide. Nesting involves the female digging a hole called a “body pit” with her flippers, and then an egg cavity. It is usually around 2 feet deep. Sea turtle eggs are flexible for maximum capacity. Life expectancy for baby sea turtles is low, so the more eggs, the better. Before leaving, the female will cover the nest with sand, making a big mess all over to throw off predators. The gender of the sea turtles depends on the temperature of the sand surrounding the nest. Cooler sand produces males and warmer sand produces females. Many remember this by the saying, “Dudes are cool and chicks are hot.” The eggs usually hatch 60 days after they are laid in the nest. They use a temporary tooth called the caruncle to break through the egg shell. They emerge from the nest during the night. Baby sea turtles are on their own from birth because the mother does not return to her nest. Male sea turtles spend their whole life at sea. This has led to scientists learning more about female sea turtles than male sea turtles. 

Why Sea Turtles are Endangered 

• Boat strikes (“Threats”): Several different species suffer injuries from boat strikes, especially those that are marine reptiles and mammals that need to travel to the surface for air.

• Injury from fishing gear (“Threats”): Nets, fishing lines, all of it poses threats to sea turtles.

• Artificial lighting (“Threats”): Since sea turtles only nest at night, artificial lighting near beaches confuses nesting females.

• Predators raiding nests (“Threats”): This is an issue for many egg laying species, and not something that is easily preventable, it’s just unfortunately a part of nature.

• Pollution (“Threats”): Pollution of the ocean and beaches is a huge problem overall. Plastic bags getting into the sea is specifically a problem for sea turtles that eat jellyfish. Plastic bags look almost identical to jellyfish when drifting in water. So, sea turtles eat them, they get stuck in their throats, and the sea turtles die.

• Beach erosion (“Threats”): Erosion of beaches limits the amount of space that sea turtles have to nest.

• Poaching (“Threats”): The hawksbill sea turtle specifically is poached

• Beach tourism (“Threats”): People visiting beaches at night can disturb both the nesting and hatching process for sea turtles.

Works Cited

Bennet, Larisa. “Sea Turtles.” Smithsonian, December 2018, https://ocean.si.edu/ocean-life/reptiles/sea-turtles.

Center for Biological Diversity. “Reptiles.” biologicaldiversity.org, https://www.biologicaldiversity.org/species/reptiles/#.

“Threats.” Sea Turtle Conservancy, https://conserveturtles.org/threats/. Accessed 7 April 2025.