DNA remnants found in six million-year-old sea turtle

Scientists have recently uncovered remnants of DNA within the fossilized remains of a sea turtle, closely related to contemporary Kemp’s ridley and olive ridley turtles, dating back six million years. 

This finding marks one of the infrequent instances where genetic material has been identified in ancient vertebrate fossils. The study is published in the Journal of Vertebrate Paleontology.

Bone cells 

The fossil, discovered along the Caribbean coast of Panama in 2015, presented notably well-preserved bone cells, known as osteocytes. 

Though the fossil is incomplete, featuring a relatively intact carapace – the turtle’s shell – but lacking the rest of the skeleton, researchers estimate the turtle would have measured about a foot (30 cm) in length during its lifetime.

DNA remnants

In some of these osteocytes, the cell nuclei remained intact and responded to a specific chemical solution. This reaction allowed the scientists to detect the remnants of DNA, essential for an organism’s development and functioning, explained study lead author Edwin Cadena, a paleontologist at the Universidad del Rosario in Bogota and the Smithsonian Tropical Research Institute.

“I want to point out that we did not extract DNA, we only were able to recognize the presence of DNA traces in the nuclei,” Cadena said. 

Given its perishable nature, DNA preservation in ancient remains is rare, but not unprecedented, with discoveries of DNA remnants dating back to around two million years in some ancient specimens from Greenland.

The only vertebrate fossils older than the newly studied turtle and showing similar DNA remnants belonged to two dinosaurs – Tyrannosaurus, from around 66 million years ago, and Brachylophosaurus, from about 78 million years ago. DNA remnants have also been reported in insects dating back tens of millions of years.

Evolutionary history 

The turtle in question belongs to the genus Lepidochelys, which includes two of the seven existing sea turtle species – the Kemp’s ridley, primarily found in the Gulf of Mexico, and the more widespread olive ridley. 

This fossil is pivotal in shedding light on the evolutionary history of this genus, which remains largely unclear. 

Due to the incomplete nature of the remains, the researchers have refrained from identifying the turtle by its species.

Biomolecular remains 

“Each fossil, each fossil site has specific conditions of preservation that in some cases could have favored preservation of original biomolecular remains such as proteins and DNA,” said Cadena.

“Maybe in the future and with more studies of this kind, we could be able at some point to sequence very small pieces of DNA and to infer things about their close relatives or involve that information in a broader molecular evolutionary study.”

Ridley turtles 

Ridley turtles, specifically those in the Lepidochelys genus, are known for their relatively small size compared to other sea turtles and unique nesting habits.

There are two species within this genus: the Kemp’s ridley turtle (Lepidochelys kempii) and the olive ridley turtle (Lepidochelys olivacea).

Size

Ridley turtles are among the smallest sea turtles, with adults usually weighing between 75 and 100 pounds.

Shell

Their carapace (upper shell) is broad and somewhat heart-shaped, typically olive-green in Kemp’s ridleys and grayish-green in olive ridleys.

Limbs

They possess flippers with clawless tips, which are adapted for swimming in open oceans.

Habitat and distribution

Kemp’s ridley is primarily found in the Gulf of Mexico and along the Atlantic coast of North America. These turtles prefer coastal habitats like bays, estuaries, and lagoons.

Olive ridley is found in tropical regions of the Pacific, Atlantic, and Indian Oceans. They are often found in open waters but migrate to coastal areas for nesting.

Nesting

One notable behavior of ridley turtles is “arribada,” a mass nesting phenomenon where thousands of females come ashore simultaneously to lay eggs. 

Olive ridley turtles are particularly known for this behavior, with significant arribada events occurring in places like Ostional, Costa Rica.

Diet

Ridley turtles are omnivores, consuming a variety of food items, including crabs, shrimp, mollusks, jellyfish, and occasionally algae and seaweed.

Conservation status

Both species of ridley turtles are listed under the IUCN Red List. Kemp’s ridley is classified as critically endangered, with major threats including habitat loss, accidental capture in fishing gear (bycatch), and climate change affecting their nesting sites.

Olive ridley is listed as vulnerable, facing threats like illegal egg collection, bycatch, and habitat destruction.

Conservation efforts 

Various conservation programs and legal protections are in place to help conserve ridley turtles, such as:

• Establishing protected areas and wildlife refuges.
• Implementing regulations and initiatives to reduce bycatch.
• Engaging in public education and awareness campaigns.

Arribadas 

The synchronized nesting events, or arribadas, are crucial for the survival of these species as it overwhelms predators, ensuring that more hatchlings can safely reach the ocean.

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