This 6-million-year-old turtle shell still has some DNA

Sea turtles have been around for at least 110 million years, yet relatively little is known about their evolution. Two of the most common sea turtles on Earth are olive ridley and Kemp’s ridley turtles that belong to a genus called Lepidochelys that could help fill in some of the gaps of sea turtle biology and evolution. A team of paleontologists not only discovered the oldest known fossil of turtle from the Lepidochelys genus, but also found some traces of ancient turtle DNA. The findings are detailed in a study published September 28 in the Journal of Vertebrate Paleontology.

[Related: 150 million-year-old turtle ‘pancake’ found in Germany.]

The DNA comes from the remains of a turtle shell first uncovered in 2015 in the Chagres Formation on Panama’s Caribbean coast. It represents the oldest known fossil evidence of Lepidochelys turtles. The turtle lived approximately 6 million years ago, curing the upper Miocene Epoch. At this time, present day Panama’s climate was getting cooler and drier, sea ice was accumulating at Earth’s poles, rainfall was decreasing, sea levels were falling.

“The fossil was not complete, but it had enough features to identify it as a member of the Lepidochelys genus,” study co-author and Universidad del Rosario in Bogotá, Colombia paleontologist Edwin Cadena tells PopSci. Cadena is also a research associate at the Smithsonian Tropical Research Institute in Panama.

The team detected preserved bone cells called osteocytes. These bone cells are the most abundant cells in vertebrates and they have nucleus-like structures. The team used a solution called DAPI to test the osteocytes for genetic material.

“In some of them [the osteocytes], the nuclei were preserved and reacted to DAPI, a solution that allowed us to recognize remains of DNA. This is the first time we have documented DNA remains in a fossilized turtle millions of years old,” says Cadena.

According to the study, fossils like this one from vertebrates preserved in this part of Panama are important for our understanding of the biodiversity that was present when the Isthmus of Panama first emerged roughly 3 million years ago. This narrow strip of land divided the Caribbean Sea and the Pacific Ocean and joined North and South America. It created a land bridge that made it easier for some animals and plants to migrate between the two continents.

[Related: Hungry green sea turtles have eaten in the same seagrass meadows for about 3,000 years.]

This specimen could also have important implications for the emerging field of molecular paleontology. Scientists in this field study ancient and prehistoric biomatter including proteins, carbohydrates, lipids, and DNA that can sometimes be extracted from fossils. 

Molecular paleontology aims to determine if scientists can use this type of evidence to determine more about the organisms than their physical shape, which is typically what is preserved in most fossils. Extracting this tiny material from bones was critical in sequencing the Neanderthal genome, which earned Swedish scientist Svante Pääbo the 2022 Nobel prize in physiology or medicine.

“Many generations have grown up with the idea of extracting and bringing back to life extinct organisms,” says Cadena. “However, that is not the real purpose of molecular paleontology. Instead, its goal is to trace, document, and understand how complex biomolecules such as DNA and proteins can be preserved in fossils.”

This new turtle specimen could help other molecular paleontologists better understand how soft tissues can be preserved over time. It could also shift the idea that original biomolecules like proteins or DNA have a specific timeline for preservation in fossils and encourage re-examining older specimens for traces of biomolecules.